Timetables

List of Abstracts, talks

Day 1: first block

Convener: Andréa Tommasi

A long section of depleted mantle peridotite

C. Johan Lissenberg¹*, Andrew M. McCaig², Susan Q. Lang³, Peter Blum⁴, Natsue Abe⁵, William J. Brazelton⁶, Rémi Coltat⁷, Jeremy R. Deans⁸, Kristin L. Dickerson⁹, Marguerite Godard¹⁰, Barbara E. John¹¹, Frieder Klein¹², Rebecca Kuehn¹³, Kuan-Yu Lin¹⁴, Haiyang Liu¹⁵, Ethan L. Lopes¹⁶, Toshio Nozaka¹⁷, Andrew J. Parsons¹⁸, Vamdev Pathak¹⁹, Mark K. Reagan²⁰, Jordyn A. Robare²¹, Ivan P. Savov², Esther M. Schwarzenbach²², Olivier J. Sissmann²³, Gordon Southam²⁴, Fengping Wang²⁵, C. Geoffrey Wheat²⁶, Lesley Anderson²⁷, Sarah Treadwell²⁸

¹School of Earth and Environmental Sciences, Cardiff University; Cardiff, United Kingdom. ²School of Earth and Environment, University of Leeds; Leeds, United Kingdom ³Department of Geology and Geophysics, Woods Hole Oceanographic Institution; Woods Hole MA USA. ⁴International Ocean Discovery Program, Texas A&M University; College Station TX, USA. ⁵Japan Agency for Marine-Earth Science and Technology; Yokohama, Japan. ⁶School of Biological Sciences, University of Utah; Salt Lake City UT, USA. ⁷Instituto Andaluz de Ciencias de la Tierra; CSIC-UGR, Spain. ⁸School of Biological, Environmental, and Earth Sciences, University of Southern Mississippi; Hattiesburg MS, USA. ⁹Department of Earth and Planetary Sciences, University of California, Santa Cruz; Santa Cruz CA, USA. ¹⁰Geosciences Montpellier, CNRS, University of Montpellier, Montpellier, France. ¹¹Department of Geology and Geophysics, University of Wyoming; Laramie WY, USA. ¹²Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution; Woods Hole MA, USA. ¹³Institute of Geosciences and Geography, Martin-Luther-University Halle-Wittenberg; Halle, Germany. ¹⁴Department of Earth Sciences, University of Delaware; Newark DE, USA. ¹⁵Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences; Qingdao, China. ¹⁶Department of Geophysics, Stanford University; Stanford CA, USA. ¹⁷Department of Earth Sciences, Okayama University; Okayama, Japan. ¹⁸School of Geography, Earth and Environmental Sciences, University of Plymouth; Plymouth, United Kingdom. ¹⁹Department of Geology, Central University of Punjab; Bathinda, India. ²⁰Department of Earth and Environmental Sciences, University of Iowa; Iowa City IA, USA. ²¹School of Molecular Sciences, Arizona State University; Phoenix AZ, USA. ²²Department of Geosciences, University of Fribourg; Fribourg, Switzerland. ²³IFP Energies Nouvelles; Paris, France. ²⁴School of the Environment, The University of Queensland; St. Lucia QLD, Australia. ²⁵Key Laboratory of Polar Ecosystem and Climate Change, Ministry of Education; and School of Oceanography, Shanghai Jiao Tong University, Shanghai 200030, China. ²⁶Global Undersea Research Unit, University of Alaska Fairbanks; Moss Landing CA, USA. ²⁷Arctic Research Support and Logistics; Washington DC, USA. ²⁸Department of Communication, University of North Dakota and Blue Marble Space Institute of Science, USA

When: Wednesday 2nd october at 9:20 am Speaker: Johan Lissenberg; https://orcid.org/0000-0001-7774-2297

The depleted mantle is a principal component of the upper mantle and drives significant volcanism. However, our knowledge of the depleted mantle has remained limited due to its severely restricted accessibility, with only local exposure of abyssal peridotites along slow- and ultraslow-spreading mid-ocean ridges and transform faults. Although abyssal peridotites have yielded important insights into mantle composition and melting, the vast majority of these rocks have been obtained by dredging. Hence, they lack the context, spatial continuity, and orientation information critical for understanding a range of processes, including the spatial scale of compositional variations, melt migration and mantle flow. As a result, many properties of the depleted mantle have been inferred from the study of its melting products: mid-ocean ridge basalts.

Here, we report the recovery of a long (1268 m) section of serpentinized abyssal mantle peridotite, interleaved with thin gabbroic intrusions, from Atlantis Massif (Mid-Atlantic Ridge) during IODP Expedition 399. The nearly continuous recovery in the principal hole (U1601C) provides an opportunity to obtain a robust and quantitative lithological, mineralogical, and structural inventory of the upper mantle.

The peridotites are dominated by harzburgite (82%; Fig. 1), with significant dunite (18%); lherzolite is rare, as are ultramafic veins (wehrlite, orthopyroxenite, (olivine) websterite). Mineralogically and geochemically, the peridotites are depleted: harzburgites have low orthopyroxene content (average 15.9%) and often lack clinopyroxene altogether. Geochemically, the peridotites have high MgO/SiO₂ and low Al₂O₃/SiO₂, falling on the depleted end of the array defined by global mantle rocks.

Orthopyroxene abundance varies significantly on scales ranging from the centimeter to hundreds of meters, forming a continuum from 0-30%. Dunite occurs as zones in harzburgite and, more frequently, in orthopyroxene-bearing dunite, and are typically tens of centimeters thick. Contacts between dunite and the surrounding peridotites are nearly always gradational, and well-defined dunite zones are an endmember of a continuum of variations in orthopyroxene content in the mantle rocks, with the full spectrum of harzburgite to dunite preserved in the core. We posit that the observed variations in mineralogy and therefore lithology are controlled by melt migration and the associated dissolution of orthopyroxene and precipitation of olivine, superimposed on the residues of relatively high degrees of melting.

The geometry of melt flow is captured by the orientation of the dunite zones. Harzburgite-dunite contacts have predominantly intermediate dips and record a ~40° discordance with the mantle fabrics. This is consistent with the formation of a network of dunite channels with variable but predominantly intermediate dips oblique to mantle upwelling as a result of melt focusing toward the ridge axis.

Going forward, the comprehensive rock record obtained during Expedition 399 provides a wealth of opportunities to make fundamental advances on our understanding of the oceanic upper mantle. This includes a determination of mantle heterogeneity on the centimeter to kilometer scale and investigations of the role of previous depletion in governing mantle composition and melting. The continuous nature of the core, and its re-orientation, will also be instrumental in studies of spatial variations in melting and melt transport, as well as research into mantle flow and the associated deformation mechanisms.

Figure 1. Harzburgites recovered from Hole U1601C during IODP Expedition 399.

Reactive melt percolation in spinel mantle harzburgite: microstructural and petrological evidence from the Wadi Tayin Massif, Oman

Battifora C.¹, Ferrando C.¹, Crispini L.¹, Godard M.², Basch V.³, Rampone E.¹

¹D.I.S.T.A.V. - University of Genova, Corso Europa 26 – 16132 Genoa, Italy ²Géosciences Montpellier – University of Montpellier, campus Triolet CC60, Place Eugène Bataillon – 34095 Montpellier, France ³Earth and Environmental Sciences Department - University of Pavia, Via Ferrata 1 - 27100 Pavia, Italy

When: Wednesday 2nd october at 9:40 am Speaker: Caterina Battifora; https://orcid.org/0009-0002-3028-5492

The Oman ophiolite provides direct sampling access to a complete sequence of oceanic lithosphere formed in a fast-spreading ridge environment. Our research aims at constraining processes ruling lithosphere accretion, by studying the composite recrystallization and melt migration history recorded in the Oman lithospheric mantle. Such evolution of magmatic and metamorphic processes occurring within the mantle from spinel- to plagioclase-facies conditions are poorly constrained to date, despite numerous studies devoted to the Moho Transition Zone (MTZ) and the oceanic crust.

Here, we present microstructural and petrological evidence of reactive melt percolation that occurred in the Oman mantle harzburgites at spinel-facies conditions.

The studied samples pertain to the upper mantle section of the Wadi Tayin massif. They were selected according to their textural features and depth from the MTZ, to observe possible variations in the modes and extent of the reactive melt percolation process as a function of depth in the mantle. The entire sample set was collected by our research team during the OmanDP core logging of the CM (Crust-Mantle transition) Sites (2017- 2018; Kelemen et al., 2020) and a field campaign along the Wadi Nassif (February 2024). The latter allowed sampling of the shallow harzburgitic mantle down to ~800 m below the MTZ, and better documenting the macro- to micro-scale distribution and interplays between melt transport and shallow mantle deformation at the crust-mantle interface.

The studied harzburgites display a porphyroclastic association of Ol+Opx±Cpx±Spl and a weak mantle foliation defined by the elongation and alignment of orthopyroxene and/or spinel grains. Opx porphyroclasts show diffuse Cpx exsolutions and are partially to completely replaced by granoblastic aggregates composed of Opx+Cpx±Spl. The granoblastic association is formed by the progressive cooling of the harzburgite at subsolidus lithospheric conditions (T= 950–1050 °C as determined by Opx – Cpx geothermometers). Both pyroxene porphyroclasts and granoblasts show lobate contacts with interstitial olivine crystals, suggesting reactive melt percolation that involved pyroxene dissolution and crystallization of new olivine from the migrating melt. Different pyroxene generations (porphyroclastic and granoblastic) exhibit similar major element compositions.

Crystallographic Preferred Orientations (CPO) of orthopyroxene show patterns characteristic of upper mantle deformation under high-T and dry conditions. In contrast, the olivine CPO varies from axial-[100] patterns, coherent with olivine from upper mantle peridotite deformed under high T and dry conditions (Tommasi et al., 2000), to axial-[010] patterns, indicating deformation in the presence of melt (Zimmerman et al., 1999). Such CPO transition is correlated with olivine modal contents increasing from 65% to 80%. Harzburgites with lower modal olivine contents show weak axial-[100] olivine CPO, whereas samples with higher modal olivine abundances have well-defined axial-[010] olivine CPO. Harzburgite modal composition and CPO evolution are interpreted as being controlled by the melt-rock ratio integrated over time similar to what proposed by (Higgie & Tommasi, 2012) for the MTZ. Harzburgites preserving lower olivine modal amount and axial-[100] CPO record lower melt-rock ratio over time compared to those characterized by stronger axial-[010] CPO symmetry and high olivine modal contents.

This study suggests that reactive melt percolation affected the Oman mantle at spinel-facies conditions, after melting and subsequent incorporation at lithospheric environment by conductive cooling. Such event, although not causing evident changes in mineral major element compositions, is clearly documented by modal and microstructural evolution of spinel harzburgites. This evolution reflects microscale variations in melt/rock ratios and indicates percolation of melts across the uppermost ~800m of mantle section, with no direct correlation with depth from the Moho Transition Zone.

References Higgie, K., & Tommasi, A. (2012). Feedback between deformation and melt distribution in the crust–mantle transition zone of the Oman ophiolite. Earth and Planetary Science Letters, 359, 61-72. https://doi.org/10.1016/j.epsl.2012.10.003

Kelemen, P. B., Matter, J. M., Teagle, D. A., Coggon, J. A., & Oman Drilling Project Science Team. 2020. Oman drilling project: Scientific drilling in the Samail ophiolite, sultanate of Oman. Proceedings of the Oman drilling Project.

Tommasi, Andréa, et al., 2000. Viscoplastic Self‐consistent and Equilibrium‐based Modeling of Olivine Lattice Preferred Orientations: Implications for the Upper Mantle Seismic Anisotropy. Journal of Geophysical Solid Earth 105, 7893–908. https://doi.org/10.1029/1999JB900411

Zimmerman, M. E., Zhang, S., Kohlstedt, D. L., & Karato, S. I. 1999. Melt distribution in mantle rocks deformed in shear. Geophysical Research Letters, 26(10), 1505-1508. https://doi.org/10.1029/1999GL900259

Formation mechanism of mantle dunitic channels in oceanic slow-ultraslow spreading centers

Li Wang¹, Qing Xiong¹*, Jian-Ping Zheng¹, Hong-Kun Dai¹, Xiang Zhou¹, Hong-Da Zheng¹

¹State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China

When: Wednesday 2nd october at 10:00 am Speaker: Li Wang; https://orcid.org/0000-0002-2750-0377

Oceanic spreading is one of the core processes of modern plate tectonics. In oceanic spreading centers, the melts formed by asthenosphere decompression upwelling can migrate quickly into oceanic crust through focused channels (Xiong et al., 2022). However, the existed dunitic channel model is mainly built based on investigations of the Oman ophiolite produced in fast spreading centers. Researchers have few knowledges on how dunitic channels formed in slow-ultraslow spreading centers. In this study, we selected the Dazhuka ophiolite, a product formed in oceanic slow-ultraslow spreading centers (Wang et al., 2024), to reveal the formation mechanism of dunitic channels. Through detailed petrographic observations and geochemical analyses, we found that these dunites can be divided as three types, Type 1 dunite, Type 2 dunite and Type 3A/3B dunite. The three types of dunites have relationships in petrogenesis. 1) From Type 1 to Type 2 dunites, the content of orthopyroxene (Opx) is gradually decreasing until Opx is all consumed, while the diameter of olivine (Ol) is gradually growing up to 2-3 cm. In addition to this, geochemical evidence also shows gradually increasing of Mg#-whole rock (0.91-0.93), Cr#-spinel (Spl; 0.46-0.83) and LREE/HREE-clinopyroxene (Cpx, 0.08-1.42). These phenomena may indicate reaction between wall-rock harzburgites and silica-unsaturated melts. 2) From Type 2 to Type 3A/3B dunite, Mg#-whole rock (0.93-0.87), Cr#-Spl (0.83-0.24), and LREE/HREE-Cpx (1.42-0.02) are decreasing, which may be caused by interaction between Type 2 dunites and silica-saturation melts. 3) The three types of dunites all exhibit amphibole growing around Cpx, and enrichment of fluid mobile elements of two kinds of pyroxenes, implying that later volatile-rich melts or fluids metasomatized these dunites.

Based on these lines of evidence, we suggest that the formation processes of the multiple types of Dazhuka dunites reveal unique dynamic characteristics of asthenosphere upwelling beneath oceanic slow-ultraslow spreading centers. In the first stage, silica-unsaturated melts were formed by stronger asthenospheric upwelling and reacted with wall-rock harzburgites to gradually formed Type 1 and Type 2 dunites. In the second stage, the upwelling of asthenosphere with the dunitic channels became weaker, ambient silica-saturated melts were focused into the dunitic channels and reacted with Type 2 dunites to form Type 3A/3B dunites. Eventually, when the dunitic channels became a part of the lithospheric mantle, the asthenospheric upwelling almost stagnated and formed volatile-rich melts or fluids to metasomatize the dunitic channels within the lithosphere. This study thus shows that asthenospheric upwelling may be pulsed under oceanic slow-ultraslow spreading center.

Keywords: Oceanic slow-ultraslow spreading centers, Dunitic channels, Melt-rock interaction, Dazhuka ophiolite in Tibet

References

Wang, L., Xiong, Q., Zheng, J.P., Dai H.K., Tian L.R., Zhou X., 2024. Multistage and diverse melt-mantle interaction in dunite-harzburgite channel systems beneath oceanic slow-ultraslow spreading centers: Evidence from the Xigaze ophiolite (Tibet). Lithos 468-469, 107501. https://doi.org/10.1016/j.lithos.2024.107501

Xiong, Q., Dai, H.K., Zheng, J.P., Griffin, W.L., Zheng, H.D., Wang, L., O’Reilly, S.Y., 2022. Vertical depletion of ophiolitic mantle reflects melt focusing and interaction in sub-spreading-center asthenosphere. Nature Communications 13, 6956. https://doi.org/10.1038/s41467-022-34781-w

Day 1: second block

Convener: José Alberto Padrón-Navarta

How dry is the Kaapvaal craton?

Jessy Dominique¹, Nathalie Bolfan-Casanova¹, Bertrand N. Moine¹, Ioana B. Radu², Ana E. Pradas del Real³, Jean-Luc Devidal¹, Dmitri A. Ionov⁴

¹Université Clermont Auvergne, Laboratoire Magmas et Volcans LMV, France ²Department of Geosciences, Swedish Museum of Natural History, Sweden ³SOLEIL Synchrotron, France ⁴Université de Montpellier, France

When: Wednesday 2nd october at 10:40 am Speaker: Jessy Dominique; https://orcid.org/0009-0006-1335-677X

Kimberlites give an exceptional insight of the deep cratonic mantle by exhuming at an extreme pace well-preserved peridotites from a wide range of depths. The fast exhumation velocity of peridotite limits diffusion of incompatible trace elements (REE, hydrogen) due to the interaction between the ascending kimberlite melt and the rock and allows to measure pristine water contents in nominally anhydrous minerals (NAMs). Water is regarded as a key parameter as it may impact, depending on its concentration, physical parameters such as rheology and partial melting. This study is focused on the petrology and water quantification of 17 spinel and garnet harzburgites scattered over a wide range of pressure (25 to 51 kbar) from Jagersfontein mine located at the rim of the Kaapvaal craton. Sampling was focused on poorly metasomatized, ultra-refractory harzburgites (Fo_92-95) which are considered to represent the most preserved cratonic archean mantle. Water content was obtained from FTIR spectroscopy on the most representative phases: olivine, orthopyroxexe, clinopyroxene and garnet. The aim of this study is to investigate the relation between petrology, water content of non-metasomatized cratonic mantle.

Nanochannels along fluid-solid interface in the Persani Mountains lithospheric mantle (Transylvania)

Thomas Pieter Lange^1,2,3,4, Péter Vancsó⁵, Zakhar Popov⁶, Mihály Pósfai⁷, Péter Pekker⁷, Csaba Szabó^1,3, István János Kovács^1,3 & Márta Berkesi^1,8

¹HUN-REN Institute of Earth Physics and Space Science, Budapest, Hungary ²Eötvös Loránd University, Doctoral School of Environmental Sciences, Budapest, Hungary ³MTA FI Lendület Pannon LitH2Oscope Research Group, Hungary ⁴Eötvös Loránd University, Lithosphere Fluid Research Lab, Budapest, Hungary ⁵HUN-REN Centre for Energy Research, Nanostructures Department, Budapest, Hungary ⁶Russian Academy of Sciences, Emanuel Institute of Biochemical Physics, Russia ⁷University of Pannonia, Nanolab, Environmental Mineralogy Research Group, Veszprém, Hungary ⁸MTA-FI Lendület FluidsbyDepth Research Group, Sopron, Hungary

When: Wednesday 2nd october at 11:00 am Speaker: Thomas Pieter Lange; https://orcid.org/0000-0002-8709-9239

The observed nanochannels are found parallel to [001] along the clinopyroxene-amphibole interface, particularly where an even number of clinopyroxene I beams transform to an odd number of amphibole I beams (Fig. 1; Veblen and Buseck, 1981). All nanochannels have slightly asymmetrical, edge-like shape in the [010] direction from the clinopyroxene towards the amphibole, associated with an additional pyribole phase in the [100] direction towards the clinopyroxene (Fig. 1). The pyribole is most likely the result of stress relaxation finalizing the shapes of the nanochannels.

The major and trace element composition of rock-forming minerals of upper mantle xenoliths from the Persani Mountains volcanic area show that the upper mantle went through significant degree of hydrous metasomatism (Vaselli et al., 1995; Falus et al., 2008; Faccini et al., 2020; Lange et al., 2023). Fraction of the infiltrating H₂O-bearing supercritical fluid was trapped in fluid inclusions hosted by olivine, orthopyroxene and clinopyroxene (Lange et al., 2023). In clinopyroxene, some fluid inclusions are associated with amphibole, which is the reaction product of the host clinopyroxene and the entrapped H2O-bearing fluid (Lange et al., 2023), based on petrographical observation. The presence of these channels implies higher diffusion velocity due to the relatively large "void" volume compared to other structural distortions along the clinopyroxene-amphibole phase boundary.

In our study, we use quantum mechanical simulations to explore the physical properties and effects of the clinopyroxene-amphibole interface that ensure the rapid element diffusion. The obtained results help us to better understand the formation and growth of nominally ‘water’-bearing minerals in deep lithospheric settings (lower crust, upper mantle) and the driving forces for significant fluid composition changes during micron- to nanoscale fluid evolution.

Figure 1. High-angle annular dark-field image of a nanochannel (‘Void’) and surrounding structure along the clinopyroxene-amphibole interface, where (010) and (100) represent two different interface planes. In the inset of I-beam geometry red, yellow and orange lines represent the amphibole, clinopyroxene, and pyribole I beams, respectively. The white edge-like shape represents the nanochannel. Amph = amphibole, Cpx = clinopyroxene, Py =pyribole.

Funding: The research was supported by the NKFIH FK-142985 and FK_132418, Nanomin, and the MTA-FI Lendület FluidsByDepth and Pannon LitH2Oscope research projects (LP2022-2/2022; LP2018-5/2023).

References Faccini, B., Rizzo, A. L., Bonadiman, C., Ntaflos, T., Seghedi, I., Grégoire, M., Ferretti, G. & Coltorti, M. (2020). Subduction-related melt refertilisation and alkaline metasomatism in the Eastern Transylvanian Basin lithospheric mantle: Evidence from mineral chemistry and noble gases in fluid inclusions. Lithos, 364, 105516. https://doi.org/10.1016/j.lithos.2020.105516

Falus, G., Tommasi, A., Ingrin, J. & Szabó, Cs. (2008). Deformation and seismic anisotropy of the lithospheric mantle in the southeastern Carpathians inferred from the study of mantle xenoliths. Earth and Planetary Science Letters, 272(1-2), 50-64. https://doi.org/10.1016/j.epsl.2008.04.035

Lange, T. P., Pálos, Z., Pósfai, M., Berkesi, M., Pekker, P., Szabó, Á., Szabó, Cs. & Kovács, I. J. (2023). Nanoscale hydrous silicate melt inclusions at the clinopyroxene-amphibole interface in a mantle xenolith from the Perșani Mountains Volcanic Field. Lithos, 454, 107210. https://doi.org/10.1016/j.lithos.2023.107210

Vaselli, O., Downes, H., Thirlwall, M., Dobosi, G., Coradossi, N., Seghedi, I., Szakács, A. & Vannucci, R. (1995). Ultramafic xenoliths in Plio-Pleistocene alkali basalts from the Eastern Transylvanian Basin: depleted mantle enriched by vein metasomatism. Journal of Petrology, 36(1), 23-53. https://doi.org/10.1093/petrology/36.1.23

The origin of deep CO₂-rich fluids in the Pannonian Basin: combined stable isotope study on fluid inclusions and dissolved gases in groundwaters

Spránitz, T.¹, Lange, T.P^1,2,3,4, Hencz, M.¹, Porkoláb, K.¹, Kővágó, Á.^2,4,5, Gelencsér, O.^2,3, Créon, L.⁶, Molnar, K.⁷, Tóth, Á.⁸, Erőss, E.⁹, Rouchon, V.¹⁰, Szabó, Cs.^1,2, Kovács, I.J.⁴, Török, K.¹¹, Berkesi, M.¹

¹MTA-EPSS FluidsByDepth Lendület Research Group, HUN-REN Institute of Earth Physics and Space Science, Csatkai Endre utca 6-8, Sopron 9400, Hungary; ²Lithosphere Fluid Research Lab, ELTE Eötvös Loránd University, Hungary; ³Doctoral School of Environmental Sciences, ELTE Eötvös Loránd University, Budapest 1117, Hungary; ⁴MTA-EPSS Lendület Pannon LitH2Oscope Research Group, HUN-REN Institute of Earth Physics and Space Science, Sopron, Hungary; ⁵Doctoral School of Earth Sciences, ELTE Eötvös Loránd University, Hungary; ⁶CAMECA, 29 Quai des Gresillons, Gennevilliers 92230, France; ⁷Isotope Climatology and Env Research Centre, HUN-REN Institute for Nuclear Research (ATOMKI), , Hungary; ⁸Copernicus Inst of Sustainable Devel, Utrecht Univ, Princetonlaan 8a, 3584 CB, Utrecht, The Netherlands ⁹József and Erzsébet Tóth Endowed Hydrogeology Chair, ELTE Eötvös Loránd University, Hungary; ¹⁰IFP Energies Nouvelles, Rond-point de l’Echangeur de Solaize BP 3, Solaize 69360, France ¹¹Supervisory Authority for Regulatory Affairs, Budapest, Hungary

When: Wednesday 2nd october at 11:20 am Speaker: Marta Berkesi; https://orcid.org/0000-0003-4380-057X

Stable isotope composition of CO₂-rich fluid is a powerful tool to trace lithosphere scale fluid processes like Earth’s degassing and global carbon cycling. Fluid inclusions (FI) encapsulating CO₂-rich fluids provide direct evidence on paleofluid migration events that took place at different levels of the deep lithosphere. On the other hand, deep fluid signature can also be detected in dissolved gases of much shallower-seated groundwaters, indicating the lithosphere scale connection of deep and shallow fluids. We present here preliminary results of the ongoing project ‘FluidsByDepth’, which aims to give contributions to the understanding of non-volcanic natural CO₂ degassing in the central Pannonian Basin. A comparative study has been carried out by determining the carbon and noble gas composition of deep lithospheric fluid (entrapped as inclusion in mantle xenoliths) and melt inclusions of asthenospheric origin as well as shallow groundwater’s dissolved gases in the Bakony-Balaton Highland Volcanic Field, Styrian Basin Volcanic Field and the Persani Mountains Volcanic Field.

A detailed characterization and thus careful selection of FI permitted the filtering out of late-stage fluid inclusions, which was also supported by the results on previous studies made on the same xenoliths in the sample series. In addition, vapor bubble of primary melt inclusions in olivine phenocrysts provided deep fluid signature from cooling alkali mafic melt. Moreover, groundwater was sampled considering the geohydrologic flow directions and hence was made at discharge areas.

He, Ne, and Ar isotope compositions of FI were estimated by crushing technique, while carbon isotope composition of CO₂ was approximated by Raman spectroscopy. One further goal of this study is to test the applicability to calculate δ¹³C-CO₂ isotope compositions of fluids based on in situ Raman spectroscopic measurements. We applied this method on fluid inclusions in xenoliths, including harzburgite, lherzolite, websterite, mafic-, felsic and metapelitic granulites, which sampled both the upper mantle and the lower to upper crust. In total 129 individual FI were selected in different host minerals, such as clinopyroxene, orthopyroxene, olivine, plagioclase and garnet.

The δ²H and δ¹⁸O stable isotopic ratio of the sampled groundwaters indicate meteoric origin and show no sign of an additional deep basin (metamorphic, magmatic) contribution. In contrast, the δ13C isotopic ratio of the dissolved CO₂ gases shows a narrow range (-5.2 – -9.6) and suggests mantle origin with slight organic sedimentary overprint. Stable isotopic ratios of dissolved helium also clearly indicate mantle origin for specific domains of the volcanic area.

Our results highlight the indirect role of preceding monogenetic upwelling channels as well as their relation to groundwater discharge areas in recent mantle degassing, which could be important contribution to deep carbon cycle.

Funding: This project was supported by the NKFIH_FK132418 as well as the MTA-EPSS Lendület (Momentum) FluidsByDepth Research group (LP2022-2/2022).

Day 1: third block

Convener: Basil Tikoff

Deformation, Rheology and Seismic Anisotropy in the Lithospheric Mantle beneath the North Anatolian Fault

Vasileios Chatzaras¹, Alexander D.J. Lusk², Utpal Singh¹, Ercan Aldanmaz³, Basil Tikoff⁴

¹The University of Sydney, School of Geosciences, Sydney, NSW, Australia, ²U.S. Geological Survey, Geosciences and Environmental Change Science Center, Denver, CO, USA ³University of Kocaeli, Department of Geology, Izmit, Turkey ⁴University of Wisconsin-Madison, Department of Geoscience, Madison, WI, USA

When: Wednesday 2nd october at 12:00 Speaker: Vasileios Chatzaras; https://orcid.org/0000-0001-9759-4754

We present constraints on the deformation conditions, rheology, and seismic properties of the lithospheric mantle beneath the North Anatolian fault zone (NAFZ). Peridotite xenoliths from the Biyikali and Çorlu volcanic centers equilibrated at temperatures ranging from 757 to 1019 °C and pressures between 1.0 to 2.1 GPa (38–75 km). The xenoliths record deformational microstructures consistent with transpressional shearing in a lithosphere-scale transcurrent fault system (Chatzaras et al., 2021). Spinel displays oblate fabric ellipsoids, consistent with flattening strain. Olivine exhibits axial-[010] and A-type crystallographic preferred orientation patterns (CPO). The obliquity of olivine CPO to the spinel lineation decreases with proximity to the NAFZ, suggesting an increase in shear strain.

Analysis by Fourier transform infrared spectroscopy indicates that constituent phases retain some OH- but bulk rock concentrations are generally restricted to <50 ppm H₂O by weight. From the rock microstructure, we determined differential stress magnitude and active deformation mechanism(s); combined with estimates of hydration state, we constrained the rheology. Recrystallized grain size piezometry shows that the mantle beneath the NAFZ sustained differential stresses of 10 to 20 MPa, largely independent of depth. The dominant deformation mechanism(s) change with depth; xenoliths extracted from shallower depths record evidence for grain size sensitive creep possibly in the presence of melt (Figure 1). At intermediate depths, both dislocation creep and grain size sensitive mechanisms were active, and we did not observe evidence for deformation in the presence of melt. The deepest samples were dominated by dislocation creep. The strong temperature sensitivity of creep mechanisms, combined with the low variability in differential stress, contributes to a stratified viscosity profile ranging from 1018 Pa s for the deepest samples, to >1022 Pa s at shallower depths (assuming a melt-free rheology) (Lusk et al., 2023). Although difficult to quantify from the rock record, melt likely reduced the viscosity of the shallow lithospheric mantle. The vertical stratification in viscosity below the NAFZ, the result of melt-present deformation and/or transitions in deformation mechanism, has important consequences for the seismic cycle of strike-slip fault systems.

We estimated the elastic wave velocities and seismic anisotropy of the peridotite xenoliths from the full elastic tensor, by accounting for the dependency of elasticity with pressure and temperature. The spatial variations in the seismic properties with distance from the NAFZ and depth, provide insights into the effect of strain gradients and rheological stratification on the seismic structure of the lithospheric mantle beneath a transcurrent plate boundary.

Figure 1. Block diagram of the North Anatolian fault zone at the Sea of Marmara area (Lusk et al., 2023). Panel (c) shows the resistivity model of Kaya et al. (2013). Panel (b) is the S-wave velocity model by Papaleo et al. (2018). Panel (d) shows the viscosity profile form the analysis of peridotite xenoliths. Electron backscattered diffraction phase maps are shown for four representative samples (field of view is 15 mm). Phase coloring is as follows: olivine—green; orthopyroxene— yellow; clinopyroxene—red; spinel—pink.

References

Chatzaras, V., Lusk, A.D., Chapman, T., Aldanmaz, E., Davis, J.R., & Tikoff, B., 2021. Transpressional deformation in the lithospheric mantle beneath the North Anatolian Fault Zone. Tectonophysics, 815, 228989. https://doi.org/10.1016/j.tecto.2021.228989

Kaya, T., Kasaya, T., Tank, S.B., Ogawa, Y., Tunçer, M.K., Oshiman, N., Honkura, Y., Matsushima, M., 2013. Electrical characterization of the North Anatolian Fault Zone underneath the Marmara Sea, Turkey by ocean bottom magnetotellurics. Geophysical Journal International, 193(2), 664–677. https://doi.org/10.1093/gji/ggt025

Lusk, A.D., Chatzaras, V., Aldanmaz, E., & Tikoff, B., 2023. Hydration state and rheologic stratification of the lithospheric mantle beneath the North Anatolian fault, Turkey. Geochemistry, Geophysics, Geosystems, 24, e2023GC011096. https://doi.org/10.1029/2023GC011096

Papaleo, E., Cornwell, D., Rawlinson, N., 2018. Constraints on North Anatolian Fault Zone width in the crust and upper mantle from S wave teleseismic tomography. Journal of Geophysical Research: Solid Earth, 123(4), 2908–2922. https://doi.org/10.1002/2017JB015386

Moa Island: Hydrogen, microstructures and petrophysical properties of an exceptionally fresh mantle sliver

Demouchy, S.¹, Barou, F.², Ishikawa, A.^3,4, Gardés, E.¹, and Tommasi, A.²

¹Laboratoire Magmas et Volcans, Université Clermont Auvergne, CNRS, IRD & OPGC, France, ²Geosciences Montpellier, CNRS & Université de Montpellier, Montpellier, France ³Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152–8550, Japan ⁴Research Institute for Marine Resources Utilization, Japan Agency for Marine-Earth Science and Technology, 2–15, Natsushima, Yokosuka, Kanagawa, 237–0061, Japan.

When: Wednesday 2nd october at 12:20 Speaker: Sylvie Demouchy; https://orcid.org/0000-0001-5023-4655

We characterize and quantify the microstructure, hydrogen concentrations, and seismic properties of a tectonically exhumed – very fresh – sliver of oceanic lithospheric mantle outcropping in the Moa Island (Leti archipelago, Timor-Tanimbar region). The 18 studied spinel peridotites (lherzolites and harzburgites) display variable degrees of olivine recrystallization (6.9-31.3 %), recorded by growth of strain‐free neoblasts onto the pre-existing deformed (coarse-porphyroclastic) microstructure. Occurrence of clinopyroxenes with interstitial (cuspate) shapes and crystal‐preferred orientations (CPOs) uncorrelated with the olivine CPO implies post-deformation crystallization by reactive melt percolation.

The olivine CPO has dominantly axial‐[010] or [100](010) patterns, similar to those usually observed in peridotitic xenoliths from oceanic mantle lithosphere. Seismic properties are calculated based on the modal compositions and CPO data for all samples. Increase in the olivine recrystallized fraction decreased the seismic anisotropy, since dynamic recrystallization produced some dispersion of the CPO but did not change the pattern acquired during the asthenospheric deformation. Average seismic velocities (mean Vp=7.9 km.s-1; mean Vs=4.5 km.s-1) and anisotropy (mean maximum S wave polarization anisotropy = 4.5%) are estimated by considering coherent orientation of the foliation and lineation of all samples.

The nominally anhydrous minerals contain small amounts of hydrogen (olivine: 18 ppm H₂O by weight; orthopyroxene: 58-175 ppm (Figure 1) H₂O wt and clinopyroxenes: 244-288 ppm H₂O wt), which yields an average bulk water content of 50 ppm H₂O wt for the Moa spinel-bearing peridotites, in agreement with previous estimates for the oceanic mantle lithosphere based on peridotitic xenoliths (Figure 1). This is the first direct measurement of the hydrogen concentrations in peridotites from an oceanic mantle lithosphere not modified by extensive serpentinization or magmatic extraction. As previously reported, no correlation between CPO patterns and hydrogen concentrations is found.

Figure 1. Compilation of hydrogen concentrations in orthopyroxenes (mostly porphyroblasts) from oceanic mantle lithosphere specimens from this study in and from Le Roux et al., (2021); Satsukawa et al., (2017); Schmädicke et al. (2011); Soustelle et al., (2010); Demouchy et al., (2015); and Demouchy and Tommasi, (2021). Hydrogen concentrations were determined by FTIR except for one study by Le Roux et al., (2021) who used SIMS.

References

Demouchy, S. and Tommasi, A., 2021. From dry to damp and stiff mantle lithosphere by reactive melt percolation atop the Hawaii plume. Earth Planet Sci. Lett., 574, 117159. https://doi.org/10.1016/j.epsl.2021.117159

Demouchy, S., Ishikawa, A., Tommasi, A., Alard, O., Keshav, S., 2015. Characterisation of the hydration in the oceanic mantle lithosphere: peridotite xenoliths from Ontong Java Plateau as an example. Lithos. 212-215, 189–201, https://doi.org/10.1016/j.lithos.2014.11.005.

Le Roux, V., Urann, B. M., Brunelli, D., Bonatti, E., Cipriani, A., Demouchy, S., Monteleone, B.D., 2021. Post-melting hydrogen enrichment in the oceanic lithosphere, Sci. Adv., 7, eabf6071. https://doi.org/10.1126/sciadv.abf6071

Satsukawa, T, Godard, M, Demouchy, S., Michibayashi, K., and Ildefonse, B., 2017. Chemical interactions in the subduction factory: New insights from an in situ trace elements and hydrogen study of the Ichinomegata and Oki-Dogo mantle xenoliths (Japan). Geochim. Cosmochim. Acta. 208, 234-267. https://doi.org/10.1016/j.gca.2017.03.042.

Schmaedicke E., Gose, J., and Will, T.M., 2011. Heterogeneous mantle underneath the North Atlantic: Evidence from water in orthopyroxene, mineral composition and equilibrium conditions of spinel peridotite from different locations at the Mid-Atlantic Ridge. Lithos, 125, 308-320. https://doi.org/10.1016/j.lithos.2011.02.014

Soustelle, V., Tommasi, A., Demouchy, S., Ionov, D. 2010. Deformation and fluid-rock interactions in supra-subduction mantle: Microstructures and water contents in peridotite xenoliths from the Avacha volcano, Kamchatka, J. Petrol., 51:363-394. https://doi.org/10.1093/petrology/egp085

Day 1: fourth block

Convener: Carlos Garrido

Unveiling mantle heterogeneity in a modern OCT: new insights from the West Iberian margin (ODP Leg 149 and 173)

Secchiari Arianna¹, Godard Marguerite², Montanini Alessandra³

¹University of Milan, Via Botticelli 23, Milan, Italy
²Géosciences Montpellier CNRS, Place Eugène Bataillon, Montpellier, France
³University of Parma, Parco Area delle Scienze 157 a, Parma, Italy

When: Wednesday 2nd october at 16:00 Speaker: Secchiari Arianna; https://orcid.org/0000-0001-6670-5458

Magma-poor ocean-continent transition zones (OCTs) encompass extensive regions of tectonically uplifted mantle. While substantial literature has explored fossil analogues (e.g., Picazo et al., 2016), recent studies on mantle sequences exhumed in modern OCTs are limited (e.g., McCarthy et al., 2020), resulting in a fragmented understanding. Key questions regarding the mechanisms, timing, and location of lithospheric breakup, melt production, and the nature of the mantle source remain debated.

The West Iberian margin (WIM) is one the best-documented continental margins worldwide, with scientific drilling initiated more than three decades ago (e.g., Boillot et al., 1989). However, peridotites from this region have remained poorly characterized due to their extensive serpentinization.

In this study, we revisit the peridotites from the OCT of the WIM, presenting new in situ data for a set of variably serpentinized samples. To capture a comprehensive view of the petrological and geochemical characteristics, we analyzed samples from three boreholes along an E-W transect, from the most proximal to the most oceanward domains: ODP Hole 1068A, 899B, and 1070A.

Hole 1068A samples are serpentinized plagioclase lherzolites (cpx ~ 10 vol.%) with Na₂O- (0.61-0.87 wt%) and Al₂O₃-rich (5.83-7.10 wt%) clinopyroxene, high spinel Cr# (0.255-0.322) and TiO₂ (0.21-0.30 wt%). These characteristics, along with clinopyroxene convex-upward REE patterns yielding negative Eu anomalies and high YbN= 15-17, resemble those of refertilized domains in the sub-continental lithospheric mantle (SCLM, e.g. Müntener et al., 2010). Geochemical modelling indicates an origin involving MORB-type melt impregnation followed by re-equilibration in the plagioclase stability field.

Hole 899B exhibits the higher degree of lithological and chemical heterogeneity. The investigated rock-types are coarse-grained peridotites, varying from spinel to plagioclase harzburgites (cpx ~ 2-8 vol.%). They show evidence of melt-rock interaction, highlighted by olivine-forming, pyroxene-dissolving microstructures. Plagioclase ± secondary orthopyroxene aggregates were also observed in sample 899-1. Mineral compositions (i.e. clinopyroxene and spinel) have moderately depleted to slightly enriched signatures (Cpx: Al₂O₃= 4.05-6.90 wt%, Na₂O= 0.23-0.63 wt%; Sp: Cr#= 0.230-0.463, TiO₂= 0.58-0.61 wt% for sample 899-1). Clinopyroxene in spinel harzburgites displays uncommon chondrite-normalized V-shaped REE patterns (LaN/SmN= 2.7-3.4, DyN/YbN= 0.5-0.6) previously recorded in mantle xenoliths from central and eastern Europe (e.g. Downes et al., 2003). In contrast, clinopyroxene in plagioclase-bearing sample shows concave-downward REE patterns, with depleted LREE segments (La_N/Sm_N= 0.02-0.03) and negative Eu anomalies, suggesting melt depletion followed by re-equilibration in the plagioclase stability field.

Hole 1070A peridotites are coarse-grained spinel harzburgites (cpx ~ 2-5 vol.%) displaying pyroxene-dissolving and olivine-precipitating microtextures. Positive correlations among melting indexes in clinopyroxene (i.e. Cr#, Al₂O₃ and Yb) suggest a residual origin for these lithologies. However, unusual Cr-Na enrichments and hump-shaped clinopyroxene REE patterns indicate open system melting in the spinel stability field, accompanied by percolation of an enriched melt. These features, though rare in modern oceans, were previously attested in some peridotites from slow- to ultraslow-spreading settings (Hellebrand & Snow, 2003; Seyler et al., 2011).

Calculated equilibrium temperatures for the WIM peridotites range within those of fossil OCTs (T_Ca-in-Opx= 921-1029 °C). They reflect a history of melt-rock interaction at high temperature (T_REE-Y= 1098-1244 °C), followed by thermal re-equilibration at lower temperatures (T_Ol-Sp= 770-813 °C).

These new data provide compelling evidence of highly heterogeneous mantle domains exposed in the OCT of the WIM. The documented heterogeneity, observed among peridotites from the same borehole and among boreholes, challenges the traditional view of OCTs as regions with simple and fixed distribution of mantle domains with well-defined major and trace element compositions. We posit that melt-rock interaction plays a key role in generating petrological and chemical heterogeneity in OCTs and the upper mantle.

References

Boillot, G., Féraud, G., Recq, M. & Girardeau, J., 1989. Undercrusting by serpentinite beneath rifted margins. Nature, 341, 523–525. https://doi.org/10.1038/341523a0

Downes, H., Reichow, M. K., Mason, P. R. D., Beard, A. D. & Thirlwall, M. F., 2003. Mantle domains in the lithosphere beneath the French Massif Central: Trace element and isotopic evidence from mantle clinopyroxenes. Chemical Geology, 200, 71–87. https://doi.org/10.1016/S0009-2541(03)00126-8

Hellebrand, E. & Snow, J. E., 2003. Deep melting and sodic metasomatism underneath the highly oblique-spreading Lena Trough (Arctic Ocean). Earth and Planetary Science Letters, 216, 283–299. https://doi.org/10.1016/S0012-821X(03)00508-9

McCarthy, A., Falloon, T. J., Sauermilch, I., Whittaker, J. M., Niida, K. & Green, D. H., 2020. Revisiting the Australian-Antarctic Ocean-Continent Transition Zone Using Petrological and Geophysical Characterization of Exhumed Subcontinental Mantle. Geochemistry, Geophysics, Geosystems, 21, 7, e2020GC009040. https://doi.org/10.1029/2020GC009040

Müntener, O., Manatschal, G., Desmurs, L. & Pettke, T., 2010. Plagioclase Peridotites in Ocean–Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere. Journal of Petrology, 51, 255–294. https://doi.org/10.1093/petrology/egp087

Picazo, S., Müntener, O., Manatschal, G., Bauville, A., Karner, G. & Johnson, C. 2016. Mapping the nature of mantle domains in Western and Central Europe based on clinopyroxene and spinel chemistry: Evidence for mantle modification during an extensional cycle. Lithos, 266–267, 233–263. https://doi.org/10.1016/j.lithos.2016.08.029

Seyler, M., Brunelli, D., Toplis, M. J. & Mével, C., 2011. Multiscale chemical heterogeneities beneath the eastern Southwest Indian Ridge (52°E-68°E): Trace element compositions of along-axis dredged peridotites. Geochemistry, Geophysics, Geosystems, 12, 9. https://doi.org/10.1029/2011GC003585

Insights into rifting of fertile mantle, Part 1: Compositions

Gordana Garapic¹, Ulrich Faul², Akihiro Tamura³, Allison Seyler¹, Tomoaki Morishita³

¹State University of New York, New Paltz, USA ²Massachusetts Institute of Technology, Cambridge, USA ³Kanazawa University, Kanazawa, Japan

When: Wednesday 2nd october at 16:20 Speaker: Gordana Garapic; https://orcid.org/0000-0002-9042-2711

The Dinaride ophiolite belt connects the Alpine peridotite bodies with those of Albania and Greece. The belt consists of numerous individual massifs and smaller outcrops. The Krivaja-Konjuh massif in central Bosnia is one of the largest peridotite complexes within the Dinarides with an area of 650 km2. The massif is embedded in a limestone melange, with structural similarities to an ocean-continent transitional setting documented for some Alpine peridotite bodies.

The lithology of the western part of the massif (Krivaja) ranges from spinel peridotites in the south and west to predominantly plagioclase peridotites in the north and east, with a central portion consisting of plagioclase dunites grading into troctolites and gabbros. The plagioclase peridotites record both melt migration/impregnation and metamorphic reaction of spinel to plagioclase.

Major and minor element compositions as well as phase proportions indicate that the Krivaja massif is fertile, with olivine phase proportions from 50 to 70%, and clinopyroxene (cpx) Na${}_2$$_2$O up to 2% at 6 - 7% Al${}_2$$_2$O${}_3$$_3$. No macroscopic (matrix) hydrous phases have been observed.

Newly acquired trace element data confirm the generally fertile nature of the massif. Trace element patterns suggest typical degrees of melt extraction of at most a few percent. Significantly, the data show that the spinel peridotites have not been refertilized after melt extraction; or metasomatized by a hydrous fluid. Some samples from the southwestern-most portion of the massif have cpx trace element patterns/contents virtually identical to the model MORB source composition DMM (Workman and Hart, 2005).

Both opx and cpx porphyroclasts have extensive exsolution lamellae indicating adjustment to lower pressures and temperatures. Porphyroclast rims and neoblasts are free of exsolution. While most of the exsolution lamellae consist of the opposite species of pyroxene, the lamellae also contain small grains of spinel and pargasitic amphibole. Both are consistent with adjustment of the pyroxene composition down temperature and pressure where the solubility of Al${}_2$$_2$O${}_3$$_3$ and H${}_2$$_2$O as well as other trace elements decreases. The trace element patterns exclude a metasomatic origin of the amphibole. The presence of amphibole therefore indicates a significant hydroxyl content of the protolith, consistent with the overall fertility of the massif.

Two-pyroxene thermometry of porphyroclasts was based on area analyses that sought to integrate the exsolution lamellae. The so obtained temperatures range up to 1300${}^{\circ }$$^{\circ}$C. For neoblasts the lowest calculated temperatures of some of the most deformed samples record temperatures near 900${}^{\circ }$$^{\circ}$C. Comparing compositions of porphyroclasts and neoblasts as a function of temperature indicates equilibration of neoblast MgO contents to the lower temperature, but not for example their Na${}_2$$_2$O content.

The high Na${}_2$$_2$O content of cpx, the occurrence of symplectites of opx and spinel after garnet together with the high temperatures suggest parts of the massif originated near the spinel to garnet transition at the base of the lithosphere. The fertile nature of the massif with near MORB source trace element composition includes a corresponding hydroxyl content. This water content affects diffusion and deformation, discussed in Part 2. Overall the massif enables observation of processes associated with rift initiation in a fertile environment.

References

Workman, R.K., Hart, S.R., 2005. Major and trace element composition of the depleted MORB mantle (DMM). Earth and Planetary Science Letters 231, 53–72. https://doi.org/10.1016/j.epsl.2004.12.00

Insights into rifting of fertile mantle, Part 2: Microstructures and Deformation

Ulrich Faul¹, Gordana Garapic², Tomoaki Morishita³

¹Massachusetts Institute of Technology, Cambridge, USA ²State University of New York, New Paltz, USA ³Kanazawa University, Kanazawa, Japan

When: Wednesday 2nd october at 16:40 Speaker: Hans Ulrich Faul; https://orcid.org/0000-0001-5036-4572

The spinel peridotites throughout the Krivaja massif, Central Bosnia, show signs of deformation by dislocation creep. However, the microstructures generally do not correspond well to the definition of a porphyroclastic texture. The grain shapes particularly of neoblasts are irregular and embayed, rather than polygonal. The largest porphyroclasts suggest pre-deformation grain sizes near 1 cm. Cpx grains in more highly deformed samples form in some cases continuous recrystallization trails extending across thin sections. Some opx porphyroclasts show similar elongation. In some instances olivine remains porphyroclastic. The most highly deformed samples can be characterized as mylonytic/ultramylonitic with grain sizes near 50 micron.

Extensive phase mixing is observable in olivine dominated areas, while phase mixing in recrystallizing cpx trails and directly adjacent to opx porphyroclasts is more limited. Analysis of the phase distribution following Heilbronner and Barrett (2014) indicates that the phase distribution particularly of the most fine-grained aggregates falls into the (chess board-like) ’ordered’ domain, interpreted to originate by a solution-precipitation process. Large strain deformation experiments with olivine and opx resulted in similarly ordered aggregates (Tasaka et al., 2017). They attributed the phase mixing to stress-driven diffusive redistribution of Mg and Fe. Consistent with this notion neoblasts show diffusive equilibration of Mg and Fe down temperature (Part 1).

Most samples have a well-developed, predominantly E-type fabrics. In some samples porphyroclasts have an E-type fabric, while neoblasts have an A-type fabric. The most highly deformed (and mixed) samples have only weak fabrics. As detailed in Part 1, the presence of exsolved amphibole indicates moderate hydroxyl contents of the mineral phases prior to exsolution. Experimentally, E-type (or axial [100](001)) fabrics have been observed in deformation of olivine with moderate water contents (Jung et al., 2006). A transition from E-type fabric to fabrics with neoblast A-type may develop due to exsolution of amphibole and consequent partitioning of hydrogen into amphibole at decreasing temperature (Part 1). The weak fabrics of the most highly deformed and mixed samples are consistent with a diffusive deformation mechanism.

The observations indicate that phase mixing based on diffusion can occur at deep lithospheric levels. Diffusion of Mg and Fe can be enhanced by the presence of hydrogen in fertile compositions (see Part 1). The mixed lithologies retard grain growth, and deform predominantly by diffusion creep. This provides a mechanism for strain localization during rift initiation in fertile mantle.

We hypothesize that the massif records rift initiation in MORB source mantle, with deformation starting at high temperatures near the lithosphere-asthenosphere boundary. Deformation continues at decreasing temperature and pressure during lithospheric thinning. The consequent mantle upwelling leads to melting and melt migration observable in the central and north-eastern parts of the massif in the form of interstitial plagioclase in finger-like domains. The microstructures of plagioclase bearing peridotites show no further sign of deformation and indicate recovery. Troctolites and gabbros together with minor pillow basalts and cherts indicate the formation of an incipient ocean basin.

References

Heilbronner, R., Barrett, S., 2014. Image Analysis in Earth Sciences: Microstructures and Textures of Earth Materials. Springer Berlin Heidelberg, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-10343-84

Jung, H., Katayama, I., Jiang, Z., Hiraga, T., Karato, S., 2006. Effect of water and stress on the lattice-preferred orientation of olivine. Tectonophysics 421, 1–22. https://doi.org/10.1016/j.tecto.2006.02.011

Tasaka, M., Zimmerman, M.E., Kohlstedt, D.L., Stünitz, H., Heilbronner, R., 2017. Rheological Weakening of Olivine + Orthopyroxene Aggregates Due To Phase Mixing: Part 2. Microstructural Development. JGR Solid Earth 122, 7597–7612. https://doi.org/10.1002/2017JB014311

Day 1: fifth block

Convener: TO SET

Tectonothermal evolution of the giant eclogitic layer from the Cabo Ortegal Complex (NW Iberian Massif): geodynamic implications

I. Novo-Fernández¹, R. Arenas², J.I. Gil Ibarguchi³, R. Albert⁴, A. Gerdes⁴, S. Sánchez Martínez², R. Díez Fernández⁵, A. Beranoaguirre⁴, A. Garcia-Casco¹

¹Dpto. Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, 18071, Granada, Spain ²Dpto. Mineralogía y Petrología, Facultad de Ciencias Geológicas, Universidad Complutense, 28040, Madrid, Spain ³Dpto. de Geología, Facultad de Ciencia y Tecnología, Universidad del País Vasco, 48940, Bilbao, Spain ⁴Frankfurt Isotope and Element Research Center, Goethe-Universität Frankfurt, 60438, Frankfurt, Germany ⁵Dpto. de Geología y Subsuelo, Centro Nacional Instituto Geológico y Minero de España (CSIC) 37001 Salamanca, Spain

When: Wednesday 2nd october at 17:20 Speaker: Irene Novo-Fernández; https://orcid.org/0000-0002-6722-5525

From the Early Devonian to the Carboniferous, the convergence between Gondwana and Laurussia led to the assembly of Pangea during the Variscan Orogeny. This orogen can be followed through Europe along the Bohemian, Armorican, French Massif Central and Iberian massifs. In the latter, the most internal zone of the orogen appears in several allochthonous complexes that contains, from bottom to top, a Late Devonian high-P and low- to intermediate-T metamorphic belt (Lower Allochthon), Cambrian and Middle Devonian ophiolitic units and an Upper Allochthon whose lower section constitutes an Early Devonian high-P and high-T metamorphic belt (Arenas et al., 2016). This metamorphic belt is constituted by ultramafic massifs, high-P granulites, eclogites, and high-P migmatitic paragneisses. The eclogites occur in boudins within the gneisses and in a 20 km long and 700 m thick layer constituting one of the largest outcrops of eclogites worldwide. In this layer, Mendia et al. (2001) distinguished three types of eclogites: common eclogites, ferrotitaniferous eclogites and kyanite-bearing eclogites (Figure 1). In this work, we have studied the three types of eclogites via thermodynamic modelling. The calculations yield metamorphic peak conditions of ~ 750-800 ºC and 23-25 kbar that indicate deep subduction of the peri-Gondwanan realm at the onset of the Variscan Orogeny (c. 400 Ma), when the eclogites were buried up to ~ 90 km depth.

Figure 1. Phase maps of the (a) common, (b) ferrotitaniferous and (c) kyanite-bearing eclogite.

References

Arenas, R., Sánchez Martínez, S., Díez Fernández, R., Gerdes, A., Abati, J., Fernández-Suárez, J., Andonaegui, P., González Cuadra, P., López-Carmona, A., Albert, R., Fuenlabrada, J.M., Rubio Pascual, F.J., 2016. Allochthonous terranes involved in the Variscan suture of NW Iberia: A review of their origin and tectonothermal evolution. Earth-Science Reviews 161, 140-178. https://doi.org/10.1016/j.earscirev.2016.08.010.

Mendia, M., Gil Ibarguchi, J.I., Ábalos, B., 2001. Evolución metamórfica P-T-d-t y significado geodinámico de la unidad eclogítica del complejo de Cabo Ortegal (NO de España). Cadernos do Laboratorio Xeoloxico de Laxe 26, 155-178.

Cabo Ortegal Complex general discussion

When: Wednesday 2nd october at 17:40 Leader: Romain Tilhac; https://orcid.org/0000-0001-5132-6228

Day 2: first block

Convener: Olivier Alard

Hydrogen Deep Water Cycle

Nathalie Bolfan-Casanova¹

¹Laboratoire Magmas et Volcans, Université Clermont-Auvergne

When: Thursday 3rd october at 9:00 am Speaker: Nathalie Bolfan-Casanova; https://orcid.org/0000-0002-1859-1107

Hydrogen, incorporated as hydroxyl, has long been recognized to be an ubiquituous trace component of most mantle minerals [1]. Xenoliths from the cratonic keels host up to 200-250 hundreds of parts per million by weight of water (ppm wt H₂O), see [2] in this meeting. While in the convecting mantle, the water content measured in Mid Ocean Ridge Basalts (MORBs) also points out to a few hundreds of ppm wt H₂O in their source at about 100 km depth. Deeper, the low seismic velocities atop the transition zone are unanimously interpreted as due to melting, especially dehydration melting, due to the high water content in the material ascending from the transition zone (estimates range from 0.2 to 1 wt% H₂O see [3]). However, this scenario could be revised in light of the recent experimental determinations of water storage in the presence of Carbon [4]. For the rest of the deeper part of the Earth, the lower mantle, there are very few constraints on the actual water content and the water storage ranges from 0.6 to 3 ocean masses depending on the storage in the main mineral bridgmanite, which is very controversial [5].

Because the Earth’s mantle gets more and more reducing with increasing pressure [6], the speciation of H is expected to change as a function of depth, going from H₂O and OH- under oxidizing conditions to H₂ under reducing conditions. And indeed, recently, we have discovered H₂ in natural omphacite from eclogite xenoliths brought up by kimberlites [7], in agreement with previous experimental observations (8]. Due to the difficulty in detecting H₂, with spectroscopy, it is possible that the H storage of the mantle has therefore been largely under-estimated in previous studies of mantle xenoliths.

References

[1] Demouchy S., and Bolfan-Casanova N. 2016. Distribution and transport of hydrogen in the lithospheric mantle: A review, Lithos, DOI:10.1016/j.lithos.2015.11.012

[2] Dominique J., Bolfan-Casanova N., Moine B., Radu B., Pradas del Real A.E., Devidal J.L., Ionov D.A. 2024. How dry is the Kaapvaal craton? 7th Orogenic Lherzolite Conference (Oviedo, Spain).

[3] Andrault D. and Bolfan-Casanova N. 2022. Mantle rain toward the Earth's surface: A model for the internal cycle of water, Phys. Earth Planet. Int. 322, https://doi.org/10.1016/j.pepi.2021.106815

[4] Bolfan-Casanova N., Martinek L., Manthilake G., Verdier-Paoletti M.et Chauvigne P. 2023. Effect of oxyen fugacity on the storage of water in wadsleyite and olivine in H and H-C fluids and implications for melting atop the transition zone, Eur. J. Min. 35(4): 549-568. https://doi.org/10.5194/ejm-35-549-2023

[5] Novella D., Demouchy S. and Bolfan-Casanova N. 2024. The Invisible Ocean: Hydrogen in the Deep Earth. vol. 20, no. 4, in press.

[6] Frost DJ and McCammon C. 2008. The Redox State of Earth’s Mantle. Ann Rev Earth Planet Sci 36: 389-420. https://doi.org/10.1146/annurev.earth.36.031207.124322

[7] Moine B., N., Bolfan-Casanova N., Radu I.B., Ionov D.A., Costin G., Korsakov A.V. , Golovin A.V., Oleinikov O.B., Deloule E., Cottin JY. 2020. Molecular Hydrogen in minerals, a clue to interpret ∂D variations in the mantle, Nature Comm, https://doi.org/10.1038/s41467-020-17442-8.

[8] Yang X and Keppler H. 2016. Molecular Hydrogen in Mantle Minerals. Geoch. Persp. Lett. Vol.2, Issue 2, P 160. https://doi.org/10.7185/geochemlet.1616.

Insights into S recycling in the mantle from high-precision isotope analysis of pyroxenite-hosted sulfides

A. Montanini¹, A. Secchiari², L. Martin³, C. Marchesi⁴, C. J. Garrido⁵, M. Fiorentini³

¹University of Parma, Parco Area delle Scienze 157a, Parma, Italy ²Milan University, Via Botticelli 23, Milan, Italy ³The University of Western Australia, 35 Stirling Highway, 6009 Perth, Australia ⁴Universidad de Granada, Avenida Fuentenueva, 18002 Granada, Spain ⁵Instituto Andaluz Ciencias de la Tierra (IACT), CSIC, Av Palmeras 4, Armilla, Spain

When: Thursday 3rd october at 9:20 am Speaker: Alessandra Montanini; https://orcid.org/0000-0002-9861-1787

Large-scale tectonic processes introduce a variety of crustal lithologies into the Earth’s mantle, leading to increasing mantle heterogeneity over time. During subduction in arc environments, recycled mantle pyroxenites may retain pristine geochemical and isotopic signatures that reflect their crustal evolution. Therefore, pyroxenites can be considered representatives of the materials transferred from deeply subducted slabs to the mantle sources of oceanic basalts, providing insights into the recycling of sulfur into the deep Earth interior. However, no S isotopic data are available in literature for recycled pyroxenite-hosted sulfides.

Here, we focus on garnet clinopyroxenites from the External Ligurian (EL) mantle sequences (N Apennines, Italy) and on selected samples from the world-famous orogenic massifs of Ronda and Beni Bousera (Betic-Rif Belt, Spain and Morocco) with the aim of integrating new sulfur isotope data into the existing pyroxenite petrogenetic models, shedding light on mechanisms of sulfur geochemical cycling in the subduction factory. Previous studies, based on major and trace elements and Nd-Hf-O isotope systematics, have shown that the EL pyroxenites derived from MORB-type gabbroic precursors, which underwent a long-term evolution into the mantle (> 1 Ga). The Ronda and Beni Bousera massifs display significant pyroxenite diversity, including the occurrence of UHP (>4.5 GPa) garnet clinopyroxenites with graphitized diamond pseudomorphs and corundum-bearing garnet clinopyroxenites. For these rocks, a wide range of elemental and Sr-Nd-Hf-Pb-O isotopic signatures have been documented. This heterogeneity has been ascribed to an origin from continental lower crust (Varas Reus et al., 2018), or from oceanic crustal protoliths that experienced variable degrees of seafloor hydrothermal alteration (Pearson et al., 1993) and mixing with pelagic sediments (Lorand et al., 2021).

Our study targets the sulfur isotopic composition of primary Fe-Ni-Cu sulfides (BMS) from these localities to provide new constraints on their protoliths and sulfur sources. In situ measurements of δ³⁴S have been carried out by SIMS (Secondary-ion mass spectrometry). The sulfide assemblage in the EL garnet clinopyroxenites consists of Ni-free pyrrhotite, pentlandite, and chalcopyrite reflecting subsolidus exsolution from high-temperature Fe–Ni–Cu MSS. Their sulfur isotope compositions range from δ³⁴S = - 2.84 to + 0.83 ‰ compared to VCDT. A garnet clinopyroxenite from Ronda, with a sulfide assemblage of chalcopyrite and pentlandite+pyrrhotite intergrowths, yields a larger interval of prevailing negative δ³⁴S values, ranging from -3.59 to +0.59. Conversely, Cu-Ni sulfides from a Beni Bousera corundum-bearing garnet pyroxenite have δ³⁴S shifted towards positive values (from -0.24 to + 4.80). Both the Ronda and Beni Bousera corundum-bearing pyroxenites belong to the Group A as defined by Varas Reus et al. (2018) based on radiogenic isotopes. They were interpreted as ancient recycled oceanic gabbros, possibly mixed with low amounts of pelagic sediments.

Whereas the sulfur isotope signature of the EL garnet clinopyroxenites overlaps depleted mantle values (−1.4±0.5, Labidi et al., 2012), in agreement with the lack of a oxygen isotope signature distinctive of hydrothermal alteration (Montanini et al., 2012), the entire range of δ³⁴S from the Betic-Rif Belt significantly deviates from depleted mantle values, possibly reflecting preservation of the S signature of altered oceanic crust. The sulfides of a diamond-facies Beni Bousera garnet clinopyroxenite (Group-B of Varas Reus et al., 2018) consist of troilite with negative δ³⁴S ranging from – 6.03 to -1.48. Desulfidation of a sedimentary pyrite component during prograde metamorphism of subducted oceanic crust under strongly reducing conditions was proposed by Lorand et al. (2021) for the origin of troilite in these samples. The recorded δ³⁴S signature is permissive (although not exclusive) of such a derivation. Alternatively, the deviation from mantle values could be attributed to involvement of recycled continental lower crust in the origin of Group-B pyroxenites (Varas Reus et al., 2018).

Referencias

Labidi, J., Cartigny, P., Birck, J. L., Assayag, N., & Bourrand, J. J. (2012). Determination of multiple sulfur isotopes in glasses: a reappraisal of the MORB δ34S. Chemical Geology, 334, 189–198. https://doi.org/10.1016/j.chemgeo.2012.10.028

Lorand, J.P., Pont, S., Labidi, J. Cartigny, P. & El Atrassi, F. (2021). Sulphide petrology and contribution of subducted sulphur in diamondiferous garnet-bearing pyroxenites from Beni Bousera (Northern Morocco). J. Petrol. 62, 1–24. https://doi.org/10.1093/petrology/egab089

Montanini, A., Tribuzio, R. & Thirlwall, M. (2012). Garnet clinopyroxenite layers from the mantle sequences of the Northern Apennine ophiolites (Italy): evidence for recycling of crustal material. Earth Planet. Sci. Lett. 351-352, 171-181. http://dx.doi.org/10.1016/j.epsl.2012.07.033

Pearson, D.G., Davies, G.R. & Nixon, P.H. (1993). Geochemical constraints on the petrogenesis of diamond facies pyroxenites from the Beni Bousera peridotite massif, North Morocco. J. Petrol. 34, 125–172. https://doi.org/10.1093/petrology/34.1.125

Varas-Reus, M.I., Garrido, C. J., Marchesi, C., Bosch, D. & Hidas, K. (2018). Genesis of ultra-high pressure garnet pyroxenites in orogenic peridotites and its bearing on the compositional heterogeneity of the Earth’s mantle. Geochim. Cosmochim. Acta 232, 303–328. https://doi.org/10.1016/j.gca.2018.04.033

Abundance and distribution of carbon in the subcontinental lithospheric mantle (SCLM)

Halimulati Ananuer^1,2, Olivier Alard¹, Sue O’Reilly², Yoann Greau^1,2

¹The Australian National University, Canberra, ACT, Australia ²CCFS, Macquarie University, Sydney, NSW, Australia

When: Thursday 3rd october at 9:40 am Speaker: Halimulati Ananuer; https://orcid.org/0000-0001-8335-1786

Carbon (C) content in basalts from a range of geodynamic contexts, and the occurrence of carbonate, graphite and diamond in mantle samples, indicate that a significant amount of carbon (with different speciation), is stored in the Earth’s mantle. However, measurement of low C content (CO2<0.1 wt.%) at both whole-rock and mineral scales is extremely challenging. Therefore, C abundance, distribution and speciation in the mantle remain poorly known. For example, current “primitive” mantle estimates for C concentration ([C]) vary from 100 to 1000 ppm.

In this study we investigated nine suites, ca 100 mantle peridotite xenoliths (spinel facies only), hosted in alkali basalts from worldwide occurrences. These sampled Proterozoic to Phanerozoic sub-continental lithospheric mantle, of varying degrees of fertility and metasomatism (modal to cryptic, ‘hydrous’ to ‘carbonatitic’). C concentrations for whole-rock and hand-picked mineral fractions were obtained using two complementary analytical methods: (i) Elemental Analyzer, providing bulk measurements at a fixed temperature ca 1700°C; and (ii) Simultaneous Thermal Analyser coupled with a Quadrupole Mass Spectrometer (STA/QMS), providing high-resolution thermograms of volatiles released (including C) from 50°C to 1300°C.

The preliminary results of this study indicate that: (i) bulk [C] content varies between 30 and 1250 µg/g, with ~ 70% of samples between 100-500 µg/g, ~ 10% less than 100 µg/g and ~ 20% more than 500 µg/g; (ii) during STA analyses, C is mostly released (80-90%) between 200 to 700°C, indicating that most of the C is not stored in the mineral lattice of the main silicates (olivine, orthopyroxene, clinopyroxene(cpx)) nor in metasomatic minerals. Indeed, C-bearing metasomatic phases have distinct C-release temperatures higher than 700°C. Carbonates have a release peak at ca 780±80°C, while apatite releases C at 1100±50°C and 1250±50°C. Graphite is oxidised and degases over 900°C. These results strongly suggest that C in the lithospheric peridotitic mantle assemblages is mainly hosted in inclusions, crystal defects, grain boundaries or interstitial components. Amphiboles (amp) and phlogopite have a significantly higher C content (449±131 ppm and 886 ppm, respectively) than coexisting silicates ([C]_Cpx= 231±116 μg/g).

Carbon is strongly linked to the geochemistry of large-ion lithophile elements (LILE) such as Ba and Sr both in cryptically and modally metasomatised samples. The relationships between Sr isotopes (⁸⁷Sr/⁸⁶Sr) and C in the whole-rock and in constituent minerals (amp, cpx) could indicate different enrichment processes related to melts/fluid with distinct sources and compositions that will be discussed at the conference.

Day 2: second block

Convener:

Petrological characteristics of subarc ultramafic xenoliths: Lanyu Island (Taiwan) in the Luzon magmatic arc

Tomoaki Morishita^1,2, Norikatsu Akizawa^3,4, Takami Araki¹, Akihiro Tamura¹, Tomoyuki Mizukami¹, Ken-Ichiro Tani⁵, Akihiro Tamura³, Akihiro Tamura³, Akihiro Tamura³

¹Kanazawa University, Kakuma, Kanazawa, Ishikawa 920‑1192, Japan ²Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Kanagawa 237-0061, Japan ³University of Tokyo, 5‑1‑5 Kashiwanoha, Kashiwa, Chiba 277‑8564, Japan ⁴Tokyo Gakugei University, 4-1-1 Nukuikita, Koganei, Tokyo 184-8501, Japan ⁵National Museum of Nature and Science, 4-1-1- Amakubo, Tsukuba, Ibaraki 305-0005, Japan

When: Thursday 3rd october at 10:20 am Speaker: Tomoaki Morishita; https://orcid.org/0000-0002-8724-6868

The Island Arc is one of the most volcanically active regions on earth and is considered a continental growth area. Petrological characteristics of the subarc mantle provide essential information for understanding the evolution of island arcs. However, it is not easy to obtain subarc mantle materials. Ultramafic xenoliths captured into the arc magma are probably the mantle material of the sub-arc. Ultramafic xenoliths in the arc setting are relatively rare compared to those on the continent and oceanic hotspot areas.

The Luzon Volcanic Arc is a north-south trend volcanic chain extending from Taiwan to the Philippines. Mantle-derived peridotite xenoliths have been reported from Lutao Island (Taiwan) and Iraya volcano, Batan and Pinatubo volcano, Luzon, in the Luzon volcanic arc (Vidal et al., 1989; Yoshikawa et al., 2016; Shellnutt et al., 2024). We report on the petrological and mineralogical characteristics of ultramafic xenoliths in the Lanyu volcanic rocks near the northern end of the Luzon Volcanic arc.

Lanyu Island is a volcanic island located about 95 km southeast of Taitung city on the main island of Taiwan. Lanyu Island belongs to the Northern Luzon Volcanic Arc on the Philippine Sea Plate (e.g., Chen et al., 1994). The ultramafic xenoliths are classified as harzburgite, lherzolite, dunite, wehrlite and pyroxene-amphibole peridotite.

Before discussing the origin of the studied ultramafic xenoliths, it is necessary to discuss the effect of the host magma and related magmatism on the changes in petrography and mineral chemistry due to the small size of the xenolith. It is assumed that the studied ultramafic xenoliths are affected by the magmatism associated with the host magma. Clinopyroxene in some samples is grayish in color, which is attributed to formation of fine-grained opaque minerals. The light rare earth elements in the grayish clinopyroxene are clearly higher than in the transparent “normal” clinopyroxene, indicating a reaction with the melt. Samples containing grayish clinopyroxene tend to coexist with spinel accompanied by fine grains in the margin. The fine grains in the spinel margin are characterized by higher Cr# and TiO₂ contents than the central part of the same spinel grain. The interstitial phase of the fine-grained spinel margin of the spinel grains is rich in Na, which is not abundant in the constituent minerals of the ultramafic xenoliths. These petrological and chemical characteristics of minerals can be explained by mineral-melt interaction in relatively late events.

Harzburgite and lherzolite are generally interpreted as residue after partial melting of primitive mantle peridotite. The chemical compositional characteristics of the Fo content of olivine and the Cr# of coexisting spinel are consistent with the chemical range of residual peridotites. The relatively higher Cr# of spinel and higher Fo content of olivine in harzburgite than in lherzolite support that harzburgite is a residue with higher degree of partial melting than lherzolite. The trace element patterns of clinopyroxene in harzburgite and lherzolite are not consistent with those expected for residues after simple partial melting and melt extraction from the primitive mantle, such as depleted MORB mantle compositions. Clinopyroxene in harzburgite and lherzolite has a higher content of LREE and Sr, i.e., incompatible elements, than clinopyroxene in the modeled residue after nearly fractional melting from the DMM composition. Information on fluid inclusions and water content in olivine will be also reported.

The clinopyroxene in other ultramafic rocks differs from the clinopyroxene in harzburgite and lherzolite. Based on mineral/melt partition coefficients, melt compositions calculated from pyroxenites in these ultramafic rocks suggest that they formed in association with volcanic activity of the host rock.

References

Chen, C.H., Liu, T.K., Yang, T.Y., Chen Y.G.,1994. Lanyu: Explanatory text of the geologic map of Taiwan, Central Geological Survey, 55 pp. (in Chinese)

Yoshikawa, M., Tamura, A., Arai, S., Kawamoto, T., Payot, B., Rivera, D., Bariso, E., Mirabuenom, M., Okuno, M., Kobayashi, T., 2016. Aqueous fluids and sedimentary melts as agents for mantle wedge metasomatism, as inferred from peridotite xenoliths at Pinatubo and Iraya volcanoes, Luzon arc, Philippines. Lithos, 262, 355-368. http://dx.doi.org/10.1016/j.lithos.2016.07.008

Shellnutt, J.G., Yeh, M.-W., Lee, T.-Y., Iizuka, Y., Chen W.-Y., Prasanth, P.M., 2024. Sub-arc mantle heterogeneity of the Northern Luzon volcanic arc: Mineral and whole rock compositional variability in mantle xenoliths from Lutao Island. Journal of Petrology 65, https://doi.org/10.1093/petrology/egae037

Vidal, Ph., Dupuy, C., Maury, R., Richard, M., 1989. Mantle metasomatism above subduction zones: Trace-element and radiogenic isotope characteristics of peridotite xenoliths from Batan Island (Philippines). Geology 17, 1115-1118. https://doi.org/10.1130/0091-7613(1989)017%3C1115:MMASZT%3E2.3.CO;2

Petrology and geochemistry of the Neoproterozoic Ophiolite of Calzadilla (SW Iberian Massif): from onset of subduction to forearc-arc collision

I. Novo-Fernández¹, N. Pujol-Solà¹, R. Arenas², R. Díez Fernández³, J.A. Proenza⁴, E. Rojo-Pérez⁵, A. Cambeses¹, S. Sánchez Martínez², G. Iglesias², A. Garcia-Casco¹

¹Dpto. Mineralogía y Petrología, Facultad de Ciencias, Universidad de Granada, 18071, Granada, Spain ²Dpto. Mineralogía y Petrología, Facultad de Ciencias Geológicas, Universidad Complutense, 28040, Madrid, Spain ³Dpto. de Geología y Subsuelo, Centro Nacional Instituto Geológico y Minero de España (CSIC), 37001, Salamanca, Spain ⁴Dpto. de Mineralogía, Petrología y Geología Aplicada, Universidad de Barcelona, 08028, Barcelona, Spain ⁵Geochronology section, Seckenberg Naturhistorische Sammlungen Dresden, Dresden, Germany

When: Thursday 3rd october at 10:40 am Speaker: Irene Novo-Fernández; https://orcid.org/0000-0002-6722-5525

In the SW Iberian Massif, the Ossa-Morena Complex contains different terranes that are key to establishing the evolution of the Ediacaran margin of Gondwana. Among them, the Ediacaran Calzadilla Ophiolite represents the remnants of a suprasubduction oceanic lithosphere that was formed during subduction of a northern ocean below Gondwana (Arenas et al., 2018). It is composed of a mantle section that contains serpentinized peridotites, chromitites, hornblendites and mafic dikes, and is overlaid by a mafic section made up of c. 540 Ma amphibolites with c. 600 Ma protoliths of boninitic affinity (Arenas et al., 2018). In this work, we study the geochemistry of the serpentinites to establish the petrological nature and the mantle processes undergone by these rocks. Additionally, a serpentinite and a mafic dike from the mantle section, together with an amphibolite from the mafic section are studied by means of thermodynamic modelling with the aim of establishing the tectonothermal evolution of the ophiolite. The results indicate that harzburgitic protoliths underwent fluid/melt-rock interactions and relatively high partial melting (~16 vol.%) in a fore-arc setting, in accordance with the boninitic crust. The thermodynamic modeling reveals that both, the mafic and mantle sections of the ophiolite underwent similar PT conditions at ~ 5 kbar and ~ 575 ºC during the metamorphic peak along a barrovian gradient. We envisage a geodynamic evolution characterized by roll-back of the subducting slab that triggered the opening of the fore-arc basin at c. 600 Ma with the partial melting of the mantle and the formation of the boninitic crust, followed by serpentinization via fluid-rock interactions. Ensuing compression led to collision and emplacement of the Calzadilla Ophiolite onto the associated Cadomian arc (Díez Fernández et al., 2019) and the development of barrovian metamorphism.

References

Arenas, R., Fernández-Suárez, J., Montero, P., Díez Fernández, R., Andonaegui, P., Sánchez Martínez, S., Albert, R., Fuenlabrada, J.M., Matas, J., Martín Parra, L.M., Rubio Pascual, F.J., Jiménez-Díaz, A., Pereira, M.F., 2018. The Calzadilla Ophiolite (SW Iberia) and the Ediacaran fore-arc evolution of the African margin of Gondwana. Gondwana Research 58, 71-86. https://doi.org/10.1016/j.gr.2018.01.015

Díez Fernández, R., Jiménez-Díaz, A., Arenas, R., Pereira, M.F., Fernández-Suárez, J., 2019. Ediacaran Obduction of a Fore-Arc Ophiolite in SW Iberia: A Turning Point in the Evolving Geodynamic Setting of Peri-Gondwana. Tectonics 38, 95-119. https://doi.org/10.1029/2018TC005224

Accretion of “young” Phanerozoic subcontinental lithospheric mantle triggered by back-arc extension

Abimbola C. Ogunyele^1,2,3, Alessio Sanfilippo^1,2, Vincent J. M. Salters⁴, Mattia Bonazzi², Alberto Zanetti²

¹Department of Earth and Environmental Sciences, University of Pavia, Via Ferrata 1, ²CNR – Istituto Geoscienze e Georisorse, Via Ferrata 1, 27100 Pavia, Italy ³Department of Earth Sciences, Adekunle Ajasin University, PMB 001 Akungba-Akoko, Nigeria ⁴National High Magnetic Field Laboratory, Department of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee, FL 32310, USA

When: Thursday 3rd october at 11:00 am Speaker: Alberto Zanetti; https://orcid.org/0000-0001-9026-1519

The subcontinental lithospheric mantle (SCLM) beneath Phanerozoic regions is mostly constituted by fertile lherzolites, which sharply contrast with cratonic mantle made of highly-depleted peridotites. The question of whether this chemical difference results from lower degrees of melting associated with the formation of Phanerozoic SCLM or from the refertilization of ancient depleted SCLM remains a subject of debate. Additionally, the timing and geodynamic environment of accretion of the fertile SCLM in many Phanerozoic regions are poorly constrained. We here document new geochemical and Nd-Hf isotopic data for orogenic lherzolite massifs from the Ivrea-Verbano Zone (IVZ), Southern Alps. Even though a few Proterozoic Re depletion ages are locally preserved in these mantle bodies, our data reveal that the IVZ lherzolitic massifs were “recently” accreted to the SCLM in the Upper Devonian (ca. 370 Ma) during Pangea amalgamation, with a petrochemical evolution characterized by low-degree (~5–12%) depletion and nearly contemporaneous pervasive to focused melt migration. The lithospheric accretion putatively took place through asthenospheric upwelling triggered by Variscan intra-continental extension in a back-arc setting related to the subduction of the Rheic Ocean. We thus conclude that the fertile sections of Phanerozoic SCLM can be accreted during “recent” events of back-arc continental extension, even where Os isotopes preserve memories of melting events in much older times.

References

Ogunyele, Abimbola C., et al. «Accretion of “Young” Phanerozoic Subcontinental Lithospheric Mantle Triggered by Back-Arc Extension—the Case of the Ivrea-Verbano Zone». Scientific Reports, vol. 14, n.o 1, mayo de 2024, p. 11805. www.nature.com, https://doi.org/10.1038/s41598-024-61763-3

Day 2: third block

Convener: Sarah Lambart

Oceanic mantle refertilization via melt-harzburgite reaction: an experimental study at 1-2 GPa

Crotti C.F.¹, Borghini G.¹, Fumagalli P.¹, Tiepolo M.¹, Klemme S.², Rampone E.³

¹Dipartimento di Scienze della Terra, University of Milano, via Botticelli 23, 20133 Milano, Italy ²Institut fuer Mineralogie, Westfalische Wilhelms Universitat Muenster, Germany ³DISTAV, University of Genova, 16132 Genova, Italy

When: Thursday 3rd october at 11:40 am Speaker: Chiara Francesca Crotti; chiarafrancesca.crotti (at) unimi (dot) it

Earth's upper mantle is characterized by wide compositional variability mainly caused by partial melting and interactions between migrating melts and peridotites. After oceanic melting, refractory peridotites may react with migrating MORB melts before their accretion to the oceanic lithosphere. Reactive melt infiltration is expected to significantly modify the mineralogy and chemistry of residual mantle and affect the isotopic evolution of large portions of lithospheric mantle. Recent experimental results have revealed that the reaction between Enriched-MORB and opx-free mantle assemblage efficiently reset the trace elements composition of clinopyroxene via dissolution and reprecipitation (Borghini et al., 2023). However, the effect of such melt-peridotite interaction in opx-bearing mantle rocks is so far poorly studied. This work aims to experimentally investigate the effect of E-MORB - harzburgite reaction at oceanic asthenosphere-lithosphere boundary (1-2 GPa; 1150-1350°C) on mantle mineral chemistry. A modeled harzburgite is simulated by mixing depleted orthopyroxene (La_N/Yb_N = 0.004), San Carlos olivine (Fo90), and a moderately evolved tholeiitic basaltic glass (X_Mg = 0.60, Na₂O = 3.36 wt%, K₂O = 0.78 wt%) with E-MORB signature (La_N/Yb_N = 5.39). High-temperature (1200-1350°C) isothermal runs simulate the effects of interaction during the reactive percolation of transient melt, whereas some experiments cooled to 1150-1200°C simulate peridotite infiltration and impregnation via crystallization of reacted interstitial melt. Isothermal experiments mostly result in orthopyroxene dissolution coupled to olivine dissolution and reprecipitation. In cooling experiments, clinopyroxene and orthopyroxene precipitate from reacted melt, associated with few modal garnets at 2 GPa and 1150°C or plagioclase at 1 GPa and 1150°C. Olivine compositions vary with temperature; they display high X_Mg (up to 0.95) and Ca at low Ni concentrations in high-T isothermal experiments, and lower X_Mg (up to 0.88), coupled to moderate Ca and variably Ni contents after cooling. Clinopyroxenes (X_Mg = 0.83-0.88) have moderate Al (up to 0.49 a.p.f.u.) and Ti (up to 0.07 a.p.f.u.). New crystallized orthopyroxenes (X_Mg = 0.84-0.89) display high Al and Ti and low Cr. Interestingly, rims of partially dissolved orthopyroxene (X_Mg = 0.86-0.91) are modified towards high Ca and Ti and low Al and Cr contents with respect to initial orthopyroxene composition. Clinopyroxenes show high convex upward LREE patterns with marked MREE/HREE fractionation. Overall, they have higher absolute REE concentrations and LREE-MREE fractionation over HREE, compared to clinopyroxenes in equivalent orthopyroxene-free reaction experiments (Borghini et al., 2023). Newly formed orthopyroxenes show variable LREE enrichments (La_N/Sm_N = 0.19-1.38) coupled to lower HREE contents (Sm_N/Yb_N = 0.24-1.68), compared to initial orthopyroxene (La_N/Sm_N = 0.04; SmN/YbN = 0.10). Crystallization of reacted melt results in orthopyroxenes with increased REE contents coupled to slightly higher LREE over HREE reflecting in-situ melt fractionation. Indeed, final experimental glasses have LREE/HREE fractionations and absolute REE concentrations increasing with the extent of reacted melt crystallization. Unusual LREE-MREE enrichment in experimental orthopyroxenes has been documented in abyssal peridotites metasomatized by infiltration and crystallization of variably evolved melts (Zhang et al., 2024). Remarkably, after high-T interaction, several rims of orthopyroxene relicts show systematic LREE enrichment suggesting that partial trace elements re-equilibration with E-MORB may occur via elements diffusion. Experimental results demonstrate that reactive melt percolation and impregnation are able to significantly refertilize residual harzburgites before their incorporation into oceanic lithosphere mantle.

References

Borghini G., Fumagalli P., Arrigoni F., Rampone E., Berndt J., Klemme S. and Tiepolo M., 2023. Fast REE re-distribution in mantle clinopyroxene via reactive melt infiltration. Geochemical Perspectives Letters 26: 40-44. https://doi.org/10.7185/geochemlet.2323

Zhang W. Q., Liu C. Z., Dick H. J. B., Mitchell R. N., Liu B. D. (2024). Oceanic mantle beneath ultraslow spreading ridges metasomatized by variably evolved melts. Contributions to Mineralogy and Petrology 179: 15. https://doi.org/10.1007/s00410-023-02093-x

Vertical depletion of ophiolitic mantle reflects melt focusing and interaction in sub-spreading-center asthenosphere

Qing Xiong^1,2, Hong-Kun Dai^1,2, Jian-Ping Zheng¹, William L. Griffin², Hong-Da Zheng¹, Li Wang¹, Suzanne Y. O’Reilly²

¹State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China ²Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC, School of Natural Sciences, Macquarie University, NSW 2109, Australia

When: Thursday 3rd october at 12:00 Speaker: Qing Xiong; https://orcid.org/0009-0000-5354-1394

Decompressional melting of asthenosphere under spreading centers has been accepted to produce oceanic lithospheric mantle with vertical compositional variations, but these gradients are much smaller than those observed from ophiolites, which clearly require additional causes. Here we conduct high-density sampling and whole-rock and mineral analyses of peridotites across a Tibetan ophiolitic mantle section (~2 km thick), which shows a primary upward depletion (~12% difference) and local more-depleted anomalies. Thermodynamic modeling demonstrates that these features cannot be produced by decompressional melting or proportional compression of residual mantle, but can be explained by melt-peridotite reaction with lateral melt/rock ratio variations in an upwelling asthenospheric column, producing stronger depletion in the melt-focusing center and local zones. This column splits symmetrically and flows to become the horizontal uppermost lithospheric mantle, characterized by upward depletion and local anomalies. This model provides insights into melt extraction and uppermost-mantle origin beneath spreading centers with high melt fluxes (Xiong et al., 2022).

References

Xiong, Q., Dai, H.K., Zheng, J.P., Griffin, W.L., Zheng, H.D., Wang, L., O’Reilly, S.Y., 2022. Vertical depletion of ophiolitic mantle reflects melt focusing and interaction in sub-spreading-center asthenosphere. Nature Communications 13, 6956. https://doi.org/10.1038/s41467-022-34781-w

Pyroxenite generation via high-pressure crystallization of a MORB-type basalt: an experimental study at 1-2.5 GPa

Giulio Borghini¹, Chiara Francesca Crotti¹, Patrizia Fumagalli¹

¹Dipartimento di Scienze della Terra “Ardito Desio”, Università di Milano, via Botticelli 23, 20133 Milano, Italy

When: Thursday 3rd october at 12:20 Presenting author: Giulio Borghini, https://orcid.org/0000-0002-1825-0097

Pyroxenites represent widespread lithological heterogeneities found in the upper mantle in various geodynamical settings. They occur as lens or layers embedded within mantle peridotites in ophiolitic or orogenic ultramafic massifs, as thin veins in abyssal peridotites and found out as lithospheric mantle xenoliths. The origin of pyroxenites can be attributed to different magmatic or metamorphic processes or a combination of them (Bodinier & Godard, 2003). Several mantle pyroxenites are believed to result from melt segregation at high-pressure conditions (P ≥ 1 GPa) of mantle-derived melts that experienced varying extent of interaction with host peridotite (Borghini et al., 2022). Parental melts of many garnet- or spinel-bearing pyroxenites, usually occurring as thick layers, have low X_Mg (X_Mg = 0.6-0.8) suggesting they could be derived either via peridotite-derived melt differentiation or partial melting of low-MgO heterogeneous mantle sources. Although several experimental studies investigated MORB-like basalt crystallization at moderate pressure (P < 1 GPa), few experiments focused on phase relations and mineral chemistry at higher pressure.

In this study, we performed piston cylinder experiments at 1-2.5 GPa and 1100-1350°C to investigate the mineralogy and the composition of mineral phases in pyroxenites formed through high-pressure crystallization of a MORB-like basalt. Our starting material is a natural glass with the composition of moderately evolved tholeiitic basalts (X_Mg = 0.60, SiO₂ = 48.6 wt%, Na₂O = 3.48 wt%, K₂O = 0.81 wt%). High temperature (T > 1200°C) isothermal experiments revealed that clinopyroxene is the liquidus phase at 1-2.5 GPa. Liquidus clinopyroxenes have X_Mg around 0.79 and 0.84 at 1.5 GPa, 1200°C and 2 GPa, 1300°C respectively, all with moderate Al and Na contents. In order to simulated close-system crystallization after deep melt segregation in the mantle, we conducted high-pressure experiments that, after a high-temperature step, were cooled down to 1100 and 1150°C. During cooling, garnet precipitates after clinopyroxene from the basaltic melt at P ≥ 1.5 GPa and T > 1200°C, in association with rutile at 2 GPa after significant extents of crystallization (residual glass < 20 wt%, at 1100°C). At 1 GPa, low-fosterite olivine (Fo_76-79 at 1200°C and Fo_65-68 at 1100°C) was observed in all runs in association with plagioclase An_45-48 at 1100°C. Upon cooling, X_Mg in clinopyroxene decreases (down to 0.66) accompanied by a slight decrease in Ca and an increase in Ti content. At increasing pressure, clinopyroxenes exhibit progressively higher Na and Al contents (Jadeite content up to 0.25 at 2.5 GPa) with decreasing Ca and Cr concentrations. Garnets record upon cooling increase of Grossular and Pyrope contents combined with a decrease of Almandine. By assuming that after cooling and resulting isobaric crystallization, residual melts are expelled through compaction, the modal abundances derived from cooling experiments can be used to determine the final mineralogy of the rocks. Pyroxenites generated through infiltration and crystallization of MORB-type basalt at P from 1.5 to 2.5 GPa are garnet clinopyroxenites having garnet modal abundance increasing with pressure from 22 to 38 wt%. Olivine-bearing clinopyroxenite, with a low abundance (≈ 7 wt%) of Ab-rich plagioclase, forms at 1 GPa. Mineral compositions and modal abundances derived from experiments have been employed to estimate the bulk compositions of pyroxenites formed through high-pressure crystallization. Computed bulk-rock compositions well resemble those of garnet- and olivine+plagioclase-clinopyroxenites documented in orogenic and ophiolitic ultramafic massifs.

References

Bodinier, J.-L. & Godard, M., 2003. Orogenic, ophiolitic and abyssal peridotites. In: Carlson, R. W. (ed.) The Mantle and Core: Treatise on Geochemistry, Vol. 2. pp 103–170 Elsevier. https://doi.org/10-1016/B0-08-043751-6/02004-1

Borghini, G., Fumagalli, P., Rampone, E., 2022. Melt-rock interactions in a veined mantle: pyroxenite-peridotite reaction experiments at 2 GPa. Eur J Mineral 34, 109-129, https://doi.org/10.5194/ejm-34-109-2022

Day 2: fourth block

Convener: Romain Tilhac

Progressive strain localization and fluid focusing in mantle shear zones during rifting: petrostructural constraints from the Zabargad peridotites, Red Sea

Andréa Tommasi¹*, Marialine Chardelin¹, and José Alberto Padrón-Navarta^1,2

¹Géosciences Montpellier, CNRS & Université de Montpellier, F-34095 Montpellier, France. ²Instituto Andaluz de Ciencias de la Tierra, CSIC, Granada, Spain.

When: Thursday 3rd october at 16:00 Speaker: Andréa Tommasi; https://orcid.org/0000-0002-6457-1852

By re-analysing with new methods 40 samples from three peridotite massifs of Zabargad island in the northern Red Sea, we document the evolution of pressure and temperature conditions and the successive influence of hydrous melts and aqueous fluids on the operation of extensional shear zones, which exhumed mantle slivers from deep lithospheric or asthenospheric depths, in a rift-to-drift setting. By coupling high-resolution mapping of the microstructure by electron backscattered diffraction with recent developments in barometry for plagioclase-bearing peridotites and thermodynamic modelling of peridotitic compositions, we (1) constrain the temporal and spatial evolution of petrological and tectonic processes in the shallow mantle during rifting and (2) document the presence of melts or aqueous fluids throughout the activity of the shear zones, unravelling substantial feedback between petrological and tectonic processes. Thermobarometry and thermodynamic modelling constrained by the microstructural observations substantiate progressive strain localization associated with shearing under decreasing pressure and temperature, from near solidus conditions at >1 GPa (in the north and central peridotite massifs) or ~0.7 GPa (in the southern massif) to < 600°C and <0.3 GPa in all three massifs. These data also support local aqueous fluid saturation in the shear zones. The higher contents of hydrous minerals in ultramylonites indicate fluid focusing in the shear zones with evidence for seawater ingress up to >10 km depth. The presence of melts or fluids enabled concurrent dislocation and dissolution-precipitation creep, resulting in weakening of the shear zones. However, fluid supply was spatially heterogeneous and likely intermittent, with equilibrium achieved only locally in the ultramylonites. The present study documents therefore how the feedback between progressive strain localization and fluid-focusing in extensional shear zones contributes to thinning and exhumation of the mantle during continental rifting and the rift-to-drift transition.

Figure 1. Evolution of microstructures and modal compositions with progressive strain localization and decrease in the pressure and temperature conditions of the deformation in the Zabargad peridotites

References

Chardelin, M., Tommasi, A., Padrón-Navarta, J.A., 2024. Progressive Strain Localization and Fluid Focusing in Mantle Shear Zones during Rifting: Petrostructural Constraints from the Zabargad Peridotites, Red Sea. Journal of Petrology 65, egae081. https://doi.org/10.1093/petrology/egae081

Width and displacement rate of a divergent plate boundary shear zone: Constraints on the maximum strength in the mantle lithosphere

Basil Tikoff¹, Vasileios Chatzaras², Julie Newman³, Martyn R. Drury⁴

¹University of Wisconsin-Madison, Department of Geoscience, Madison, WI, USA ²The University of Sydney, School of Geosciences, Sydney, NSW, Australia ³Texas A&M University, Department of Geology and Geophysics, College Station, TX, USA ⁴Utrecht University, Department of Earth Sciences, Utrecht, Netherlands

When: Thursday 3rd october at 16:20 Speaker: Basil Tikoff; https://orcid.org/0000-0001-6022-7002

The Turon de Técouère massif of the French Pyrenees preserves a Cretaceous, magma-poor hyperextended plate margin within the mantle lithosphere. The massif records a: 1) An inferred ~40 m-wide mylonite zone at conditions of ~1000° C, ~1 GPa, and strain rates of ~10^-11 1/sec; 2) An inferred ~40 m-wide ultramylonite zone for conditions of ~850° C, ~0.6 GPa, and strain rates of ~10^-11 1/sec; and 3) A ~40 m-wide ultramylonite zone for conditions of ~700° C, ~0.5 GPa, and strain rates of ~4 x 10^-12 1/sec (Newman et al., 2021). We utilize the unique relationship between displacement rate, strain rate, and shear zone thickness, assuming simple shearing. Using the displacement rates determined from tectonic analyses and the strain rate estimates determined from microstructural analysis, we are able to predict the width (within a factor of 4) of the shear zone. This result establishes the Turon de Técouère shear zone as the exposed Cretaceous plate boundary. The inferred ~40 m-wide ultramylonite zone, deformed at 850° C, records the highest differential stress values (~200 MPa) and thus corresponds to the strongest part of the lithospheric mantle. Stress at lower temperatures decreases due to reaction softening (Newman et al., 1999; Newman and Drury, 2010). The study indicates that constant displacement rate boundary conditions is the best approach to consider plate-boundary shear zones, as both the stresses and strain rates vary during deformation.

References

Newman, J., Chatzaras, V., Tikoff, B., Wijbrans, J.R., Lamb, W.M., Drury, M.R. 2021. Strain Localization at Constant Strain Rate and Changing Stress Conditions: Implications for Plate Boundary Processes in the Upper Mantle. Minerals 11, 1351, https://doi.org/10.3390/min11121351

Newman, J., Drury, M.R., 2010. Control of shear zone location and thickness by initial grain size variations in upper mantle peridotites. Journal of Structural Geology 32, 832–842. https://doi.org/10.1016/j.jsg.2010.06.001

Newman, J., Lamb, W.M., Drury, M.R., Vissers, R.L.M., 1999. Deformation processes in a peridotite shear zone: reaction-softening by an H2O-deficient, continuous net transfer reaction. Tectonophysics 303, 193–222. https://doi.org/10.1016/S0040-1951(98)00259-5

Mantle-melt interactions related to lithospheric break-up at magma-poor rifted margins

Harmony Suire^1-2, Marc Ulrich¹, Gianreto Manatschal¹

¹Institut Terre Environnement Strasbourg, Université de Strasbourg, UMR7063, 5 Rue René Descartes, 67000 Strasbourg, France ²Centre National de Recherche Scientifique-Délégation Alsace, 23 Rue du Loess, 67200 Strasbourg, France

When: Thursday 3rd october at 16:40 Speaker: Harmony Suire; https://orcid.org/0009-0002-3745-0542

The study of dredged and drilled magmatic and mantle rocks from the Western Iberia and conjugate Newfoundland margins together with those from the fossil Alpine Tethys and the present-day Australia-Antarctica margins (Ballay et al., submitted), led to the development of models for the tectono-magmatic evolution of magma-poor rifted margins. However, it remains unclear to what extend the nature, origin, and history of the subcontinental mantle lithosphere (SCLM) controls the magmatic budget and mantle-melt reactions during mantle exhumation and what are the pressure-temperatures (P-T) conditions and rates of exhumation in these systems.

In this study, we use a set of yet little explored dredged and drilled samples from the Northwestern and Southwestern Iberia margin, that include the Galicia bank, the Iberia and newfoundland ODP sites and the Lion and Dragon seamounts from Tore-madeira ridge. New petrological and geochemical data including in-situ major and trace element concentrations and Nd-Hf isotopic compositions of minerals from ultramafic and mafic rocks were obtained to characterize mantle-melt interactions and identify the source(s) of melts. These data bring new constraints on the nature of the SCLM, the recorded mantle-melt interactions and on the nature of percolating melts during the early steps of oceanization. These results are discussed in terms of inheritance and mantle-melt processes at the scale of the southern North Atlantic and to define their control on the tecton-magmatic processes occurring during breakup at these magma-poor rifted margins.

Day 2: fifth block

Convener: Sylvie Demouchy

Fingerprinting metasomatic agent in the Styrian Basin mantle xenoliths via 3D Raman mapping of complex inclusions

Myovela, J.L.^1,2,5*, Aradi, L.E^3,4, Spránitz, T^3,5, Zoltán Taracsák⁶, Hegedűs, M.⁷, Konečný, P.⁸, Kovács, J.^1,9, Berkesi, M.^3,5

¹University of Pécs, Department of Geology and Meteorology, Hungary ²University of Dodoma, Department of Geology, Tanzania ³Lithosphere Fluid Research Lab, Eötvös Loránd University, Lithosphere Fluid Research Lab, Hungary ⁴Archaeometry Laboratory, National Archaeological Institute, Hungarian National Museum, Hungary ⁵HUN-REN Institute of Earth Physics and Space Science (EPSS), MTA EPSS Lendület FluidsByDepth Research Group, Hungary, ⁶Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, United Kingdom ⁷Department of Materials Physics, Eötvös Loránd University, Hungary ⁸State Geological Institute of Dionýz Štúr, Slovakia ⁹Environmental Analytical and Geoanalytical Research Group, Szentágothai Research Centre, University of Pécs, Hungary

When: Thursday 3rd october at 17:20 Speaker: Justine Myovela; https://orcid.org/0000-0002-3753-1863

The Styrian Basin, located in the transition zone between the Pannonian Basin and the Eastern Alps, is believed to have formed above a lithospheric wedge affected by subduction as strongly supported by geophysical data (Aradi et al., 2017; Qorbani et al., 2015). The Late Miocene-Pliocene alkali basalts sampled the subcontinental lithospheric mantle beneath the basin, bringing mantle xenoliths to the surface (e.g., Aradi et al., 2020). These xenoliths are amphibole-rich, indicating extensive modal metasomatism at mantle pressures. We performed 3D Raman mapping, Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM), Electron Microprobe Analysis (EMPA), and thermodynamic modeling to gain insights into the composition and nature of metasomatic agent recorded in the fluid and melt inclusions. Inclusions in this study are primary and pseudosecondary, hosted in orthopyroxene, clinopyroxene and amphibole. The fluid inclusions are irregular to negative crystal-shaped (3-100 μm), whereas melt inclusions are glass-rich with rounded to negative crystal shapes (4-15 μm).

Figure 1. Representative Raman 3D maps showing the distribution and volume percentages of phases. (A) Distribution of CO₂–rich fluid phase and solid phases. (B) Distribution of liquid H₂O.

3D Raman mapping on these inclusions has revealed complex phase assemblages comprising fluid and solid phases. The fluid phases are CO₂ (up to 98.4 mol%) and H₂O (up to 50.8 mol%). Solid phases include magnesite, silicate glass, pyrite, talc, anhydrite, and nahcolite, all showing variable volume properties (Figure 1). The silicate glass in the melt inclusions is H₂O-bearing (∼2 wt%) and compositionally evolved (trachyandesitic composition; 54.31-60.65 wt% SiO₂) relative to the host basalt of the studied xenoliths. We revealed pargasitic amphiboles quenched in the SiO₂-rich glass in the melt inclusions that co-entrapped with the CO₂-H₂O-rich fluid phase, suggesting that an immiscible SiO₂-rich melt and CO₂-H₂O-rich was circulating in the mantle wedge above the subducted slab during amphibole formation. An immiscible CO₂-H₂O-rich fluid and SiO₂-rich melt is suggested as a metasomatic agent that modified the subcontinental lithospheric mantle beneath the Styrian Basin. We propose that immiscibility could strongly enhance amphibole formation at mantle depths. Moreover, based on thermodynamic modeling, the revealed CO₂-rich fluids can co-exist with trachyandesitic melt with ~1 wt. % CO₂ and 2 wt% H₂O at P-T conditions (950-1030 °C, 1.2-1.6 GPa) ideal for forming the studied xenoliths. Consequently, the fluid-saturated composition in the melt inclusions could reflect the equilibrium state of the studied xenoliths.

Acknowledgements & funding: The authors owe thanks to Zoltán Dankházi and Ábel Szabó (Department of Materials Physics, Eötvös Loránd University, Hungary) for their help during the FIB-SEM analysis. This work was financially supported by the NKFIH_FK research fund nr. 132418 and MTA FI FluidsByDepth Lendület Research Group (LP2022-2/2022). The authors are grateful to the Doctoral School of Earth Sciences of the University of Pécs for partly funding this work

References

Aradi, L. E., Bali, E., Patkó, L., Hidas, K., Kovács, I. J., Zanetti, A., Garrido, C. J., Szabó, C. 2020. Geochemical evolution of the lithospheric mantle beneath the Styrian Basin (Western Pannonian Basin). Lithos, 378-379, 105831. https://doi.org/10.1016/j.lithos.2020.105831

Aradi, L. E., Hidas, K., Kovács, I. J., Tommasi, A., Klébesz, R., Garrido, C. J., Szabó, C., 2017. Fluid-Enhanced Annealing in the Subcontinental Lithospheric Mantle Beneath the Westernmost Margin of the Carpathian-Pannonian Extensional Basin System. Tectonics, 36(12), 2987-3011. https://doi.org/10.1002/2017TC004702

Qorbani, E., Bianchi, I., & Bokelmann, G. 2015. Slab detachment under the Eastern Alps seen by seismic anisotropy. Earth and Planetary Science Letters, 409, 96-108. https://doi.org/10.1016/j.epsl.2014.10.049

Fluid mobile elements and volatile behaviour during serpentinite dehydration

Asetre, Jo Hannah¹, Alard, Olivier^1,2,3, Ezad, Isra¹, Foley, Stephen^1,2

¹School of Natural Sciences, Macquarie University, North Ryde, New South Wales, Australia ²Research School of Earth Sciences, Australian National University, Acton, ACT, Australia ³Géosciences Montpellier, Université de Montpellier, Place Eugène Bataillon, Montpellier, France

When: Thursday 3rd october at 17:40 Speaker: Jo Hannah Asetre; https://orcid.org/0000-0002-7581-7758

Geochemical exchanges occurring along subduction zones regulate the mobility and long-term cycle of life-essential elements (e.g. C, H, O, S, F, Cl) between the exospheres and the deep earth. The capture, transfer, and eventual release of volatiles from the seafloor to subduction depths are a product of key metamorphic reactions, as initiated by the dehydration of the plunging slab. In particular, the prograde metamorphism and dehydration journey of serpentinites (Mg₃Si₂O₅(OH)₄) acts as a filter to either release volatiles and fluid-mobile elements (FME’s) back to the mantle wedge and/or the surface, or sequester them further down to the deep mantle contributing to the evolution of the Earth’s mantle composition (Fig. 1). The composition and volatile contents of the fluids liberated by serpentine breakdown reactions have thus critical implications on the secular equilibrium of Earth’s reservoirs, the emplacement of critical elements, and the oxidation of the mantle wedge.

However, the specific systematics and relative contribution of serpentinites in the global geochemical cycle remains to be fully understood. The nature, composition, and abundances of volatiles within the fluids released during deserpentinization reactions remain largely unconstrained. How efficient are these reactions in releasing fluids? Do these reactions produce oxidized or reduced fluids? Is S significantly extracted from the slab components, and if so, at which stage/depth does this occur? Do water and halogens enhance element transfer? All these questions remain highly debated in the literature (Benard et al., 2018; Evans and Frost, 2021; Kendrick et al., 2018; Marchesi et al., 2013; Pettke and Bretscher, 2022; Piccoli et al., 2019).

Figure 1: Serpentinites and their role in the subduction factory.

By employing in-situ mineral trace element analyses on high-pressure serpentinites and metaperidotites recording unique subduction histories, we have investigated the geochemical signatures associated with serpentine dehydration at various depths. Enrichments in key FME’s reinforce the importance of surviving hydrous phases (i.e. chlorite, humites) as a carrier of these elements down to 150 km depth. Preliminary results from high-pressure experiments also highlight the volatile-halogen transfer mechanisms at key temperatures and pressures where serpentine breakdown occurs. Volatile release and retention in the subducting slab is a complex and protracted process that merits further investigation. Ultimately, our study solidifies the significance of serpentinites towards the larger geochemical “diet” of subduction zones that govern the crossroads between the surface reservoirs and the deep mantle.

References

Bénard, A., Koga, K.T., Shimizu, N., Kendrick, M.A., Ionov, D.A., Nebel, O. and Arculus, R.J., 2017. Chlorine and fluorine partition coefficients and abundances in sub-arc mantle xenoliths (Kamchatka, Russia): Implications for melt generation and volatile recycling processes in subduction zones. Geochimica et Cosmochimica Acta, 199, 324-350 https://doi.org/10.1016/j.gca.2016.10.035

Evans, K.A. and Frost, B.R., 2020. Deserpentinization in subduction zones as a source of oxidation in arcs: A reality check. Journal of Petrology, 62(1), 1-32. https://doi.org/10.1093/petrology/egab016

Kendrick, M.A., Scambelluri, M., Hermann, J. and Padrón-Navarta, J.A., 2018. Halogens and noble gases in serpentinites and secondary peridotites: Implications for seawater subduction and the origin of mantle neon. Geochimica et Cosmochimica Acta, 235, 285-304. https://doi.org/10.1016/j.gca.2018.03.024

Marchesi, C., Garrido, C.J., Padrón-Navarta, J.A., Sánchez-Vizcaíno, V.L. and Gómez-Pugnaire, M.T., 2013. Element mobility from seafloor serpentinization to high-pressure dehydration of antigorite in subducted serpentinite: Insights from the Cerro del Almirez ultramafic massif (southern Spain). Lithos, 178, 128-142. https://doi.org/10.1016/j.lithos.2012.11.025

Pettke, T. and Bretscher, A., 2022. Fluid-mediated element cycling in subducted oceanic lithosphere: The orogenic serpentinite perspective. Earth-Science Reviews, 225, 103896. https://doi.org/10.1016/j.earscirev.2021.103896

Piccoli, F., Hermann, J., Pettke, T., Connolly, J.A.D., Kempf, E.D. and Vieira Duarte, J.F., 2019. Subducting serpentinites release reduced, not oxidized, aqueous fluids, Sci. Rep., 9, 19573. https://doi.org/10.1038/s41598-019-55944-8

The Rise and Fall of Ultramafic-Rich Mélanges in Cold to Hot Subduction Zones: Implications for Chemical Geodynamics

Anna M. Rebaza¹ , Ananya Mallik¹, Emily H.G. Cooperdock², Bridgett I. Holman¹

¹Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721. ²Department of Earth, Environmental & Planetary Sciences, Brown University, 324 Brook Street Providence, RI 02912.

When: Thursday 3rd october at 18:00 Speaker: Anna M. Rebaza; https://orcid.org/0000-0003-3062-7659

Buoyant ultramafic-rich (serpentine- or chlorite-rich) mélange diapirs in sediment-starved subduction channels may play a crucial role in transporting slab material to arc sources, thereby profoundly impacting arc magma diversity, mantle heterogeneity, and deep Earth cycling. However, it is unclear whether they preserve the overall buoyancy from deep forearc to subarc depths (~65-95 km), what conditions are required to form diapirs and their potential fate in subduction zones. Previous experimental and thermodynamic modeling predicts the buoyancy and diapir formation of chlorite-rich mélanges at those depths. Nevertheless, a gap in our understanding of mass transfer by serpentinerich mélanges still needs to be addressed. Here, (1) we experimentally constrain the partial melting behavior and the associated density transformations of a serpentine-rich matrix (5-10 wt.% H₂O) with minor sediments (9:1 ratio) at forearc (~65 km) and sub-arc (~95 km) depths (2-3 GPa and 800-1250ºC). (2) Evaluate the pMELTS software’s potential to reproduce our experimental data and to be used by future studies for similar systems. (3) Assess the buoyancy of a diverse suite of ultramafic-rich mélanges by exploring density contrasts relative to the overriding mantle, and the crucial interplay of viscosities and thicknesses of the mélange thickness at different subduction rates for diapir growth. (4) We discuss the fate of ultramafic-rich mélanges and the potential chemical geodynamic scenarios for various element groups (e.g., LILEs, HFSEs, volatiles) in subduction zones. Serpentine-rich mélanges have high solidus (1050-1100 ºC) and require either its diapiric rise into the hotter mantle wedge or interactions with a hotter asthenosphere through slab tears to partially melt and produce basaltic melts. Otherwise, the serpentine-rich mélange channel only dehydrates to form a denser peridotite lithology. We observe that pMELTS, a widely used tool, has limitations in reproducing phase equilibria and melting behavior under similar conditions to as our study. Serpentine-rich and chlorite-rich mélanges lose buoyancy at ~800ºC and ≥1000ºC, respectively. As they ascend, they potentially encounter a buoyancy barrier due to thermal equilibration and mineralogical transformation in the hotter mantle, leading to stalled or failed diapirs. The onset of diapirs in cold and fast subduction zones requires mélanges that may sometimes be thicker than those observed by field and geophysical studies, while hot and slow subduction zones generally require thinner mélanges. Partial melting of the mélange (caused by a diapiric rise or slab tear) does not fractionate the initial LILE/HFSE signature and transfer to the signature intact to arc magma sources. Dehydration fractionates the initial LILE/HFSE signature, releasing aqueous fluids with a high LILE/HFSE signature to the overriding mantle. This mantle-fluid interaction may favor the formation of F-OH-Ti-clinohumite-type rocks, holding back further HFSEs and releasing aqueous fluids with a higher LILE/HFSE signature and transferred to arc magma sources. Given high LILE/HFSE ratio is a ubiquitous arc magma signature, but slab tears are not, and diapirism in ultramafic-rich mélanges is highly conditional, this study corroborates that aqueous fluids are the predominant mass transfer agents in sediment-starved subductions rather than diapirs.

Day 3: first block

Convener: Marguerite Godard

Mantle Olivine Geochemistry

Olivier Alard^1,2,3, Marina Veter³, Ananuer “Alanur” Halimulati^1,3, Sylvie Demouchy⁴, Stephen F. Foley^1,3, Suzanne Y. O’Reilly³

¹Research School of Earth Sciences, The Australian National University, Canberra, Au. ²Géosciences Montpellier, CNRS & Université de Montpellier, Fr ³ARC Centre of Excellence for Core to Crust Fluid Systems and GEMOC, Macquarie University, Au. ⁴Laboratoire Magma & Volcans, CNRS & Université Clermont Auvergne, Fr

When: Friday 4th october at 9:20 am Speaker: Olivier Alard; https://orcid.org/0000-0002-0832-6625

Olivine has been, for some time now, the derelict mineral of the lithosphere for geochemists. Indeed because of its “tight” crystalline structure, olivine is extremely depleted in trace elements ca 100 times less than usual key peridotite minerals (cpx, gt). Yet, olivine forms more than 60% of the mantle and thus dictates the rheology and geophysical properties of the entire Earth’s upper mantle. Therefore, while geophysicists study an olivine mantle, geochemists focus on a pyroxene mantle.

By spicing up our plasma with trace molecular gas, a significant increase in ionisation efficiency is achieved, and typical detection limits for Lu, Eu, La, or Nb are now between 0.02 and 0.05 ppb. Thanks to this remarkable improvement, we can finally assess the full ultra-trace-element composition of olivine, including the rare-earth, high-field-strength and large-ion lithophile elements, as well as the abundance of moderately siderophile and chalcophile elements.

Olivines that equilibrated in the spinel or garnet facies from peridotite xenoliths entrained in alkali basalts and kimberlites and in peridotites from orogenic massifs have been investigated. Modally and cryptically metasomatised samples have been also investigated. The results show significant variations in trace-element content depending on coexisting phases, equilibrium conditions, melting degree and metasomatic processes. For instance, Yb and Lu show a clear correlation with melting degrees. These demonstrate that olivine trace element contents and relative abundances provide reliable fingerprints to characterise the composition of the lithospheric mantle. Olivine geochemistry thus provides a new and sensitive tool to increase the resolution of chemical tomography models of the mantle lithosphere and could provide a more straightforward link to seismic and magnetotelluric data.

However, the content and relative abundance of highly incompatible elements such as LREE or LILE are irrespective of melting degree and metasomatism type (e.g., hydrous vs. carbonatitic; modal vs. cryptic). Further, their abundances in olivine are not in ‘equilibrium’ with those measured in coexisting silicates (e.g., cpx, opx or amphibole). The mechanisms of incorporation of those highly incompatible elements and their significance will be further discussed.

Tracing potential sources for oceanic basalts using iron isotope systematics

Marianne Richter¹, Carlos Garrido¹, Romain Tilhac¹, Stephan König¹

¹Instituto Andaluz de Ciencias de la Tierra, CSIC, 18100 Armilla, Granada, Spain

When: Friday 4th october at 9:40 am Speaker: Marianne Richter; https://orcid.org/0000-0003-2048-1713

Ultramafic rocks are a major constituent of the Earth’s Mantle, distributed heterogeneously throughout it. These lithologies exhibit distinct chemical signatures due to chemical modification through enrichment and depletion processes during subduction, metasomatism and delamination of subcontinental lithospheric mantle. When these lithologies melt at mid-ocean ridges or in ocean island settings, they produce a basaltic melt with mixed geochemical and isotopic signatures. Radiogenic (Sr-Nd-Hf-Pb) and stable isotope, as well as trace element systematics can be used to trace different source lithologies.

This study aims to identify potential source lithologies for oceanic basalts using the Fe stable isotope system, in conjunction with radiogenic and trace element systematics and thermodynamic modelling. Previous studies on Fe isotope systematics in ocean floor basalts suggested pyroxenite is a contributor to the observed heavy isotopic compositions in these settings (e.g., Nebel et al., 2019; Gleeson et al., 2020; Richter et al., 2021; Soderman, et al., 2024). However, to which extend is not known. Past research focused on the Fe isotopic composition of mantle xenoliths and individual pyroxenite and peridotite lithologies (e.g. Williams et al., 2005; Weyer and Ionov 2007; Poitrasson et al., 2013; Williams and Bizimis 2014), but studies on exhumed ultramafic rocks and/or from recycled source lithologies are sparse. We, therefore, focus on ultra-high-pressure rocks from Ronda (Spain) and Beni Bousera (Marocco) to evaluate whether pyroxenites from recycled sources could be a potential source for oceanic basalts.

We analyzed thirteen garnet-pyroxenites from both regions, previously studied by Varas-Reus et al. (2018), for Fe isotopes (reported here as δ⁵⁷Fe permille, which is the permille deviation from the IRMM-524) at the Andalusian Earth Science Institute in Granada, Spain. The sample set has been subdivided by Varas-Reus et al. (2018) into three different groups. Group A garnet-pyroxenites, derived from oceanic crust recycling, display a large range from depleted (-0.159 ± 0.032‰) to enriched (+0.205 ± 0.007‰) δ⁵⁷Fe values. Group B samples, influenced by continental lower crust recycling, range from depleted MORB mantle (+0.048 ± 0.024‰) to enriched/heavy (+0.254 ± 0.025‰) Fe isotope values. Group C, classified as a hybride lithology, comprises a single sample with δ⁵⁷Fe of +0.050 ± 0.009‰, reflecting depleted MORB mantle value. Results show a positive correlation of Fe isotope with ⁸⁷Sr/⁸⁶Sr ratios, with Group B pyroxenite being more enriched in radiogenic Sr than Group A pyroxenites. Varas-Reus et al. (2018) reported similar trends with Sr-Isotope and Ca-Tschermak pyroxene, which is a proxy for eclogite recycling versus lower crust recycling. A potential link between eclogite and lower crust recycling and the observed Fe isotope composition will be further discussed in this contribution.

References

Craddock, P. R., Dauphas, N., 2011, Iron isotopic compositions of geological reference materials and chondrites. Geostandards and Geoanalytical Ressearch 35, 101–123. https://doi.org/10.1111/j.1751-908X.2010.00085

Gleeson, M.L.M., Gibson, S.A., Williams, H.M., 2020, Novel insights from Fe-Isotopes into the lithological heterogeneity of Ocean Island Basalts and plume-induced MORBs, Earth and Planetary Research Letters 535, 116114. https://doi.org/10.1016/j.epsl.2020.116114

Nebel., O., Sossi, P.A., Benard, A., Arculus, R.J., Yaxley, G.M., Woodhead, J.D., Davies, D.R., Ruttor, S., 2019, Reconciling petrological and isotopic mixing mechanisms in the Pitcairn mantle plume using stable Fe isotopes, Earth and Planetary Science Letters 521, 60-67. https://doi.org/10.1016/j.epsl.2019.05.037

Poitrasson, F., Delpech, G., Gregoire, M., 2013, On the iron isotope heterogeneity of lithospheric mantle xenoliths: implications for mantle metasomatism, the origin of basalts and the iron isotope composition of the Earth, Contributions to Mineralogy and Petrology 165, 1243-1258.

Richter, M., Nebel., O., Schwindinger, M., Nebel-Jacobsen, Y., Dick., H.J.B., 2021, Competing effects of spreading rate, crystal fractionation and source variability on Fe isotope systematics in mid-ocean ridge lavas, Scientific Reports 11, 4123.

Soderman, C.R., Matthews, S., Shorttle, O., Jackson, M.G., Ruttor, S., Nebel, O., Turner, S., Beier, C., Millet, M-A., Widom, E., Humayun, M., Williams, H.M., 2021, Geochimica et Cosmochimia Acta 292, 309-332. https://doi.org/10.1016/j.gca.2020.09.033

Varas-Reus, M.I., Garrido, Ca.J., Marchesi, C., Bosch, D., Hidas, K., 2018, Genesis of ultra-high pressure garnet pyroxenites in orogenic peridotites and its bearing on the compositional heterogeneity of the Earth’s mantle, Geochimica et Cosmochimica Acta 232, 303-328. https://doi.org/10.1016/j.gca.2018.04.033

Weyer, S., Ionov, D.A., 2007, Partial melting and melt percolation in the mantle: The message from Fe isotopes, Earth and Planetary Science Letters 259, 119-133. https://doi.org/10.1016/j.epsl.2007.04.033

Williams, H.M., Peslier, A.H., McCammon, C., Halliday, A.N., Levasseur, S., Teutsch, N., Burg, J.-P., 2005, Systematic iron isotope variations in mantle rocks and minerals: The effects of partial melting and oxygen fugacity, Earth and Planetary Science Letters 235, 435-452. https://doi.org/10.1016/j.epsl.2005.04.020

Williams, H.M., Bizimis, M., 2014, iron isotope tracing of mantle heterogeneity within the source regions of oceanic basalts, Earth and Planetary Science Letters 404, 396-407. https://doi.org/10.1016/j.epsl.2014.07.033

-White, W.M., 2015, isotopes, DUPAL, LLSVPs and Anekantavada, Chemical Geology 419, 10-28. https://doi.org/10.1016/j.chemgeo.2015.09.026_

Stability and chemistry of rutile and titanite in metamafic rocks

Inês Pereira¹, Emilie Bruand², Kenneth Koga³, Christian Nicollet⁴, Alberto Vitale Brovarone^5,6,7

¹Universidade de Coimbra, Centro de Geociências, Departamento de Ciências da Terra, Coimbra, Portugal ²Geo-Ocean laboratory, Université Bretagne Occidentale, CNRS, Rue Dumont d’Urville, 29280, Plouzané, France ³Institut des Sciences de la Terre d’Orléans, 1A Rue de la Férollerie – CS 20066F-45071 Orléans Cedex 2, France ⁴Université Clermont Auvergne, Laboratoire Magmas et Volcans, Campus universitaire des Cezeaux, 6 Av. Blaise Pascal, 63170 Aubière, France ⁵Department of Biological, Geological, and Environmental Sciences, Università degli Studi di Bologna, Bologna, Italy ⁶Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, 4 Place Jussieu, 75005 Paris, France ⁷Istituto di Geoscienze e Georisorse, Consiglio Nazionale delle Ricerche, Pisa, Italy

When: Friday 4th october at 10:00 am Speaker: Inês Pereira; https://orcid.org/0000-0001-9028-2483

Rutile and titanite are valuable petrochronometers, since both can be dated using U-Pb and Zr concentrations are calibrated as geothermometers. In order to use these minerals as tracers of crustal gradients, a better understanding regarding their stabilities is required, as well as the development of single-grain barometry. Previous experimental studies using MORB compositions (Liou et al, 1998) established that titanite is more stable at LT-LP and rutile at HP (> 12 kbar). Despite these, the natural occurrence of rutile at LP (< 12 kbar) and titanite at HP (> 20 kbar) indicates strong uncertainties on our current understanding about their stabilities in mafic lithologies, particularly under conditions typical of subduction zones. To understand their phase stabilities and how their chemistries may reflect their peak P-T conditions, we conducted a set of experiments using piston-cylinder apparatus and we studied the chemistry of rutile and titanite using a set of metamafic rocks with MORB composition (Pereira et al., 2023), which experienced variable P-T metamorphism, by EPMA and LA-ICPMS.

We present the results of a set of 30 experiments that were conducted under water-saturated conditions, using a cold pressure-seal capsule technique, with pressures ranging between 12 and 23 kbar, and temperatures between 400 and 750 °C (Fig. 1A). We tested multiple starting materials, with bulk rock powders yielding different Ti/Ca values, and resourcing to mineral seeds to work as nuclei for mineral overgrowth (e.g. rutile, titanite, kaersutite, wollastonite). Due to the challenging LT experiments, equilibrium is not attained, but dissolution and precipitation features are often observable (Fig. 1B). We show that when Ti/Ca is high (0.20), titanite seeds become unstable and start reacting with the basalt bulk rock powder while rutile is stable, even at lower pressures (14 kbar), and when Ti/Ca is low (0.15), titanite seeds appear metastable, even at high pressures (19 kbar) and low temperatures (< 600 °C). This is in agreement with petrological observations recorded in some natural samples (i.e. peak titanite reported in blueschist and low-T eclogite rocks; Vitale Brovarone et al., 2011). We found that water content as well as Ti/Ca ratios appear to influence the stability of these rutile and titanite in mafic systems, influencing the stable Ti-phase at HP conditions (Fig. 1C). Additionally, we present mineral chemical results for rutile and titanite from natural samples of metamafic rocks formed at HT-LP and LT-HP metamorphic conditions. In our rock dataset, we found rutile stable in ocean-floor amphibole-bearing gabbros, formed at low pressures (< 2 kbar). These pressure conditions are lower than experimental constraints suggest for rutile stability, which is formed during retrograde reactions due to excess Ti released from Ti-amphiboles. Rutile is also found in eclogitic metagabbros from the Western Alps and can be chemically distinguished from LP rutile. Blueschist metagabbros from the Western Alps and eclogitic metabasalts from Corsica have titanite stable instead of rutile. High-pressure titanite from these metamafic rocks exhibits La depletion and low La/SmN values, distinct from titanite from amphibolite-facies mafic rocks. We compare our mineral chemical data with literature data and propose La/SmN or Nb together with Yb and V to distinguish HP titanite from titanite formed under other P–T settings, and Nb/V to distinguish LP from HP metamafic rutile.

These experimental results and natural observations indicate that rutile and titanite stabilities are a function of P-T and bulk-rock composition, illustrating the necessity of using geochemical tracers for the development of the single Ti-phase PT-t recorder, especially relevant in detrital rocks.

Figure 1. Stability of rutile, titanite and ilmenite, A. in terms of P-T space based on publish and new experimental petrology; B. EDS-SEM maps of mineral phases of experiments depicted in A., and C. as a function of Ti/Ca and Ca/Al bulk-rock composition (Pereira et al., 2023).

Funding: This work was supported by the French Government Laboratory of Excellence initiative ANR-10-LABX-0006, by the Fundação para a Ciência e Tecnologia under grants numbers UIDB/00073/2020, UIDP/00073/2020 and a fellowship to IP (doi.org/10.54499/2021.01616.CEECIND/CP1656/CT0006), and by the European Union (ERC, FINGER-PT, 101117053).

References

Liou, J. G., Zhang, R., Ernst, W. G., Liu, J. & McLimans, R. 1998. Mineral parageneses in the Piampaludo eclogitic body, Gruppo di Voltri, western Ligurian Alps. Schweizerische Mineralogische und Petrographische Mitteilungen 78, 317–335.

Pereira, I., Bruand, E., Nicollet, C., Koga, K. T., & Vitale Brovarone, A. 2023. Ti-Bearing Minerals: from the Ocean Floor to Subduction and Back. Journal of Petrology, 64(7), egad041 https://doi.org/10.1093/petrology/egad041

Vitale Brovarone, A., Groppo, C., Hetényi, G., Compagnoni, R. & Malavieille, J. 2011. Coexistence of lawsonite-bearing eclogite and blueschist: phase equilibria modelling of Alpine Corsica metabasalts and petrological evolution of subducting slabs. Journal of Metamorphic Geology 29, 583–600. https://doi.org/10.1111/j.1525-1314.2011.00931.x

Day 3: second block

Convener: Vasileios Chatzaras

Partial melting and mantle-melts interactions at the Diamantina zone: insights on the mantle evolution during lithospheric break-up

Mélanie Ballay¹, Marc Ulrich¹, Gianreto Manatschal¹

¹Institut Terre et Environnement de Strasbourg – ITES/Géols- UMR 7063 CNRS

When: Friday 4th october at 10:40 am Speaker: Mélanie Ballay; https://orcid.org/0000-0001-8669-9171

How continents break and how, when, and how much magma is produced during lithospheric breakup is yet little understood. A main reason is that answering to these questions requires direct access to rocks recording the magmatic processes during breakup, which is only the case for few places, among which the present-day Iberia and fossil Alpine Tethys ocean-continent transitions (OCT) are the best investigated. Studies from these sites showed evidence for partial melting, percolation and refertilization of inherited subcontinental mantle that allowed to develop a conceptual model to explain the magma-mantle evolution during lithospheric thinning and breakup at magma-poor rifted margins. Here we present bulk-rock and mineral major and trace element concentrations of mantle rocks and basalts dredged along the Diamantina OCT (SW Australia). Our results are interpreted in terms of mineral-melt exchanges in peridotites, element partitioning during refertilization and partial melting processes, and reconstruction of thermo-barometric equilibrium conditions in the subcontinental mantle of the Diamantina OCT. Our preliminary results show that similarly to Tethys OCTs, two distinct mantle domains occur in the Diamantina zone: an inherited mantle formed of Sp-lherzolites equilibrated at T REE Cpx−Opx ~960°C, and a refertilized mantle domain of Pl-lherzolites with higher equilibrium temperatures (T REE Cpx−Opx ~1270°C) highlighting the entrapment of percolating melts in the plagioclase stability field (~7 kbar). The pyroxene speedometry indicates similar cooling rates than those calculated for the Tethyan subcontinental mantle (10^-2 to 10^-3°C/yr). Basaltic rocks have tholeiitic to alkaline compositions (Na₂O+K₂O ~ 3-5 wt.% at SiO₂ < 50 wt.%). The REE-in-plagioclase-clinopyroxene thermometer indicates crystallization temperatures of ~1180°C. These results suggest that lithospheric break-up in the Diamantina Zone is preceded by exhumation of subcontinental mantle from the Sp- to Pl-stability field in the presence of a high geothermal gradient. Thus, despite of the different inheritance and the proximity to the Kerguelen plume, the mantle evolution in the Diamantina zone is compatible with the Alpine model. We therefore consider that refertilization is a first order process that occurs at all magma-poor margins, independently of the pre-rift evolution. Nevertheless, how, and when partial melting occurred and how magmatic products evolved during exhumation remain unclear and need further investigations.

The lithospheric mantle beneath central Mongolia: constraints from spinel-bearing peridotite xenoliths and high-pressure experiments

Beltrame M.¹*, Ziberna L.¹, McCammon C.², Masotta M.³, Venier M.¹, De Felice A.¹, Majgsuren Y.⁴, De Min A.¹

¹Departement of Mathematics, Informatics and Geosciences, University of Trieste, Italy ²Bayerisches Geoinstitut, Universität Bayreuth, Germany ³Departement of Earth Sciences, University of Pisa, Italy ⁴Geoscience Center, School of Geology and Mining, Mongolian University of Science and Technology, Ulaanbaatar, Mongolia

When: Friday 4th october at 11:00 am Speaker: Marco Beltrame; https://orcid.org/0009-0000-2227-4887

When dealing with spinel peridotite xenoliths, it is difficult to accurately determine which lithospheric portions they come from, as there are currently no accurate geothermobarometric methods to estimate their depth of formation. Therefore, it is essential to characterize in the best possible way the few fragments found in different sampling campaigns in alkaline magmatic provinces also with the help of geophysical data.

One of the regions where the lithospheric mantle is still poorly investigated is the Central Asian Orogenic System, particularly the area of central Mongolia. Here, we focus on a suite of spinel-bearing mantle xenoliths from different volcanic structures located within and off a small graben in the Mongolian region of Mandakh-Mandal-Gobi. These xenoliths are hosted in alkaline lavas with an age of 71 - 51 Ma (Yarmoluk et al., 2019) and are mainly lherzolites, with minor amounts of harzburgites and pyroxenites. Texture is generally protogranular with some samples showing reactions between melt and crystals. Major element compositions of mineral phases are almost homogeneous within individual xenoliths. In contrast REE and incompatible element patterns show significant variations: xenoliths from the last magmatic event belong to a mantle which is characterized by a more complex grade of heterogeneity.

Due the lack of precise, accurate and well tested geobarometers for spinel peridotites, experiments were performed with the aim to test the performance of the available methods (e.g., Ca exchange between olivine and clinopyroxene; Köhler & Brey, 1990) and to find precise P and T of mineral phase equilibria in our natural xenoliths. Experiments were conducted with a piston cylinder press at the Bayerisches Geoinstitut (BGI) of the University of Bayreuth. The P-T window spans from 10 to 20 kbar and from 1000 to 1200°C. Two starting compositions were used, both reflecting the bulk composition of one xenolith: one sample made up of a mixture of synthetic oxides, the other consisting of powdered hand-picked minerals.

All these data are being used to achieve the following goals: i) to understand whether the xenoliths derive from just below the crust-mantle transition zone or rather represent fragments that have been sampled throughout the mantle section in which spinel peridotites are stable; these observations would help understanding the mechanisms that drive the fracturing and entrapment of mantle peridotites during deep-seated magmatism; ii) to understand the vertical and lateral variations in lithospheric composition through time in a region that underwent collisional processes followed by extensive anorogenic-type magmatism.

References

Köhler, T.P., Brey, G.P., 1990. Calcium exchange between olivine and clinopyroxene calibrated as a geothermobarometer for natural peridotites from 2 to 60 kb with applications. Geochimica et Cosmochimica Acta 54, 2375–2388. https://doi.org/10.1016/0016-7037(90)90226-B

Yarmolyuk, V.V., Kudryashova, E.A., Kozlovsky, A.M., 2019. Late Stages in the Evolution of the Late Mesozoic East Mongolian Volcanic Areal: Rock Age and Composition. Dokl. Earth Sc. 487, 773–777. https://doi.org/10.1134/S1028334X19070298

Petrology and geochemistry of Careón Ophiolite mantle (NW Iberian Massif): geodynamic implications

G. Iglesias1, A. Garcia-Casco2, S. Sánchez Martinez1, I. Novo-Fernandez2, P. Andonaegui1 and R. Arenas1

1 Dpto. de Mineralogía y Petrología e Instituto de Geociencias (UCM, CSIC), Universidad Complutense, 28040 Madrid, Spain. 2 Dpto. de Mineralogía y Petrología and Inst. And. Ciencias Tierra (UGR, CSIC), Universidad de Granada, 18071 Granada, Spain.

When: Friday 4th october at 11:20 am Speaker: Gabriel Iglesias; https://orcid.org/0009-0007-2056-1867

The Careón Ophiolite is one of the best-preserved Devonian supra-subduction zone ophiolites (c. 395 Ma) exposed along the Variscan Orogen. We present the first detailed petrographic and geochemical study of the serpentinized peridotites forming the mantle section of the Careón Ophiolite. We focus on revealing their primary mineralogical nature and composition as well as modelling the recorded partial melting/refertilization processes. The ultramafic section of the Careón Ophiolite is formed by harzburgites and metasomatic dunites and olivine-rich harzburgites. Two groups of harzburgites have been identified showing typical compositions of abyssal and fore-arc peridotites. On the other hand, two main high-temperature petrogenetic events have influenced the geochemical characteristics of the peridotite protoliths. During a first event of likely Cambrian age, the original mantle section underwent low melting/refertilization degrees under anhydrous conditions in a peri-Gondwanan margin in a back-arc setting. A second event, of likely Devonian age in a fore-arc setting, was characterized by higher degrees of partial melting/refertilization under hydrous conditions. Metasomatic replacive dunites and olivine-rich harzburgites formed during both events by interaction of peridotite with hydrous and anhydrous basaltic melts via incongruent orthopyroxene dissolution mechanism. The dynamic evolution from a Cambrian back-arc setting to a Devonian fore-arc setting is consistent with the previously described peri-Gondwanan Variscan evolution and with geochemical characteristics and magmatic ages of the mafic rocks in the mafic-ultramafic units of the NW Iberian Massif.

Figure 1. (a) Anhydrous melting/refertilization model for low-depleted harzburgites. (b) Hydrous melting/refertilization model for high-depleted harzburgite group. (c) Geodynamic model to Gondwana margin during Devonian.

Day 3: third block

Convener: Marco A. Lopez-Sanchez

Iceland plume sustained by upwelling of melt-depleted, buoyant peridotites

Sanfilippo A.^1,2, Stracke A.³, Genske F.³, Scarani S.¹, Cuffaro M.⁴, Basch V.^1,2, Borghini G.⁵, Brunelli D.^4,6, Ferrando C.^1,7, Peyve A.A.⁸, Ligi M.⁹

¹Department of Earth and Environmental Sciences, University of Pavia, Via Ferrata 1, Pavia, Italy ²Istituto di Geoscienze e Georisorse - CNR, U.O. Pavia, Italy ³Institut für Mineralogie, Universität Münster, Germany ⁴Istituto di Geologia Ambientale e Geoingegneria - CNR, Roma, Italy. ⁵Dipartimento di Scienze della Terra, Università degli Studi di Milano, Italy. ⁶Dipartimento di Scienze Chimiche e Geologiche, Università di Modena e Reggio Emilia, Modena, Italy. ⁷Dipartimento di Scienze della Terra, dell’Ambiente e della Vita, Università di Genova, Genova, Italy. ⁸Geological Institute, Russian Academy of Sciences, Moscow, Russia. ⁹Istituto di Scienze Marine - CNR, Bologna, Italy.

When: Friday 4th october at 12:00 Speaker: Alessio Sanfilippo; https://orcid.org/0009-0007-2056-1867

Anomalies in seafloor elevation locally characterize mid-ocean ridges, which, as for example at Iceland, are characterized by a thicker-than-normal, and compositionally distinct basaltic crust. In these locations, the underlying mantle is considered to be hotter and chemically more fertile that the adjacent sub-ridge mantle (Schilling, 1973). However, with the exception of rare mantle xenoliths in some OIB-derived lavas (Bizimis et al., 2007), direct mantle peridotites from upwelling mantle plumes have never been sampled.

In this contribution, we present petrological and geochemical data on peridotites from the Charlie Gibbs Transform Zone (53°N), which is located at the southern end of the Reykjanes ridge south-west of Iceland. Studied peridotites are subdivided in two categories: residual harzburgites and melt-reacted lherzolites. The clinopyroxene (cpx) in harzburgites have depleted LREE compositions, and MORB-like Nd-Hf isotope ratios (εHf=14-32 and εNd=10-16). In contrast, the lherzolite cpx have extremely radiogenic Hf isotope ratios (up to 463), but low εNd (6.8). The REE patterns of the harzburgite cpx indicate a residual character generated by 7%-10% partial melting of a DM-like peridotite. The very low M-REE associated with highly radiogenic Hf compositions of the cpx in lherzolites, on the other hand, require an ancient (>1.5 Ga) chemical depletion followed by recent interaction with enriched melts. These observations suggest that the lherzolites formed through refertilization of ancient harzburgites, i.e., they are ‘secondary’. Based on seismic velocity anomalies and gravity models we relate the peridotites from the Charlie Gibbs Transform Zone to the ‘Iceland plume’, which has fundamental implications for the geodynamic evolution of Reykjanes ridge south of Iceland, and the regional distribution of MORB with Icelandic affinity.

References

Schilling, J. G., 1973. Iceland mantle plume: Geochemical study of Reykjanes Ridge. Nature 242, 565–571, https://doi.org/10.1038/242565a0.

Bizimis, M., Griselin, M., Lassiter, J.C., Salters, V.J.M. & Sen, G., 2007. Ancient recycled mantle lithosphere in the Hawaiian plume: Osmium–Hafnium isotopic evidence from peridotite mantle xenoliths. Earth and Planetary Science Letters 257, 259-273, https://doi.org/10.1016/j.epsl.2007.02.036.

Multi-stage growth of gabbronorite lenses within harzburgitic mantle of the Purang ophiolite (South Tibet): Implications for melt migration at oceanic slow/ultraslow-spreading centers

Long-Fei Xue¹, Qing Xiong¹

¹State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China

When: Friday 4th october at 12:20 Speaker: Long-Fei Xue; https://orcid.org/0009-0008-5120-5491

The formation of oceanic lithosphere at oceanic spreading centers is a crucial process in plate tectonics, which has the world's largest magma systems, as conduits, transferring matter and energy from the deep Earth to the surface. At the oceanic spreading centers, the mantle melt extraction system develops deep in the asthenosphere to lithosphere, and determines the composition and structure of the oceanic lithosphere (Xiong et al., 2022). The studies of surface-exposed relics like ophiolites, geophysical explorations and numerical modelling are three main ways to reveal its formation and operation mechanisms. However, these efforts have yielded models of mantle melt extraction for fast spreading centers, while those beneath slow to ultraslow oceanic spreading centers remain elusive.

In this study, we chose the Purang ophiolite, a typical slow-spreading product, in the western segment of the Yarlung Zangbo suture zone in southern Tibet. A network of gabbronorite intrusions in its northwest, spanning an area of approximately 10×30 m², has been observed and characterized in detail here. The width of gabbronorite veins ranges from several centimeters to 1-2 m. These veins exhibit intruding fractures within the highly serpentinized harzburgite wall rocks, with clearly delineated contact relationships and the presence of thin pyroxenite boundary layers. The gabbronorite intrusions show two forms: single-stage and multi-stage. Single-stage veins crystallized from a single episode of melt, with gradual changes of mineral grain size and whole-rock major element compositions from the vein's edge to its core. Conversely, multi-stage veins resulted from multiple melt intrusions, showcasing abrupt shifts in mineral grain sizes and higher Al₂O₃ and Na₂O₃ contents in subsequent magma stages, along with lower CaO and Mg#. Both types of intrusions exhibited whole-rock REE/CI chondrite values of less than 1, indicative of significant depletion. Additionally, a marginal increase in overall trace element abundances from the intrusion's edge to its core suggests magma evolution within the small magma channel. For the later-stage intrusions, mineral CSD analysis suggests a residence time within magma transport channels through fractures of at least ~270 days.

In summary, the gabbronorite intrusion system represents the outcome of mafic magma intrusion and evolution along fracture surfaces within the hydrated harzburgite mantle. In slow to ultraslow oceanic spreading centers, fractures in the shallow mantle emerge as vital conduits for magma transport.

Figure 1. The photos of gabbronorite veins in field. (a) A distant view of the gabbronorite intrusions. (b) A close-up photo of gabbronorite veins outcrop, which intruded fractures within the serpentinized harzburgite. (c) A single-stage gabbronorite vein. The pyroxenes and plagioclase aggregates within the vein gradually grew larger from the edge to the center (along the red arrows). (d) A multi-stage gabbronorite vein. The gabbronorite (I) was intruded by relatively finer-grained gabbronorite (II), and in the gabbronorite (II), the size of minerals also shows an increasing tendency from the edge to the core.

References

Xiong, Q., Dai, H.K., Zheng, J.P., Griffin, W.L., Zheng, H.D., Wang, L., O’Reilly, S.Y., 2022. Vertical depletion of ophiolitic mantle reflects melt focusing and interaction in sub-spreading-center asthenosphere. Nature Communications 13, 6956. https://doi.org/10.1038/s41467-022-34781-w

Time-progressive mantle-melt evolution in an intra-oceanic arc: Evidence from the Albanide-Hellenide ophiolites

Emilio Saccani¹, Costanza Bonadiman¹, Yildirim Dilek², Adonis Photiades³

¹Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, Via Saragat 1, 44122 Ferrara, Italy ²Department of Geology and Environmental Earth Sciences, Miami University, 118 Shideler Hall, Patterson Av., Oxford, OH 45056, USA ³Department of General Geology and Geological Mapping, Institute of Geology and Mineral Exploration (IGME), Olympic Village, 13677 Acharnae, Attica, Greece

When: Friday 4th october at 12:40 Speaker: Emilio Saccani; https://orcid.org/0000-0001-9879-2795

The Albanide-Hellenide belt includes two parallel NNW-SSE trending belts of Jurassic Tethyan ophiolites: the Vardar zone in the east and the Mirdita-Subpelagonian-Pelagonian (MSP) zone in the west (Saccani et al., 2011, 2017). The MSP ophiolites feature, in turn, two distinct belts with different lithostratigraphy and geochemistry: the western and eastern MSP ophiolites (Fig. 1a).

The western MSP ophiolites consist of relatively thin (3–4 km) ophiolite sequences with lherzolitic mantle tectonites, overlain by gabbros and basalts with N-MORB geochemical affinities, as well as SSZ-type ophiolites like medium-Ti basalts (MTB) and boninites. The eastern MSP ophiolites consist of thicker (10–12 km) ophiolite sequences showing SSZ affinities with abundant harzburgitic mantle tectonites and gabbronorite-basalt series with IAT and boninitic affinities. MOR-type rocks like pl-peridotites, troctolites, and gabbros are also present. The MSP mantle lherzolites and harzburgites show varying degrees of depletion and increasing LREE enrichment with increasing whole-rock depletion. Younging ages are generally observed from the western to the eastern MSP sequences.

We present an overview of the time-progressive chemical evolution at a regional scale of the upper mantle in response to the production of different lava types (mantle depletion) and the progressive addition of subduction-derived chemical components (mantle enrichment) in a Jurassic intra-oceanic arc system in the Neo-Tethyan Albanides-Hellenides. This study is based on field-grounded geochemical studies and REE semi-quantitative modeling. These data are used to propose the following tectonomagmatic model for the evolution of the Albanide-Hellenide ophiolites (Fig. 1).

1. During Mid Jurassic, the Neo-Tethys mid-ocean spreading lasting since Mid Triassic changed to a compressional regime and a subduction started close to the mid-ocean ridge (Fig. 1a). The western-type N-MORBs were the first magmas to erupt after subduction initiation and were generated by 10%–20% partial melting of a depleted MORB mantle source. Melt extraction left mantle residua of moderately depleted lherzolites (Fig. 1b).

2. Meanwhile, subduction initiation of the Tethyan slab triggered upwelling of the residual MORB mantle toward the forearc where it experienced 5%–8% partial melting without any subduction influence, producing MTB magmas along with fading N-MORBs. The extraction of MORB and MTB melts left depleted (cpx-poor) lherzolites and harzburgites as the mantle residua (Fig. 1c).

3. Following subduction initiation, continuing slab sinking and retreat promoted the enrichment in fluid-mobile subduction components of the mantle wedge peridotites. Cpx-poor residual lherzolites were then enriched to moderate extents in LREEs and Th. Their partial melting (~10%–20%) generated IAT magmas and depleted harzburgites as the residual mantle. Minor volumes of boninitic magmas may have also been generated at this stage (Fig. 1d).

4. Fading MORB magmatism produced limited volumes of melts that migrated to shallow levels in the mantle wedge and pooled out at the upper mantle–lower crust transition and formed the MORB-type mafic-ultramafic cumulates (Fig. 1d) currently exposed in the eastern-type ophiolites (Hoeck et al., 2002; Saccani and Tassinari, 2015).

5. With continued subduction, fluid influx from the subducting slab increased, then the depleted mantle residual after previous multistage melt extractions underwent significant LREE and Th enrichment and high degree (15%–25%) partial melting, producing large volumes of the youngest boninitic rocks. The residual mantle after boninitic melt extraction was represented by extremely refractory harzburgites (Fig. 1e).

Our study of the upper mantle peridotites and extrusive sequences in the Albanide-Hellenide ophiolites presents significant insights into the evolution at a regional scale of ophiolitic magmas and their mantle sources in a SSZ environment. Our model provides an ideal case study to probe in four dimensions the chemical geodynamic evolution of the oceanic upper mantle both in ancient and modern SSZ tectonic environments.

Figure 1. Two-dimensional tectonic diagrams, depicting the tectonomagmatic evolution of mantle peridotite and volcanic rocks in the Albanide-Hellenide Jurassic ophiolites. See text for explanations, abbreviations, and numbered tectonomagmatic steps.

References

Dilek, Y., Furnes, H., Shallo, M., 2007. Suprasubduction zone ophiolite formation along the periphery of Mesozoic Gondwana. Gondwana Res 11, 453–475. https://doi.org/10.1016/j.gr.2007.01.005

Saccani, E., Beccaluva, L., Photiades, A., Zeda, O., 2011. Petrogenesis and tectono-magmatic significance of basalts and mantle peridotites from the Albanian-Greek ophiolites and sub-ophiolitic mélanges. New constraints for the Triassic–Jurassic evolution of the Neo-Tethys in the Dinaride sector. Lithos 124, 227–242. https://doi.org/10.1016/j.lithos.2010.10.009

Saccani, E., Tassinari, R., 2015. The role of MORB and SSZ magma-types in the formation of Jurassic ultramafic cumulates in the Mirdita ophiolites (Albania) as deduced from chromian spinel and olivine chemistry: Ofioliti 40, 37–56. https://doi.org/10.4454/ofioliti.v40i1.434

Saccani, E., Dilek, Y., Photiades, A., 2017. Time-progressive mantle-melt evolution and magma production in a Tethyan marginal sea: A case study of the Albanide-Hellenide ophiolites. Lithosphere, 10, 35-53. https://doi.org/10.1130/L1602.1131

List of Abstracts, posters

(Hydrous) melt assisted mantle exhumation – case study from Puke, Mirdita Ophiolite (Albania)

Jakub Mikrut¹, Magdalena Matusiak-Małek¹, Benoit Ildefonse², Andrea Tommasi², Georges Ceuleneer³, Michel Grégoire³, Kujtim Onuzi⁴

¹Institute of Geological Sciences, University of Wrocław, Wrocław, Poland ²Géosciences Montpellier, CNRS & Université de Montpellier, Montpellier, France ³Géosciences Environnement Toulouse, CNRS, Toulouse III University, CNES, IRD, Toulouse, France ⁴Institute of GeoSciences, Tirana, Albania

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #1 Presenting author: Jakub Mikrut, https://orcid.org/0000-0002-1825-0097

Since the discovery of Oceanic Core Complexes (OCC) at the Mid-Atlantic Ridge, numerous structures of similar nature have been recognized worldwide (e.g., Whitney et al. 2013). Some of the first OCC identified in ophiolitic complexes were Puke and Krabbi massifs in the Mirdita Ophiolite, Albania (Nicolas et al. 1999). An advantage of the Mirdita complex is its undisturbed, oceanic tectonics allowing studies of primary contacts and spatial relationships between lithologies. This study focuses on microtextural and geochemical features of Puke peridotites and related rocks to reconstruct its primary characteristic and exhumation conditions.

The primary lithology of Puke is a coarse-grained, anhydrous spinel harzburgite, which currently outcrops only in its eastern section (Fig. 1a). The olivine is partially recrystallized, displaying polygonal shapes. Crystallographic Preferred Orientations (CPO) of olivine are well-defined (J index>2.5). Textures of harzburgites record high temperature deformation, compatible with asthenospheric flow conditions.

The dominant lithology in the Puke massif is, however, plagioclase-bearing lherzolitic mylonite, which grade into ultramylonites in the western part of the massif. Plagioclase occurs in all mylonites in various proportions, forming trails, continous layers, or being part of aggregates. Amphibole occurs only in some sections of the massif. Pyroxenes are present as porphyroclasts and also as interstitial grains organised in trails or laminae, or forming pyroxene±olivine aggregates. Porphyroclasts often exhibit corroded rims, especially when surrounded by fine-grained aggregates. Olivine CPO weakens from the mylonites to the ultramylonites towards the west (J index decreasing from 2.18 to 1.05). In all peridotites serpentinization postdates deformation. Ubiquitous gabbroic and pyroxenitic veins are up to 10 or more cm thick and are concordant with the host peridotite’s foliations.

Differences between coarse-grained harzburgites and mylonites are also clearly visible in mineral chemistry. Olivine is Fo-richer in harzburgites compared to mylonites (90.5-91 vs. 86-90.5, respectively). Spinel and pyroxenes are TiO₂-poor in harzburgites and TiO₂-rich in mylonites (<0.1 and 0.1-1.9 wt.% in spinel and <0.2 and 0.2-1.1 wt.% in clinopyroxene, respectively). The pyroxenes in harzburgites are HREE-poor (Lu_N<0.4 in orthopyroxene and <1.1 in clinopyroxene) compared to pyroxenes in mylonites (Lu_N>0.8 in orthopyroxene and Lu_N <1.1 in clinopyroxene; Fig. 1 b). Plagioclase is present only in mylonites and shows a wide range of An contents reaching very high values (75-95 %). Interestingly, chemical composition of the flaser gabbros caping peridotites and concordant gabbro veins in the peridotites perfectly mimics trace elements contents in pyroxenes and anorthite in plagioclase for mylonites.

Early works considered Puke as a plagioclase lherzolite massif, but detailed structural investigations recognized the secondary nature of plagioclase related to melt percolation and widespread mylonitization (Nicolas et al. 1999). The presented structural and geochemical data confirms the impregnative character of the Puke massif. Harzburgites evolved in spinel facies conditions and experienced high melt extraction degree, but were later affected by mineralogical and chemical enrichment by percolating melts. This magmatic event was syn-deformational, and magmatic impregnation blurred the primary fabrics of the harzburgites. The percolating melt had characteristics similar to MORB with various content of water as suggested by changes in modal content of impregnative amphibole, and by the high anorthite content of plagioclase. The intensity of deformation increases westward, and transition from coarse-grained harzburgites to more fertile mylonites is gradational. The presented structural and chemical data is in agreement with the scenario of OCC formation proposed by Nicolas et al. (2017). As our results show lithological similarities with the Othris massif in Greece (Dijkstra et al. 2001), we suggest that similar processes may have shaped lithological mantle in other sectors of the Pindos-Mirdita Ocean.

Figure 1. (a) Geological map of western Mirdita ophiolite with sampling locations, arrow marks direction of increasing melt impregnation (map modified after Nicolas et al. 2017). (b) Trace elements contents of clinopyroxene in Puke rocks.

References

Nicolas, A., Boudier, F., Meshi, A., 1999, Slow spreading accretion and mantle denudation in the Mirdita ophiolite (Albania). Journal of Geophysical Research 104, 15155–15167. https://doi.org/10.1029/1999JB900126

Whitney, D., Teyssier, C., Rey, P., Buck., W., 2013. Continental and oceanic core complexes. GSA Bulletin, 125, 273-298. https://doi.org/10.1130/B30754.1

Nicolas, A., Meshi, A., Boudier, F., Jousselin, D., Muceku, B., 2017. Mylonites in ophiolite of Mirdita (Albania): Oceanic detachment shear zone. Geosphere 13, 136–154. https://doi.org/10.1130/GES01383.1

Dijkstra, A.H., Drury, M.R., and Vissers, R.L., 2001, Structural petrology of plagioclase-peridotites in the West Othrys Mountains (Greece): Melt impregnation in mantle lithosphere. Journal of Petrology, 42, 5–24. https://doi.org/10.1093/petrology/42.1.5

The Phase Transformation of Sediment-Rich Mélanges from Deep Forearc to Sub-Arc Depths

Bridgett I. Holman¹, Anna M. Rebaza¹, Ananya Mallik¹, Emily H. G. Cooperdock²

¹University of Arizona, Tucson, Arizona USA
²Brown University, Providence, Rhode Island USA

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #2 Presenting author: Bridgett I. Holman, https://orcid.org/0009-0003-8197-261X

Mélange channels are a noteworthy feature of subduction zones as they are variable in composition and control elemental mass fluxes (e.g., carbon, hydrogen, nitrogen, etc.) thus contributing to diversity of arc magmas and heterogeneity of the mantle. Constraining the phase equilibria of sediment-rich mélanges is necessary to understand the role of subducted sediments in deep volatile cycling and in mass transfer to arc lava sources. While pure sedimentary subducted lithologies has been previously explored, the phase equilibria of the full spectrum of compositional variety for subducted mélanges has yet to be constrained.

Here, we investigate the phase equilibria of a sediment-rich mélange (~60 wt. % SiO₂, volatile-free) in a mixture consisting of 90% sedimentary composition (mud-shale) analogous to the Franciscan complex in San Simeon, USA mixed with 10% of an ultramafic-rich mélange (serpentinite and chlorite schists) from the Kampos Belt in Syros, Greece at fore-arc to sub-arc depths (2-3 GPa, 700-1150 °C) and at 5 and 10 wt. % bulk water contents using piston cylinder apparatus.

Partial melting of subducted hydrous-shaley rich matrix yields hydrous dacitic to rhyolitic melts (64-79 wt.% SiO₂ volatile-free, Mg# 32-55) coexisting with orthopyroxene, amphibole, mica, quartz, and garnet. The solidus at 5 wt.% H₂O is just below 700°C at 2 GPa and at 800°C at 3 GPa. For 10 wt. % H₂O and at both 2 and 3 GPa, the solidus is just below 700°C. Hydrous minerals such as amphiboles and K-micas are stable up to 1000°C and 800°C at 2 GPa and 1000°C and 900°C at 3 GPa, respectively. In contrast to purely sediment mélanges (~76 wt. % SiO₂ starting, volatile-free) melting under the same conditions (2 GPa and 1000°C), our slightly mafic mélange yields alkali-depleted melt, with 2x less Na₂O and K₂O and 3-4x more enriched in MgO and FeO. Such melt compositions are in the range of ~15% of global natural arc lavas for wt. % SiO₂ (volatile-free).

The results of our experimental study demonstrate there are volatile-hosting minerals (micas, amphiboles) in the mélange channel of the subducting slab as well as in the residues of partial melting of the mélange in the sub-arc mantle in intermediate to hot subduction zones. This indicates the potential for deep cycling of volatiles during subduction of shale-rich mélanges. Furthermore, the partial melts from such shale-rich mélanges act as metasomatic agents by creating pyroxenitic bodies during partial melting, furthering mantle heterogeneity (Rebaza et al., 2023). Lastly, within the compositional range of existing natural arc lavas, subducted sediment-rich mélanges yield silica-rich melts, which if erupted nearly unchanged through pyroxenitic channels[1], diversify primary arc magma chemistries.

References

Rebaza, A.M., Mallik, A., Straub, S.M., 2023. Multiple Episodes of Rock-Melt Reaction at the Slab-Mantle Interface: Formation of High Silica Primary Magmas in Intermediate to Hot Subduction Zones. Journal of Petrology 64, egad011. https://doi.org/10.1093/petrology/egad01

Highly Siderophile Elements (HSE) and Os Isotopes in pyroxenite-peridotite associations from Northern Apennine (Italy) veined mantle

Giulio Borghini¹, Ambre Luguet², Carlotta Ferrando³, Alex Di Raimondo³, Roberto Cabella³, Elisabetta Rampone³

¹Dipartimento di Scienze della Terra “Ardito Desio”, Università di Milano, via Botticelli 23, 20133 Milano, Italy ²Institut fur Geowissenschaften, Rheinische Friedrich-Wilhems-Universitat, Bonn 53115, Germany ³Dipartimento di Scienze della Terra, dell’Ambiente e della Vita, Università di Genova, Corso Europa 26, 16132 Genova, Italy

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #3 Presenting author: Giulio Borghini, https://orcid.org/0000-0002-1825-0097

Deep infiltration and high-pressure reactive crystallization of melt at lithosphere-asthenosphere boundary conditions is one of the main mechanisms of mantle heterogeneities formation,mostly related to the emplacement of pyroxenites. During crystallization of pyroxenitic layers or lenses, melts may also percolate locally the adjacent peridotites and affect their chemical and isotopic compositions. Pyroxenite-peridotite sequences from External Liguride mantle bodies of Northern Apennines ophiolites (Italy) represent one of the most studied examples of veined mantle. Here, MORB-like peridotites experienced intrusion of tholeiitic basalts that crystallized garnet-bearing pyroxenite layers and modified via reactive percolation (coupled to chromatographic effect) the major and trace element composition of the host peridotite. Over time, this introduced small-scale Sm-Nd and Lu-Hf isotope mantle heterogeneities and produced Hf-Nd enriched mantle domains, potentially representing the heterogeneous mantle source of Enriched-MORBs (Borghini et al., 2021). Spinel- to plagioclase-facies subsolidus exhumation allowed the preservation of chemical and isotopic gradient related to the pyroxenite emplacement. In this study we investigate the distribution and major element composition of sulphides together with Highly Siderophile Elements (HSE) and Re-Os isotopes compositions in whole-rock and single sulphides along well-studied pyroxenite-peridotite profiles. In the profile, we thus distinguished the country peridotite (free of pyroxenites at 1mt scale) and the host peridotite, adjacent to pyroxenite layer. Distribution of Base Metal Sulphides does not vary among the lithologies. Few sulphide grains are enclosed in spinel- or plagioclase-facies minerals whereas most of them consist of interstitial grains associated with the plagioclase-bearing neoblastic association. Country peridotites contain sulphides consisting of pentlandite. Sulphides in the pyroxenites are all polyphase grains made by pentlandite, pyrrhotite and sporadically chalcopyrite with pyrrhotite being volumetrically predominant. In host peridotites, sulphides are also polyphase but pyrrhotite is always volumetrically lesser than pentlandite. In pyroxenites pentlandite composition has homogeneous Ni/(Ni+Fe) values between 40 and 45 but this value increases progressively from wall-rock to host to country peridotites (up to 60), as effect of interaction with pyroxenitic melt.

Peridotites show HSE concentration comparable to the Earth’s Primordial Mantle and within the range of HSE documented in many orogenic peridotites. Pyroxenites have lower Os, Ir and Ru contents and show higher variation in Pt, Pd and Re abundances. We have distinguished three groups of pyroxenites. The group I pyroxenites have the highest Al₂O₃ contents (Al₂O₃ = 14.44-16.46 wt%) and show the strongest fractionation between the IPGE (Os, Ir, Ru) and the PPGE (Pt and Pd) and Re. Group II pyroxenites have moderate Al₂O₃ contents (Al₂O₃ = 12.75-13.86 wt%) and exhibit an intermediate fractionation between the IPGE and the PPGE and Re. Pyroxenites of group III with the lowest Al₂O₃ contents (Al₂O₃ = 9.94-12.87 wt%) show the highest Os-Ir-Ru concentrations, low Pt-Pd contents resulting in HSE patterns rather smooth and close to those of the peridotites.

Present-day ¹⁸⁷Os/¹⁸⁸Os ratios of peridotite define a narrow PUM-like range (0.1242-0.1345) with host peridotites having homogeneous ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁷Re/¹⁸⁸Os ratios and distal peridotite showing more variable Os isotopic ratios. Pyroxenites exhibit more radiogenic Os isotope ratios from 0.1547 to 0.8035. They decrease at increasing Os concentration from Group I (0.2498-0.4269) to Group II (0.2121-0.2401) and Group III (0.1547-0.2308). Pyroxenites display ¹⁸⁷Re/¹⁸⁸Os ratios systematically higher than those of peridotites, with Group I showing values higher than Group II and III. HSE and Os isotopes evidence a trend of progressive increase of melt-peridotite reaction, going from Group I pyroxenites, very similar to silicate melts, to Group III pyroxenites, the latter being the most similar to peridotites. Single sulphide analyses along a selected pyroxenite-peridotite traverse reveal HSE and Os isotopes gradients resulting from interaction between infiltrating melt and host peridotite.

References

Borghini, G., Rampone, E., Class, C., Goldstein, S., Cai, Y., Cipriani, A., Hofmann, A.W., Boulge, L., 2021. Enriched Hf-Nd isotopic signature of veined pyroxenite-infiltrated peridotite as a possible source for E-MORB. Chemical Geology 586, 120591. https://doi.org/10.1016/j.chemgeo.2021.120591

Transition from lithospheric to asthenospheric mantle sources during Early Mesozoic magmatism of the Southern Alps: Evidence from alkali-rich dykes intruding the Ivrea–Verbano Zone (Italy)

Abimbola C. Ogunyele^1,2,6, Mattia Bonazzi², Tommaso Giovanardi³, Maurizio Mazzucchelli^2,3, Vincent J.M. Salters⁴, Alessandro Decarlis⁵, Alessio Sanfilippo^1,2, Alberto Zanetti^2,1

¹Department of Earth and Environmental Sciences, University of Pavia, Via Ferrata 1, 27100 Pavia, Italy ²CNR – Istituto Geoscienze e Georisorse, Via Ferrata 1, 27100 Pavia, Italy ³Department of Chemical and Geological Sciences, University of Modena and Reggio Emilia, Via Campi 103, 41125 Modena, Italy ⁴National High Magnetic Field Laboratory, Department of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee, FL 32310, USA ⁵RICH Center, Earth Sciences Department, Khalifa University of Science and Technology, P.O. Box 12788, Abu Dhabi, United Arab Emirates ⁶Department of Earth Sciences, Adekunle Ajasin University, PMB 001 Akungba-Akoko, Nigeria

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #4 Presenting author: Alberto Zanetti, https://orcid.org/0000-0001-9026-1519

The Ivrea-Verbano Zone (IVZ) in the westernmost sector of the Southern Alps is an iconic upper mantle to lower continental crust sequence of the Adriatic Plate and provides a geological window into the tectono-magmatic events that occurred at the Gondwana–Laurussia boundary from Late Paleozoic to Early Mesozoic. In this contribution, we document new geochemical and Nd-Sr-Hf-Pb isotopic data for Early Mesozoic alkali-rich dyke swarms which intruded the Finero Phlogopite Peridotite (northern IVZ) to provide geological constraints on the nature, origin and evolution of Early Mesozoic magmatism in the Southern Alps.

The studied dykes are amphibole-phlogopite-bearing and show geochemical features varying between two end-member groups. A dyke group is characterized by HFSE-poor, Al-rich amphibole (Al2O3 up to 16 wt%) with high LILE and LREE contents, high radiogenic ⁸⁷Sr/⁸⁶Sr(i) (0.704732 to 0.704934) and low radiogenic Nd isotopes (eNd(i) from –0.1 to –0.7), which support the occurrence of significant amounts of recycled continental crust components in the parental mantle melts and impart an overall ‘‘orogenic-like” affinity. This dyke group was largely derived from metasomatized lithospheric mantle sources. The second group is HFSE-rich with Al-poorer amphibole enriched in LILE and LREE, low radiogenic ⁸⁷Sr/⁸⁶Sr(i) (0.703761–0.704103) and higher radiogenic Nd isotopes (eNd(i) from +3.4 to +5.4) pointing to an ‘‘anorogenic” alkaline affinity and asthenospheric to deep lithospheric mantle sources. Some dykes show both orogenic and anorogenic affinities, providing evidence that the orogenic-like magmatism in the IVZ predates the alkaline anorogenic magmatism.

The Finero dyke swarms therefore record a geochemical change of the Early Mesozoic magmatism of the Southern Alps from orogenic-like magmatism, typical of post-collisional settings, to anorogenic alkaline magmatism, common in intraplate to extensional settings, and places a temporal correlation of Early Mesozoic magmatism in the IVZ to those in the eastern and central sectors of the Southern Alps.

References

Ogunyele, Abimbola Chris, et al. Transition from «orogenic-like» to «anorogenic» geochemical affinity in Mesozoic post-collisional magmatism: evidence from alkali-rich dykes from Ivrea-Verbano Zone (Southern Alps). mayo de 2022, pp. EGU22-13428. NASA ADS, https://doi.org/10.5194/egusphere-egu22-13428

Deconvolving mantle lithologic compositions through geochemical modeling of oceanic basalts

Lynne J. Elkins¹, Sarah Lambart²

¹University of Nebraska-Lincoln, Lincoln, NE, USA ²University of Utah, Salt Lake City, UT, USA

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #5 Presenting author: Lynne J. Elkins, https://orcid.org/0000-0002-0903-7091

While oceanic basalts can provide geochemical clues to the nature of the convecting mantle, they are also the ultimate, integrated products of multiple processes and factors, including continuous, progressive melt generation, source heterogeneity, magma transport, melt-rock chemical interactions, and homogenization and mixing. Deconvolving those signals to decipher the nature of the asthenosphere is complex, but recent developments in geochemical melt modeling provide new avenues to further investigate the mantle origins of lavas. The pyUserCalc package (Elkins and Spiegelman 2021) provides reproducible, open-source, and accessible computational modeling tools for tracking the geochemical products of partial melting by reactive porous flow. The package includes methods for calculating chemical equilibrium and disequilibrium melt transport through a decompressing solid matrix, for both trace element concentrations and uranium-series disequilibria. Additional tools are available to determine the compositions of two-dimensional integrated melts in a triangular melting regime, the effects of radioactive decay during magma transport through the lithosphere, and outcomes of traditional dynamic melting.

In combination with the models Melt-PX for calculating the melting behavior of two lithologies in thermal equilibrium (Lambart et al. 2016) and pMELTS for determining coexisting mineral modes during the melting process (Ghiorso et al. 2002), we have developed a workflow for determining the trace element concentrations and U-series disequilibria in partial melts produced by decompression of a multi-lithologic mantle. We have conducted systematic modeling of melting for a heterogeneous mantle, considering 0-50% pyroxenite in the solid source, four mafic pyroxenite compositions (G2, KG1, M7-16, and MIX1G) in thermal equilibrium with ambient peridotite, two mantle potential temperatures (1300 and 1400 ºC), and a range of solid mantle upwelling rates and reference melt porosity fractions. We find that on a global spatial scale, the mantle melting regime must contain measurable and variable quantities of pyroxenitic rocks (Figure), and those pyroxenites must span the range from silica-deficient to silica-excess compositions (Elkins & Lambart, accepted). The upper mantle is thus heterogeneous across many length scales and a wide range of compositions. Our modeling outcomes also require a variety of magma transport mechanisms, including both equilibrium and disequilibrium porous flow, implying that magma transport occurs across a range of channelized and porous flow networks in the decompressing mantle.

Figure 1. Uranium-series disequilibrium activity ratios in calculated partial melts of silica-excess, G2 (average mid-ocean ridge basalt) pyroxenite, for a range of solid mantle upwelling rates (solid lines) and maximum porosities (dashed lines), and for a mantle potential temperature of 1300 ºC. Global mid-ocean ridge data are shown for comparison. Of the scenarios tested in this study, G2 pyroxenite partial melts are best able to reproduce oceanic basalt compositions with elevated (²³⁰Th/²³⁸U) and low (²²⁶Ra/²³⁰Th) and (²³¹Pa/²³⁵U) (Elkins and Lambart, accepted, and references therein).

References

Elkins, L. Lambart, S., accepted. Uranium-series disequilibria in MORB, revisited: A systematic numerical approach to partial melting of a heterogeneous mantle. Accepted in Volcanica. Preprint: https://doi.org/10.22541/essoar.170289974.42837909/v1

Elkins, L., Spiegelman, M., 2021. pyUserCalc: A revised Jupyter notebook calculator for uranium-series disequilibria in basalts. Earth and Space Science 8(12), e2020EA001619. https://doi.org/10.1029/2020EA001619

Ghiorso, M.S., Hirschmann, M.M., Reiners, P.W., Kress, V.S., 2002. The pMELTS: A revision of MELTS for improved calculation of phase relations and major element partitioning related to partial melting of the mantle to 3 GPa. Geochemistry, Geophysics, Geosystems 3(5), 1-35. https://doi.org/10.1029/2001GC000217

Lambart, S., Baker, M.B., Stolper, E.M., 2016. The role of pyroxenite in basalt genesis: Melt-PX, a melting parameterization for mantle pyroxenites between 0.9 and 5 GPa. Journal of Geophysical Research: Solid Earth 121(8), 5708-5735. https://doi.org/10.1002/2015JB012762

Mechanical and metasomatic evolution of a developing mantle wedge from subduction initiation to obduction

Alissa J. Kotowski¹, Andrew Keats¹, Hester Smit¹, Jippe van Broekhoven¹, Matthew Tarling², Emily Cooperdock³, Marguerite Godard⁴, Oliver Plümper¹, Martyn Drury¹, Eric Hellebrand¹

¹Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Utrecht, the Netherlands ²Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, Canada ³Department of Earth, Environmental, and Planetary Sciences, Brown University, Rhode Island, USA ⁴French National Centre for Scientific Research (CNRS), Géosciences Montpellier, France

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #6 Presenting author: Eric Hellebrand, https://orcid.org/0000-0002-0903-7091

Geodynamic models suggest that formation of plate boundary shear zones requires a mechanically weak material. Peridotites occupying the proto-plate boundary hanging wall position are inherently strong, and experience cooling during subduction initiation from >1000˚C to < 500˚C over ~10 Myr, which should resist strain localization. Serpentinites are commonly credited for lithosphere-scale strain localization and considered prerequisites for subduction initiation. However, geologic observations suggest that proto-interface temperatures exceed 550˚C at 20-30 km, making conditions too hot for serpentine to form. Thus, both olivine and serpentine’s contribution to plate boundary formation is unclear. Here we present structural and geochemical data constraining the mechanical and metasomatic evolution of a fossilized subduction interface exposed on Mont Albert (Québec, Canada) to evaluate the high-temperature deformation history of the dry mantle, and the relative timing, extent, and effects of serpentinization across a developing interface.

Mont Albert is an incomplete Ordovician ophiolite complex that records subduction initiation and obduction onto the Laurentian margin during the Taconian Orogeny (~450-500 Ma). Field structural observations and optical and electron microscopy demonstrate that spinel peridotites occupy distributed shear zones exhibiting mylonitic to ultramylonitic fabric evolution under increasingly wetter conditions with proximity towards the paleo-plate contact. Olivine Crystallographic Preferred Orientation (CPO) changes from stronger A- and D-type in mylonites, then to weaker AG- and B-type in ultramylonites, corresponding with grain size reduction from ~60-80 µm to ≤20 µm. We interpret the fabric transition to result from dynamic recrystallization accommodated by dislocation creep, with a switch to grain-size sensitive diffusion-accommodated creep, and that grain size remains small due to phase mixing with primary orthopyroxene and growth of secondary hydrous phases (e.g., chlorite and amphibole). Olivine paleopiezometry returns paleo-flow stresses of 100-160 MPa and 100-135 MPa for mylonites and ultramylonites, respectively, and does not trend clearly with respect to fabric type, which suggests roughly iso-stress deformation through time.

Immediately at the paleo-plate contact, a ~10-20 m thick zone of heavily serpentinized ultramylonites (i.e., the “contact zone”; 75-90% serpentine) are characterized by sub-mm to cm-scale intercalations of fine-grained, strongly-aligned layers of lizardite (confirmed with Raman spectroscopy), Fe-oxide-rich (hematite and magnetite), and relict olivine-rich layers. No antigorite was identified. Undeformed lizardite mesh textures are extremely common, and some ‘boudins’ comprising aggregates of coarser-grained lizardite mesh with grain boundaries decorated by hematite appear to be statically serpentinized equivalents of high-temperature boudinage of coarser-grained olivine layers. We interpret that the majority of the contact ultramylonite serpentinization occurred post-kinematically.

Geochemical analyses across the 60 m mantle transect (n=9) and farther away (+300-800 m, n=4) show very limited trends with distance from the contact or degree of serpentinization. Major elements and HREE demonstrate variable melt depletion (~5-15%) of the mantle protolith; Ce, Sr, and Pb are only subtly enriched at the contact compared to structurally distal samples; and LREE and other fluid mobile elements do not vary systematically between samples. Overall, this supports minimal chemical overprinting during serpentinization, and that limited fluid flow occurred late in the shear zone history at low temperatures (<300˚C).

Our structural, microstructural, and geochemical analyses together demonstrate that an early high-temperature phase of distributed ductile deformation became increasingly localized due to the introduction of high-T hydrous phases, likely during initial plate boundary formation. Olivine flow laws suggest an order of magnitude increase in strain rate approaching the paleo-plate contact, which we suggest reflects switches in deformation mechanism rather than stress, assisting in early stages of strain localization. Serpentinites did not facilitate strain localization during plate boundary development, nor facilitate obduction. Rather, serpentinites followed from focused fluid flow along an established lithosphere-scale structure and effectively ‘locked’ the shear zone, forcing obduction-related strain to localize elsewhere.

EXCITE2: a European network providing free-of-charge access to x-ray, electron, and ion imaging facilities

E. Hellebrand¹, G. ter Maat¹, H. Vogel¹, S. Walter¹, R.J.F. Wessels¹, O. Plümper¹

¹Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Utrecht, the Netherlands

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #7 Presenting author: Eric Hellebrand, https://orcid.org/0000-0003-1909-6279

The EXCITE² Network is transforming Earth and environmental material science by uniting 19 research facilities across 12 European countries. This network provides access to advanced imaging technologies, allowing scientists to study Earth materials at scales from nanometers to centimeters. This research offers insights into environmental toxicity, critical metal extraction for renewable energy, and long-term storage of climate-altering gases. EXCITE² fosters interdisciplinary collaboration and supports a sustainable future in Europe. It also provides training for new researchers and introduces innovative services like AI and advanced imaging to enhance problem-solving. The initiative drives scientific excellence and benefits society. While the emphasis of EXCITE² is on projects with societal relevance, basic research questions continue to be eligible for support as well. Routine workhorse instruments are accessible, such as EPMA for quantitative spot analysis, (FEG-)SEM for basic imaging and large-area EDS mapping, in justified cases after first conducting X-ray tomography to identify the most appropriate 2D slice from a 3D starting material. Among the more specialized instruments are a nanoSIMS at Utrecht University, and an AtomProbe at Curtin University. Access to EXCITE can be requested by applying to our next transnational access call opening in November 2024. Interested? Have a look on the EXCITE website (https://excite-network.eu) – and apply!

Conjunction of sub-oceanic and sub-arc mantle peridotites: Revisiting the Uenzaru peridotite complex in the northern Hidaka metamorphic belt, Hokkaido, Japan

Toru Yamasaki¹, Gen Shimoda¹

¹Institute of Geology and Geoinformation, Geological Survey of Japan (AIST), 1-1-1 Higashi, Tsukuba Central 7, Ibaraki, 305-8567 Japan

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #8 Presenting author: Toru Yamasaki, https://orcid.org/0000-0002-9300-0338

The high-T/P-type Hidaka metamorphic belt (HMB) in Hokkaido, northern Japan, comprises metamorphic rocks up to granulite facies, along with intrusive and ultramafic rocks. It interprets an Eocene to Miocene island-arc-type crustal section that shallows towards the east. To the west of the HMB, the Poroshiri ophiolite extends approximately 70 km in length and <2 km in width, exposing a nearly complete oceanic crust-mantle section that shallows to the west. The HMB and Poroshiri ophiolites are bound by a thrust at the deepest lithologies.

The Uenzaru peridotite complex, the northernmost peridotite body along the thrust, is a steeply dipping sheet approximately 800 m wide. It lies between the metagabbro of the Poroshiri ophiolite and the pelitic granulites of the HMB. The western part mainly comprises harzburgite, exhibiting metamorphism with abundant amphiboles and complete absence of clinopyroxene. Veins (<1 m wide) penetrate the peridotite and are fully metamorphosed into amphibolite. The eastern part mainly consists of fresh spinel lherzolite and plagioclase lherzolite, along with pyroxenite and gabbro veins/bands, resembling lithologies found in the Horoman peridotite complex, the largest peridotite body in the HMB (Murota and Arai, 1988). Harzburgite, spinel lherzolite and plagioclase lherzolite exhibit different preferred orientations of olivine (Komatsu, 1975). Based on geological position, and the differences between the eastern and western parts, and the similarity of the eastern part to the Horoman complex, it is suggested that the western part belongs to the Poroshiri ophiolite and the eastern part to the HMB (Komatsu, 1975; Murota and Arai, 1988).

In this study, we investigate the petrological and geochemical characteristics of peridotites and their attributes and origins by revisiting the Uenzaru Complex. The sample at the eastern end comprises plagioclase-lherzolite, transitioning to spinel-lherzolite, and then to harzburgitic rock, where pyroxenes have completely transformed into amphiboles. Harzburgitic rocks occur at a point identified in previous studies as the boundary between the eastern and western regions. The compositional relationship between spinel Cr# and olivine Fo suggests that the eastern peridotites are petrogenetically related to the gabbro of the Poroshiri ophiolite. The western sample showed a wide range of spinel Cr# = 0.10-0.43, consistent with Horoman peridotites. Conversely, the Cr#–Fo relationship in the harzburgitic amphibolite at the boundary between the eastern and western parts was more similar to that of the eastern part. The relationship between MnO and Cr # in spinel showed a similar trend. The REE pattern of amphiboles across the area shows significantly low abundance and a leftward decreasing pattern in the western part, a spoon- or U-shaped pattern at the boundary to the eastern part, and relatively high abundance with an LREE-depleted pattern in the easternmost part. Comparing these patterns with those of the clinopyroxene, the eastern pattern aligns with that of the mafic cumulate (olivine Fo₈₉) of the Poroshiri ophiolite, while the boundary exhibits a similar spoon- or U-shaped pattern in the eastern sample. From the plagioclase lherzolite of the Horoman peridotite body, clinopyroxene with a relatively high content and an LREE-depleted pattern has been reported, whereas spoon- or U-shaped patterns have been reported for spinel lherzolite and harzburgite (Takazawa et al., 1992). Therefore, the trace elements of the amphiboles in the Uenzaru Complex reflect the REE pattern of clinopyroxene, indicating that the area east of the boundary belongs to the HMB.

The attribution of the eastern and western regions, previously based mainly on descriptive features, was confirmed by trace element mineral chemistry. The Uenzaru Complex represents a rare area where sub-oceanic and sub-arc mantle peridotite contact occurs in a continuous outcrop.

Figure 1. Rare earth element patterns for amphiboles and clinopyroxenes from the Uenzaru peridotites and reference samples.

References

Komatsu, M., 1975. Recrystallization of the high alumina pyroxene peridotite of the Uenzaru area in Hidaka Province, Hokkaido, Japan. Jour. Geol. Soc. Japan 81, 11–28. https://doi.org/10.5575/geosoc.81.11

Murota, Y., Arai, S., 1988. Petrological notes on deep-seated and related rocks (6) Petrological characteristics of primary peridotites from the Uenzaru complex, the Hidaka belt, Hokkaido, Japan. Ann. Rep., Inst. Geosci., Univ. Tsukuba 14, 64–68.

Ross, K., Elthon, D., 1993. Cumulates from strongly depleted mid-ocean-ridge basalt. Nature 365, 826–829. https://doi.org/10.1038/365826a0

Takazawa, E., Frey, F.A., Shimizu, N., Obata, M., Bodinier, J.L, 1992. Geochemical evidence for melt migration and reaction in the upper mantle. Nature 359, 55–58. https://doi.org/10.1038/359055a0

Molecular H in lithospheric mantle, a clue to interpret ∂D variations

Moine Bertrand N.¹, Bolfan-Casanova Nathalie¹, Dominique Jessy¹, Radu Ioana-Bogdana²

¹Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France ²Swedish Museum of Natural History | Box 50007, SE-104 05, Naturhistoriska riksmuseet, Stockholm, Sweden

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #9 Presenting author: Bertrand N. Moine, https://orcid.org/0000-0003-2780-2882

Hydrogen is a ubiquitous trace component of nominally anhydrous minerals (NAMs) in the upper mantle, estimated to amount to 1-7 times the total mass of the oceans. So far, hydrogen was known to exist in NAMs in the form of OH, with storage capacity varying with depth. However, studies of cratonic mantle xenoliths indicate that ƒO₂ ranges from FMQ -2 to -4.5 (Goncharov et al.2012), conditions under which a substantial amount of hydrogen may be present in a reduced form (H₂). We show that molecular hydrogen (H₂) coexists with OH in natural omphacite in eclogite xenoliths from the Siberian craton (3-4 GPa, 1000 °C), suggesting that H₂ plays a role in water recycling to deep mantle (Moine et al. 2020). The presence of H₂ can explain the extremely negative values due to the positive fractionation between fluids and H₂ and these counter-intuitive relationships can be explained by the coexistence of OH and H₂ in minerals. This better explains the relations between ∂D estimates for MORB-source convective upper mantle (about -50‰, Clog et al., 2013) and glass inclusions in phenocrysts from plume basalts (down to -218‰, Hallis et al., 2015) that were previously attributed to a primitive deep reservoir or to contamination by recycled partially dehydrated oceanic crust. Since molecular H₂ is most likely to be the dominant form of hydrogen in the reduced deep mantle, it follows that H isotopic fractionation should be controlled by equilibria involving H₂-bearing minerals rather than H₂O- or OH-bearing minerals.

Reference

Goncharov, A. G., Ionov, D. A., Doucet, L. S. & Pokhilenko, L. N. 2012. Thermal state, oxygen fugacity and C-O-H fluid speciation in cratonic lithospheric mantle: New data on peridotite xenoliths from the Udachnaya kimberlite, Siberia. Earth and Planetary Science Letters 357-358, 99-110. https://doi.org/10.1016/j.epsl.2012.09.016

Moine B.N., Bolfan-Casanova N., Radu I.B., Ionov D.A., Costin G., Korsakov A.V., Golovin A.V., Oleinikov O.B., Deloule E., Cottin J.Y. (2020) Molecular hydrogen in minerals as a clue to interpret ∂D variations in the mantle, Nature Communications 111 (art. Number 3604). https://doi.org/10.1038/s41467-020-17442-8

Clog, M., Aubaud, C., Cartigny, P. & Dosso, L. 2013. The hydrogen isotopic composition and water content of southern Pacific MORB: A reassessment of the D/H ratio of the depleted mantle reservoir. Earth and Planetary Science Letters 381, 156-165, https://doi.org/10.1016/j.epsl.2013.08.043

Hallis, L. J. et al. 2015. Evidence for primordial water in Earth’s deep mantle. Science 350, 795. https://doi.org/10.1126/science.aac4834

The composition and origin of sulfides in peridotites – insight from from Ruddon’s Point xenoliths (Fife, Scotland)

Hubert Mazurek¹, Magdalena Matusiak-Małek¹, Hannah S.R. Hughes², Brian J.G. Upton³

¹Institute of Geological Sciences, University of Wroclaw, Poland ²Camborne School of Mines, University of Exeter, United Kingdom ³School of GeoSciences, University of Edinburgh, United Kingdom

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #10 Presenting author: Hubert Mazurek, https://orcid.org/0000-0002-3003-0302

Sulfides are accessory minerals in the mantle peridotites, but they control the chalcophile, siderophile and platinum group elements (PGE) budget in lithospheric mantle (LM) rocks. Permian mafic volcanic rocks occurring in southern terrains of Scotland (United Kingdom) host a variety of LM xenoliths abundant in sulfides giving insight into metal migration through upper lithospheric mantle. A xenolith suite from Ruddon’s Point (Fife, Scotland) basalts contains four major textural groups of peridotites: (1) protogranular and (2) porphyroclastic lherzolites, (3) equigranular wehrlites and (4) lherzolites transitional between protogranular and equigranular peridotites. The melt-rock reaction of LM beneath S Scotland with alkaline melts resulted in clinopyroxene crystallization (wehrlitization) and decrease forsterite content (Fo) in olivine from primitive (protogranular and porphyroclastic) lherzolites (Fo_88.5–90.0) through transitional to equigranular (Fo_80.05–85.0) peridotites (Matusiak-Małek et al., 2022).

The sulfides in the peridotites occur as oval to elongated or irregular inclusions in olivine, ortho- and clinopyroxene or as interstices between these phases. Sulfide abundances increase from the transitional lherzolites (mean = 9 × 10-3 vol. ‰), through equigranular (2.6 × 10-2 vol. ‰) and porphyroclastic peridotites (2.9 × 10-2 vol. ‰) to protogranular lherzolites (5 × 10-2 vol. ‰). Phases composing sulfide grains present in all textural groups are represented by pentlandite (Pn) and chalcopyrite (Ccp). Protogranular and transitional peridotites contain also minor amounts of pyrrhotite (Po), whereas porphyroclastic lherzolites occasionally contain millerite (Mlr) and covellite (Cv). The sulfides from the equigranular and protogranular peridotites are more enriched in Cu-, and depleted in Ni-phases (Po₀Pn₇₁Ccp₂₉ and Po₄Pn₆₈Ccp₂₇, respectively) in comparison to sulfides from the porphyroclastic and transitional peridotites (Po₀Pn₈₀Ccp₂₀ and Po₆Pn₈₃Ccp₁₂, respectively). The Cu/(Cu+Fe) ratio is homogenous in sulfides of all the groups, whereas the Ni/(Ni+Fe) ratio is homogenous only in transitional and equigranular peridotites (0.64–0.65 and 0.55–0.59, respectively) in contrast to porphyroclastic and protogranular ones (0.54–0.68 and 0.52–0.64, respectively). There is no significant difference in trace element concentrations in sulfides, except lower Co and higher Zn (up to 4894 and 2214 ppm, respectively) in the protogranular lherzolites compared to transitional peridotites (30090 and 1391 ppm, respectively).

The more primitive protogranular and porphyroclastic lherzolites are characterized by the highest sulfide abundances in comparison to the sulfides from metasomatized equigranular wehrlites, with no major differences in sulfide mineral and chemical (major and trace elements) composition between lithologies. Therefore, activity of the alkaline silicate melts responsible for wehrlitization of the more primitive lherzolites has likely no influence on the sulfide enrichment in the LM beneath S Scotland. The presence of Cv and Mlr in porphyroclastic lherzolites implies later-stage alteration by hydrothermal, post-volcanic activity after eruption of basaltic lavas.

References

Matusiak-Małek, M., Kukuła, A., Matczuk, P., Puziewicz, J., Upton, B.J.G., Ntaflos, T., Aulbach, S., Grégoire, M., Hughes H.S.R., 2022. Evolution of upper mantle and lower crust beneath Southern Uplands and Midland Valley Terranes (S Scotland) as recorded by peridotitic and pyroxenitic xenoliths in alkaline mafic lavas. 4th EMAW TOULOUSE 2021 Book of Abstracts.

Combined δ¹⁸O-δ^44/40Ca Geochemistry of Eclogites from the Navajo Volcanic Field, Colorado Plateau, USA

Joshua Munro¹, Jaime D Barnes¹, John C Lassiter¹, Aaron M Satkoski¹

¹Department of Earth and Planetary Sciences, University of Texas, Austin, Texas 78712, USA.

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #11 Presenting author: Joshua Munro, https://orcid.org/0000-0003-0162-554X

The Navajo Volcanic Field (NVF) in the Colorado Plateau, USA, contains eclogite xenoliths associated with peridotite and garnetite xenoliths, all hosted within serpentinized ultramafic breccia diatremes. These diatremes erupted approximately 30 Ma, sampling portions of the altered lithospheric mantle and fragments of material transported from subduction zones to the west. These eclogites are generally accepted to represent fragments of subducted oceanic crust, either from subducted Proterozoic crust (Wendlandt et al., 1993; Smith et al., 2004) or the Cretaceous Farallon slab (Helmstaedt & Doig, 1975; Usui et al., 2003). Previous work has shown that δ¹⁸O and δ^44/40Ca values can be distinct tracers of recycled crustal material given that continental crustal material and altered oceanic crust have δ¹⁸O-δ⁴⁴Ca values that vary significantly from the mantle value (δ¹⁸O ≈ 5.5±0.2‰, Eiler (2001), and δ^44/40`Ca ≈ 0.95±0.05‰, Kang et al. (2019)). However, only a few studies have investigated the Ca isotope geochemistry of eclogites, and with the rising application of non-traditional isotopes, it is necessary to fully understand the effect of subduction and metasomatism on the Ca isotope system. Previous work was focused on the high-temperature (~900°C to 1300°C), Proterozoic eclogites of the Kaapvaal Craton (Chen et al., 2020; Smart et al., 2021) – hence, it is beneficial to investigate the contrasting low-temperature (~700°C to 800°C) eclogites of the Colorado Plateau.

Sixteen eclogites from the Garnet Ridge diatreme, NVF, were analysed for their δ18O and δ44/40Ca values (Figure). All δ18O values for mineral separates were above expected values in equilibrium within normal mantle peridotite (~5.5 to 6.0‰ for garnet, ~5.5 to 6.2‰ for pyroxene), varying from 6.6 to 9.8‰ for garnet (mean 8.0 ± 1.1‰, n = 16) and 6.6 to 9.7‰ for clinopyroxene (mostly omphacite, 7.8 ± 1.0‰, n = 16). Calculated whole-rock δ¹⁸O values are higher than mantle values, but within the altered oceanic crust range (Figure). Measured δ^44/40Ca values vary from 1.68 to 1.98‰ for garnet, 0.76 to 1.06‰ for clinopyroxene, and 0.71‰ for lawsonite. The ∆^44/40Ca grt-cpx values range from 0.71 to 0.92‰, some of the highest reported in the literature. Calculated whole-rock eclogite δ44/40Ca values range from 0.87 to 1.38‰, which overlap with the altered oceanic crust field, but are above the typical δ^44/40Ca value for average mid-ocean ridge basalt (~0.85‰).

The garnet δ¹⁸O and δ^44/40Ca values for NVF eclogites are higher than most values determined in previous work for Kaapvaal Craton eclogites (diamonds in Figure; Chen et al., 2020; Huang et al., 2022; Smart et al., 2021). In addition, the NVF garnet, clinopyroxene, and whole-rock all have positively correlated δ¹⁸O-δ^44/40Ca values, suggesting involvement of a high-δ¹⁸O, high-δ^44/40Ca component. Although the exact nature of this component is unclear, it is most likely that the δ¹⁸O-δ^44/40Ca correlation is from recycled surface material affected by low-temperature alteration processes.

The eclogite ∆^44/40Ca grt-cpx values in our work and literature are negatively correlated with estimated eclogite temperature. At typical mantle temperatures (>1200°C), the ∆^44/40Ca grt-cpx value is small (<0.2‰), whereas large fractionation factors are expected during low-temperature processes (e.g., ∆^44/40Ca grt-cpx ≈ 0.92‰ at 575°C). From this, it is possible that the large inter-mineral Ca-isotope fractionation in the NVF eclogites is due to the low-temperature of equilibration during low-angle subduction of the Farallon plate. Additionally, the exceptionally high garnet δ^44/40Ca values are due primarily to inter-mineral fractionation effects. Previous studies have used ab-initio predictions and numerical diffusion models to determine mineral δ^44/40Ca fractionation parameters as a function of temperature (Antonelli et al., 2019, and references therein; Chen et al., 2020). These models were tested against data for eclogite, pyroxenite, and peridotite garnet-clinopyroxene pairs, and shown to accurately predict real-world ∆^44/40Ca grt-cpx - temperature variation. This furthers the ability to reliably predict and interpret the temperature effect on Ca isotope fractionation in garnet-clinopyroxene pairs.

Figure 1. Plot showing δ¹⁸O-δ^44/40Ca isotope data for Navajo Volcanic Field eclogite garnet (open circles) and whole rock (filled circles), compared to eclogite garnets and whole-rock from the Kaapvaal Craton (open and filled triangles; Chen et al., 2020; Smart et al., 2021; Huang et al., 2022) and major relevant reservoirs (mantle, altered oceanic crust, fresh mid-ocean ridge basalt/MORB and sea water). NVF whole-rock data is calculated from mineral separate data. Whole-rock δ¹⁸O values are used for our samples, whereas only garnet δ¹⁸O values are available for Kaapvaal samples. Since the difference between whole-rock and garnet δ¹⁸O value is typically <0.2‰ at high temperatures, the values are comparable to our whole-rock δ¹⁸O values. Figure modified after Smart et al., (2021) and references therein.

References

Antonelli, M. A., Schiller, M., Schauble, E. A., Mittal, T., DePaolo, D. J., Chacko, T., Grew, E. S., & Tripoli, B. (2019). Kinetic and equilibrium Ca isotope effects in high-T rocks and minerals. Earth and Planetary Science Letters, 517, 71–82. https://doi.org/https://doi.org/10.1016/j.epsl.2019.04.013

Chen, C., Huang, J. X., Foley, S. F., Wang, Z., Moynier, F., Liu, Y., Dai, W., & Li, M. (2020). Compositional and pressure controls on calcium and magnesium isotope fractionation in magmatic systems. Geochimica et Cosmochimica Acta, 290, 257–270. https://doi.org/10.1016/j.gca.2020.09.006

Eiler, J. M. (2001). Oxygen Isotope Variations of Basaltic Lavas and Upper Mantle Rocks. Reviews in Mineralogy and Geochemistry, 43(1), 319–364. https://doi.org/10.2138/gsrmg.43.1.319

Helmstaedt, H., & Doig, R. (1975). Eclogite nodules from kimberlite pipes of the Colorado Plateau -samples of subducted Franciscan-type oceanic lithosphere. In L. H. Ahrens, J. B. Dawson, A. R. Duncan, & A. J. Erlank (Eds.), Physics and Chemistry of the Earth (pp. 95–111). Pergamon.

Huang, J., Huang, J. X., Griffin, W. L., & Huang, F. (2022). Zn-, Mg and O-isotope evidence for the origin of mantle eclogites from Roberts Victor kimberlite (Kaapvaal Craton, South Africa). Geology, 50(5), 593–597. https://doi.org/10.1130/G49780.1

Kang, J. T., Ionov, D. A., Zhu, H. L., Liu, F., Zhang, Z. F., Liu, Z., & Huang, F. (2019). Calcium isotope sources and fractionation during melt-rock interaction in the lithospheric mantle: Evidence from pyroxenites, wehrlites, and eclogites. Chemical Geology, 524, 272–282. https://doi.org/10.1016/j.chemgeo.2019.06.010

Smart, K. A., Tappe, S., Woodland, A. B., Greyling, D. R., Harris, C., & Gussone, N. (2021). Constraints on Archean crust recycling and the origin of mantle redox variability from the δ44/40Ca – δ18O – fO2 signatures of cratonic eclogites. Earth and Planetary Science Letters, 556. https://doi.org/10.1016/j.epsl.2020.116720

Smith, D., Connelly, J. N., Manser, K., Moser, D. E., Housh, T. B., McDowell, F. W., & Mack, L. E. (2004). Evolution of Navajo eclogites and hydration of the mantle wedge below the Colorado Plateau, southwestern United States. Geochemistry, Geophysics, Geosystems, 5(4). https://doi.org/10.1029/2003GC000675

Usui, T., Nakamura, E., Kobayashi, K., Maruyama, S., & Helmstaedt, H. (2003). Fate of the subducted Farallon plate inferred from eclogite xenoliths in the Colorado Plateau. Geology, 7, 589–592. https://doi.org/10.1130/0091-7613(2003)031%3C0589:FOTSFP%3E2.0.CO;2

Wendlandt, E., DePaolo, D. J., & Baldridge, S. W. (1993). Nd and Sr isotope chronostratigraphy of Colorado Plateau lithosphere: implications for magmatic and tectonic underplating of the continental crust. Earth and Planetary Science Letters, 116, 23–43. https://doi.org/10.1016/0012-821X(93)90043-9

Ancient, buoyant mantle under the Sierra Leone Ridge

Sani C.^1,2, Sanfilippo A.², Stracke A.¹, Genske F.¹, Ferrando C.³, Borghini G.⁴, Peyve A.⁵, & Ligi M.⁶

¹Institut für Mineralogie, Universität Münster, Corrensstraße 24, D-48149 Münster, Germany ²Dipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia, via Ferrata 1a, 27100 Pavia Italy ³Department of Earth, Environmental and Life Sciences, University of Genova, 16132 Genova, Italy ⁴Department of Earth Sciences "Ardito Desio", University of Milano, 20133 Milano, Italy ⁵Geological Institute, Russian Academy of Science, Pyzhevsky Iane 7, 119017 Moscow, Russia ⁶Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, via Gobetti 101, 40129 Bologna, Italy

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #12 Presenting author: Camilla Sani, camilla.san (at) uni-muenster (dot) de

The Sierra Leone ridge is the equatorial portion of the Mid Atlantic Ridge (MAR) between the St Paul Fracture Zone (0 - 1°N) in the south and the Doldrums fracture zone (7-9°N) in the north. At ca. 80 Ma two submarine plateaus, the Sierra Leone Rise and Cerera Rise in the African and South American plates were created during a period of excess melt production under the Sierra Leone ridge. Based on the occurrence of MORB with exceptionally enriched geochemical and Sr-Pb-Nd isotopic compositions (Schilling et al., 1994), it has been suggested that the excess melt production was caused by interaction of the Sierra Leone ridge with a nearby plume, the Sierra Leone mantle plume, which may now be centered at ~1.7° N along the MAR.

In this study we present major-trace element concentrations and Hf-Nd isotope ratios measured in abyssal peridotites and MORB from 0-9°N along the MAR. We show that high Hf isotope ratios are preserved in clinopyroxenes from these abyssal peridotites (ƐHf= 12-54), mirrored by comparatively high ƐHf in the associated erupted basalts (ƐHf= 13-19.5). These features show that the mantle under the Sierra Leone ridge has been melted extensively several 108-109 years before remelting under the present MAR. However, most of the peridotites retain low Ɛ_Nd and enrichments in LREE, indicating recent interaction with incompatible element enriched melts. Two samples do not show signs of recent melt-rock reaction, e.g. they have Lu/Hf up to 0.4. Yet, these samples have high Hf coupled with low Nd isotope ratios, locally extending to Ɛ_Nd of -1.5. These isotopic signatures show that some portion of the SL mantle experienced ancient melting but also concurrent reaction with incompatible element enriched melts. Based on our new MORB data we argue that this ancient, refertilized peridotite is the main source of basalts erupted in the region, in addition to melts from additional incompatible element enriched lithologies, probably from interspersed recycled crust.

We therefore suggest that the elevated topography and abundant magmatism that characterizes this portion of the MAR since 80 Ma might have been triggered by extensive remelting of peridotite that has melted extensively several 108-109 years ago, and thus became light and compositionally buoyant. The excess buoyancy of this melt-depleted peridotite has increased the rate of upwelling, and thus the amount of melt production per time under the MAR, and may thus be responsible for the formation of the Sierra Leone and Cereara Rise plateaus, as well as the shallow depth of the Sierra Leone ridge. Periods of excess melt production, and thus thick oceanic crust at mid-ocean ridges may thus be caused by changes in the compositional buoyancy of the passively upwelling sub-ridge mantle, which lead to periods of excess melt production and differences in the chemical and isotopic composition of the generated basalts without having to postulate interaction of the ridge with nearby mantle plumes.

References

Schilling, J.G., Hanan, B.B., McCully, B., Kingsley, R.H. and Fontignie, D., 1994. Influence of the Sierra Leone mantle plume on the equatorial Mid‐Atlantic Ridge: A Nd‐Sr‐Pb isotopic study. Journal of Geophysical Research: Solid Earth, 99(B6), pp.12005-12028. https://doi.org/10.1029/94JB00337

Origin of Moho-transition-zone and deep-seated dunites in the Troodos and UAE ophiolites

Zohar Segall¹, Bar Elisha¹, Itai Haviv¹, Mohammed Ali², Yaron Katzir¹*

¹Dept. of Earth and Environmental Sciences, Ben Gurion University of the Negev, Be’er Sheva, Israel ²Dept. of Earth Sciences, Khalifa University, Abu-Dhabi, United Arab Emirates

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #13 Presenting author: Yaron Katzir, https://orcid.org/0000-0003-1999-3746

Dunites, rocks composed of nearly pure olivine, are traditionally considered as cumulates, formed by settling and accumulation of olivine crystals from magma. An alternative mechanism to form a dunite is the dissolution of orthopyroxene by mantle-melt reaction during porous flow of basaltic melts through mantle harzburgites. This is highlighted by 1-to-10-meter wide dunite ‘channels’ within peridotite of the Wadi Tayin section of the Oman ophiolite, which track the pathways of ascending melts through the mantle (Kelemen et al., 1995). Nevertheless, voluminous dunites in the Oman ophiolite occur as hundreds of meters thick monotonous sequences at the bottom of the Moho Transition Zone (MTZ), e.g., the top of Masqad mantle diapir, comprising both ‘replacive’ and ‘cumulate’ dunites (Abily and Ceuleneer, 2013). In the Troodos ophiolite, Cyprus, dunite also occurs in variable modes: (i) 10-50 m wide and up to 500 m long parallel dunite bands, thought to represent reaction between percolating melts and the host tectonite peridotites (Batanova and Sobolev, 2000), and (ii) extensive dunitic provinces, hundreds of meters thick, at the transition between mantle peridotite and the lower crustal cumulates, considered historically as part of the cumulates that mark the petrologic Moho. Here we measure the chemical compositions of olivine and spinel in MTZ and deep-seated dunites from the Troodos ophiolite and compare them with MTZ dunites from Nahwa, UAE, at the top of the mantle section of the Oman ophiolite, to shed light on the origin of dunite at various mantle levels.

Deep seated and MTZ Troodos and Nahwa dunites are of high petrographic resemblance. They are partially to heavily serpentinized and mostly comprise medium grained (3-5 mm) sub-hexagonal to rounded olivine grains (≥95%) and opaque subhedral chromite (Cr-spinel, ±3%). Interstitial CPX with cuspate edges, reminiscent of melt percolation, occurs in a few samples. Olivine in Troodos MTZ dunites is magnesian (Mg#=0.89-0.91), and its NiO content is consistently high (0.15-0.4 wt%) and is positively correlated with Mg#. MnO content, however, decreases while Mg# increases. The correlative trends can be accounted for either by fractional crystallization of olivine from a melt or as residual olivine in peridotite after variable degrees of melt extraction. Olivine of the deep-seated Troodos dunites overlaps the high Mg# (0.90-0.91) - high NiO (0.35-0.45 wt%) edge of this trend, suggesting a replacive origin. The MTZ Nahwa olivine also shares this highly refractory character, but olivine of pyroxenite-veined dunites from Nahwa has much lower Mg#, ~0.86, indicating interaction with mafic melts. Most of the dunites studied here plot within the olivine-spinel mantle array (OSMA; Arai, 1994), indicating replacive origin, but some MTZ Troodos and Masqad dunites have lower olivine Mg#, suggesting either cumulative origin or later interaction with melts represented by pyroxenites veins.

Mineral compositions of dunites of the Troodos and UAE ophiolites reflect complex evolution affected by (1) partial melt extraction, (2) melt/rock ratio during interaction with ascending melts, and (3) the composition of ascending melts. The extent of partial melt extraction is best evaluated by spinel-Cr#, however in replacive dunites spinel composition is considered to reflect the composition of percolating melt. Dunites from Nahwa show lower spinel-Cr# and Mg# than neighbouring harzburgite, while deep seated dunites from Troodos show exactly the opposite. This can be accounted for by percolation of variable parental melts: relatively Mg-poor, Al- and Ti-rich tholeiitic melt vs. Mg-rich, Ti-poor parental boninitic melts, respectively (Pearce et al., 2000).

Figure 1. Spinel-Cr# [Cr/ (Cr+Al)] vs. (a) spinel-Mg# [Mg/(Mg+Fe)] and (b) spinel TiO2 content in dunites and host peridotites from the Troodos and Oman-UAE ophiolites. Modelled array of spinel composition during partial melting of lherzolite and composition of spinel in eruptive rocks of boninitic and tholeiitic magmas are from Pearce et al. (2000).

References

Abily, B., Ceuleneer, G., 2013. The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both? Geology 41, 67-70. https://doi.org/10.1130/G33351.1

Batanova, V.G., Sobolev, A.V., 2000. Compositional heterogeneity in subduction-related mantle peridotites, Troodos massif, Cyprus. Geology, 28 .55-58. https://doi.org/10.1130/0091-7613(2000)28%3C55:CHISMP%3E2.0.CO;2

Kelemen, P.B., Shimizu, N., Salters, V.J., 1995. Extraction of mid-ocean-ridge basalt from the upwelling mantle by focused flow of melt in dunite channels. Nature, 375, 747-753. https://doi.org/10.1038/375747a0

Pearce, J.A., Barker, P.F., Edwards, S.J., Parkinson, I.J., Leat, P.T., 2000. Geochemistry and tectonic significance of peridotites from the South Sandwich arc–basin system, South Atlantic. Contributions to Mineralogy and Petrology 139, 36-53. https://doi.org/10.1007/s004100050572

Composition of primary magmas from the Cézallier volcanic province, French Massif Central: insight from melt inclusions and implications for the nature of mantle sources

Laurine Barreau¹*, Didier Laporte¹, Nicolas Cluzel¹, Federica Schiavi¹, Jean-Luc Devidal¹

¹CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, Université Clermont Auvergne, F-63000 Clermont-Ferrand, France

When (1 min. presentation): Wednesday 2nd October at 12:40 Poster location: Panel #14 Presenting author: Laurine Barreau, https://orcid.org/0009-0002-2645-3019

Determining the composition of primary magmas at the origin of a volcanic province is important for obtaining information on the nature of its mantle source or on melting conditions. Melt inclusions are small droplets of magma trapped during the growth of the host crystal. The advantage of melt inclusions is that they retain the original composition of the liquid at the time of entrapment and may preserve the memory of deep processes that are difficult to access by analyzing bulk rocks at the surface. Thus, melt inclusions contained in the first crystals formed in magmas that are little or not differentiated are good candidates to approach the composition of primary magmas. A study of melt inclusions trapped in Mg-rich olivine crystals (forsterite contents in the range 83-89) was carried out on three volcanoes from the recent part of the Cézallier volcanic province in the French Massif Central: Sarran, Mazoires, and La Godivelle (erupted in the last 200 ka). The presence of mantle xenoliths (mostly spinel lherzolites) in the deposits indicates a rapid ascent of magmas from the mantle to the surface without prolonged storage at depth. The study of melt inclusions in the Cézallier province thus enables us to address the question of the genesis of silica-undersaturated alkaline magmas in the context of continental intraplate volcanism.

The major elements and the volatile elements (F, Cl, S) in the melt inclusions were analyzed with the electron microprobe. Raman spectroscopy was used to measure H₂O and CO₂ in silicate glasses and to characterize the CO₂-bearing phases in the shrinkage bubbles of the melt inclusions. Finally, the trace element patterns of the trapped glasses and their hosts crystals were analyzed using LA-ICP-MS.

The compositions of the glasses trapped in the melt inclusions plot in the field of basanites, basalts and trachy-basalts with SiO₂ contents ranging from 43.3 to 50.8 wt% and Na₂O+K₂O from 4.8 to 8 wt%. The glasses have particularly high CO₂ contents: up to 1.8 wt% dissolved CO₂. These values are minimum values, as CO₂ is also present as a fluid phase in the shrinkage bubble and as microcrystals of carbonates (Mg-calcite, nahcolite, ferromagnesite) that cover the bubble walls. These high CO₂ contents imply that the mantle at the origin of these magmas was enriched in carbon. The Cl/F ratio and the trace element patterns of the melt inclusions provide information on the melting processes and the nature of the mantle source of the Cézallier volcanic products. Various models have been proposed to explain the formation of silica-undersaturated alkaline magmas in continental intraplate settings. The new data obtained from the study of melt inclusions in the Cézallier volcanoes will be placed in the context of these models. In particular, we will attempt to assess the contribution of the metasomatized lithosphere and the asthenosphere to the production of primary magmas in the Cézallier volcanic province.

Source mineralogy during continental rifting

Emily Cunningham¹, Autumn Hartley¹, Sarah Lambart¹*, Pengyuan Guo², IODP Expedition 396 scientists³

¹University of Utah, Department of Geology and Geophysics, Salt Lake City, UT 84112, USA ²Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China ³IODP Expedition 396 science party.

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #15 Presenting author: Sarah Lambart, https://orcid.org/0000-0002-3636-7950

International Ocean Discovery Program Expedition 396 (Planke et al., 2023) sampled the continent-ocean transition of the Mid-Norwegian Margin, an area that produced extensive magmatism during the last break-up phase of Pangea. The presence of a mantle plume has been suggested to explain this excess magmatism. However, based on mantle potential temperatures obtained from basalt compositions collected during the expedition, a pure thermal anomaly appears insufficient to explain the excess magmatism.

Because the major element compositions of the basalt indicate that mafic lithologies/pyroxenites may have contributed to magma genesis in the area (Herzberg and Asimow, 2015; Yang et al., 2019), we developed a model to better constrain the nature of the mantle source. Lang and Lambart (2022) demonstrated that, once combined, ratios of first row transition elements in basalts can be powerful tracers for the mineralogy of the source. Our new model builds on this proof of concept and uses a Monte Carlo approach to determine the mineralogical assemblages capable of reproducing the following basalt elemental ratios: Mn/Fe, Zn/Fe, Mn/Zn, Sc/Zn, Ga/Sc. Those element ratios were chosen due to their high sensitivity to the source mineralogy and low sensitivity for fractionation processes.

When applied to basalts collected during Expedition 396, our model shows a clear change in the mineralogy of the source between sites considered to represent the initiation (U1566; olivine-rich) and peak (U1571 and U1572; clinopyroxene±garnet-rich) of rifting. These results could be consistent with an increase of the fraction of pyroxenite in the source. However, using a modified version of Melt-PX (Lambart et al., 2016), we find that even at a high mantle potential temperature, an active upwelling regime can only tolerate a small fraction of pyroxenite in the source before losing its buoyancy and ability to produce the excess magmatism. Hence, our findings may reflect cryptic mineralogical variations with time (Mourey et al., 2022). Alternatively, the remobilization of underplated basaltic material could also result in a geochemical signature consistent with the presence of pyroxenite in the source (Rooney et al., 2017). In this presentation, we will discuss the geodynamic implications of these findings.

References

Herzberg C., Asimow P. D., 2015. PRIMELT 3 MEGA. XLSM software for primary magma calculation: peridotite primary magma MgO contents from the liquidus to the solidus. Geochemistry, Geophysics, Geosystems, 16(2), 563-578. https://doi.org/10.1002/2014GC005631

Lang O. I., Lambart S., 2022. First-row transition elements in pyroxenites and peridotites: A promising tool for constraining mantle source mineralogy. Chemical Geology, 612, 121137. https://doi.org/10.1016/j.chemgeo.2022.121137

Lambart S., Baker M. B., Stolper E. M., 2016. The role of pyroxenite in basalt genesis: Melt‐PX, a melting parameterization for mantle pyroxenites between 0.9 and 5 GPa. Journal of Geophysical Research: Solid Earth, 121(8), 5708-5735. https://doi.org/10.1002/2015JB012762

Mourey A. J., Shea T., Lynn K. J., Lerner A. H., Lambart S., Costa F., Oalmann J., Lee R.L., Gansecki, C., 2022. Trace elements in olivine fingerprint the source of 2018 magmas and shed light on explosive-effusive eruption cycles at Kīlauea Volcano. Earth and Planetary Science Letters, 595, 117769. https://doi.org/10.1016/j.epsl.2022.117769

Planke S., Berndt C., Alvarez Zarikian C.A., & the Expedition 396 Scientists, 2023. Mid‐Norwegian Margin Magmatism and Paleoclimate Implications, Proceedings of the International Ocean Discovery Program (Vol. 396). https://doi.org/10.14379/iodp.proc.396.106.2023

Rooney T. O., Lavigne A., Svoboda C., Girard G., Yirgu G., Ayalew D., Kappelman J., 2017. The making of an underplate: Pyroxenites from the Ethiopian lithosphere. Chemical Geology, 455, 264-281. https://doi.org/10.1016/j.chemgeo.2016.09.011

Yang Z. F., Li, J., Jiang Q. B., Xu F., Guo S. Y., Li Y., Zhang J., 2019. Using major element logratios to recognize compositional patterns of basalt: Implications for source lithological and compositional heterogeneities. Journal of Geophysical Research: Solid Earth, 124(4), 3458-3490. https://doi.org/10.1029/2018JB016145

The heterogeneous subridge mantle of the Piedmont–Ligurian ocean: Mantle melting events from Triassic to Jurassic ages

Ferrando, Carlotta¹, Basch, Valentin², Borghini, Giulio³, Genske, Felix⁴, Stracke, Andreas⁴, Rampone, Elisabetta¹*

¹Department of Earth, Environmental and Life Sciences, University of Genova, 16132, Genova, Italy. ²Department of Earth and Environmental Sciences, University of Pavia, 27100, Pavia, Italy. ³Department of Earth Sciences "Ardito Desio", University of Milano, 20133, Milano, Italy. ⁴Institut für Mineralogie, Universität Münster, Germany.

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #16 Presenting author: Elisabetta Rampone

Geochemical constraints on abyssal and ophiolitic peridotites merge to the consensus of an extremely heterogeneous composition of the Earth’s subridge mantle, which generate melts that form the overlying oceanic crust. Remarkable isotopic disequilibrium has been documented between mantle peridotites and associated magmatic crust, questioning their genetic relationship. Likewise, mantle-crust isotopic disequilibrium has been reported in the Alpine-Apennine ophiolites, which record the entire oceanic evolution and ultra-slow seafloor spreading of the Jurassic Piedmont–Ligurian Ocean (PLO). Peridotite-gabbro associations from Lower Platta, Civrari, Chenaillet, Lanzo South, Internal Liguride (IL) and Monte Maggiore (MM) ophiolites have been inferred to represent more internal oceanic settings composed of an isotopically heterogeneous subridge mantle. Model ages of the most depleted domains yield Permian mantle melting, whereas isochrones from the least depleted domains document middle Jurassic ages of the last melting and melt migration events (Rampone and Hofmann, 2012; Rampone and Sanfilippo, 2021). However, Permian model ages rely on the composition of the Depleted Mantle (DM) source and no errorchrones are available to date. Overall, the lack of internally consistent iso- or errorchrones for many PLO ophiolitic domains impedes to determine reliable equilibrium ages and, therefore, to decipher the nature of the mantle heterogeneity and the timing of melting events.

Here, we determined Hf and new Nd isotopic compositions of clinopyroxene and orthopyroxene separates from mantle peridotites in two internal PLO domains to better understand the evolution of the oceanic lithosphere and the nature of the mantle during Jurassic seafloor spreading. We selected mantle rocks exposed in the (i) MM ophiolite, as proxy of true Jurassic oceanic lithosphere showing isotopic equilibrium between peridotite and gabbroic rocks (Rampone et al. 2009), and (ii) IL ophiolite, as the most isotopically depleted endmember of the PLO subridge mantle, interpreted as evidence of Permian depletion events, in disequilibrium with the associated Jurassic crust. MM and IL peridotites preserve petro-geochemical evidence of mantle melting (strong LREE depletion). At MM we also investigated pyroxenite layers, records of deep melt segregation processes (Basch et al., 2019); both pyroxenites and host peridotites were subsequently affected by melt migration processes from spinel- to plagioclase-facies conditions.

MM and IL mantle rocks plot along the OIB-MORB array in the ƐHf-ƐNd space, supporting the heterogeneous nature of the oceanic mantle of the PLO, ranging from DM-like (i.e., MM with ƐHfi(160 Ma) ≈ 13 and ƐNdi(160 Ma) ≈ 8) to more depleted isotopic compositions (i.e., IL with ƐHfi(160 Ma) ≈ 20 and ƐNdi(160 Ma) ≈ 12.5). All clinopyroxenes and orthopyroxenes from MM samples align at Jurassic errorchrones (from ~145 Ma to ~170 Ma), confirming previous data. Pyroxenites align along the same errorchron as the country peridotites for Hf isotopic compositions. The Nd isotopic compositions of plagioclase-pyroxenites are coherent with all peridotites suggesting complete re-equilibration with migrating melts, whereas the spinel-pyroxenites align subparallel to all other mantle samples but at higher Nd isotopic compositions. This suggests that pyroxenites initially formed from melts with a relatively more depleted isotopic signature than later migrating reactive melts. The IL mantle peridotites have more depleted isotopic signature (i.e., higher Hf-Nd isotopic compositions) and define errorchrones at relatively older ages, from Middle Triassic (~245 Ma) to lower Jurassic (~190 Ma), in contrast with the previously estimated Permian model ages of depletion, computed considering a “normal” DM mantle source. Notably, the Nd isotopic composition of the MM spinel pyroxenites overlap with the IL mantle rocks, suggesting a genetic correlation between pyroxenite-forming melts and IL mantle, in terms of isotopic signature of the DM source.

Analyses of both clinopyroxene and orthopyroxene separates have proven to be crucial in estimating reliable equilibrium ages, revealing that PLO subridge melting events likely initiated in Triassic ages, at expenses of isotopically heterogeneous asthenospheric mantle sources. The variation in isotopic signature among the analyzed samples suggests that at least two major isotopically distinct DM domains composed the PLO subridge mantle. The higher Hf-Nd isotopic composition of the IL rocks indicates that ‘highly’ depleted mantle persisted beneath the ridge, likely derived from past depletion events.

Figure 1 – ¹⁴⁷Sm/¹⁴⁴Nd vs ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Lu/¹⁷⁷Hf vs ¹⁷⁶Hf/¹⁷⁷Hf of clinopyroxene and orthopyroxene separates from MM and IL mantle rocks compared with literature data on other Alpine-Apennine Ophiolites and abyssal peridotites (compilation by Tilhac et al., 2022; McCarthy and Müntener, 2015; Sani et al., 2023). Previous Nd isotopic data on MM mantle rocks are from Rampone et al. (2009).

References

Basch V., Rampone E., Borghini G., Ferrando C., Zanetti A., 2019. Origin of pyroxenites in the oceanic mantle and their implications on the reactive percolation of depleted melts. Contributions to Mineralogy and Petrology, 174: 97. https://doi.org/10.1007/s00410-019-1640-0

Rampone, E., Hofmann, A. W., Raczek, I., 2009. Isotopic equilibrium between mantle peridotite and melt: Evidence from the Corsica ophiolite. Earth Planet. Sci. Lett. 28, 601–610. https://doi.org/10.1016/j.epsl.2009.10.024

Rampone, E., Hofmann, A.W., 2012. A global overview of isotopic heterogeneities in the oceanic mantle. Lithos, 148, 247–261. https://doi.org/10.1016/j.lithos.2012.06.018

Rampone, E., Sanfilippo, A., 2021. The heterogeneous Tethyan oceanic lithosphere of the Alpine Ophiolite. Elements, 17, 23–28. https://doi.org/10.2138/gselements.17.1.23

Compositional variability of the oceanic crust as a function of spreading rate: insights from the ultra-slow spreading Gakkel Ridge (Arctic Ocean)

Scarani, S.¹, Sanfilippo, A.¹, Basch, V.¹, Genske, F.², Stracke, A.²

¹Department of Earth and Environmental Sciences, University of Pavia, Via Ferrata 1, Pavia, Italy ²Institut für Mineralogie, Universität Münster, Germany

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #17 Presenting author: Sarah Scarani, sarah.scarani01 (at) universitadipavia (dot) it

Gakkel Ridge is the slowest spreading ridge on Earth and extends over ~1.800 km in the Artic Basin. It features a long ridge segment without transform faults, with a full spreading rate ranging from 14.6 mm.yr-1 near the Lena Trough in the west to 6 mm.yr-1 towards the active rifting in the Laptev Sea in the east (Michael et al., 2003).

Bathymetric and gravity data display considerable variation in crustal thickness and basalt chemistry (Yang et al 2024), suggesting the occurrence of three distinct tectono-magmatic provinces along its length: i) the Western Volcanic Zone (WVZ), ii) the central Sparsely Magmatic Zone (SMZ), and iii) the Eastern Volcanic Zone (EVZ).

Extensive sampling during the AMORE expedition in 2001 (HLY0102 and PS59) recovered gabbros from all these provinces, therefore allowing a detailed assessment of the composition of the lower oceanic crust at the slowest spreading ridge on Earth. In this contribution we present petrological data of gabbroic lithotypes from the entire spreading sections.

The gabbros from the SMZ range from troctolites to oxide gabbros and felsic veins, while in the WVZ and EVZ gabbros are characterized by lack of both the most evolved and most primitive lithologies. Important compositional and textural differences characterize the lower gabbroic crust from the WVZ, SMZ and EVZ: i) Fe-Ti oxides and Ti-pargasite are ubiquitous in gabbros from the SMZ, even in the most primitive lithologies such as troctolites, and nearly absent in WVZ gabbros; ii) olivine crystals are rounded granular in the SMZ and interstitial in the WVZ; iii) the preferred orientation of minerals is evident in troctolites and olivine gabbros from the SMZ, but only for olivine in WVZ; iv) vermicular orthopyroxenes are present only in the WVZ primitive lithologies.

Along with these textural characteristics, we highlight differences in mineral chemistry between WVZ, SMZ and EVZ: i) plagioclase from SMZ and EVZ has lower Anorthite (An), lower FeO and higher K2O contents, while clinopyroxene has higher Na₂O contents, with respect to WVZ; ii) there is a covariation in clinopyroxene Mg-numbers (Cpx Mg#) and plagioclase An contents, with lower values of Plg An# in the SMZ and EVZ gabbros relative to those in WVZ, at a given Cpx Mg#; iii) clinopyroxene in primitive gabbros from the SMZ and EVZ has low Ti2O contents, suggesting early saturation of Ti-Fe oxides. Comparison with gabbros from other mid-ocean ridges with different spreading rates reveals that the lower oceanic crust at the Gakkel Ridge is compositionally distinct from that at fast-spreading ridges (i.e., East Pacific Rise and Pito Deep) and from the slow-spreading Mid-Atlantic Ridge, but similar to those of the ultra-slow Southwest Indian Ridge. The intrinsic compositional variability of gabbros from Gakkel Ridge, however, exceeds that of gabbros from the Southwest Indian Ridge, suggesting that not only the spreading rate but also the composition of the mantle source plays a fundamental role in determining the chemistry of the lower oceanic crust.

References

Michael, P. J., Langmuir C. H., Dick, H. J. B., Snow J. E., Goldstein S. L., Graham, D. W., Lehnertk,K., Kurras, G., Jokatq, W., Mühe R., & Edmonds, H. N., 2003. Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge, Arctic Ocean. Nature 423, https://doi.org/10.1038/nature01704.

Yang, A.Y., Langmuir, C.H., Michael, P. J., 2024. The significance of recycled oceanic mantle lithosphere beneath the Arctic Gakkel Ridge. Earth and Planetary Science Letters 626, 118553, https://doi.org/10.1016/j.epsl.2023.118553.

Nature of the mantle-crust transition in the Finero Complex (Ivrea-Verbano Zone, Southern Alps)

Maria Lopez^1,2, Alberto Zanetti^2,1, Andréa Tommasi³, Antonio Langone¹, José Alberto Padrón-Navarta⁴, Alain Vauchez³, Michele Matteo Cosimo Carafa⁵

¹Department of Earth and Environmental Sciences (DSTA), University of Pavia, Via Ferrata 1, I-27100 Pavia, Italy ²Institute of Geosciences and Earth Resources (IGG), Italian National Research Council (CNR), Via Ferrata 1, 27100 Pavia, Italy ³Geosciences Montpellier, CNRS, Place Eugène Bataillon 34095 Montpellier Cedex05, France ⁴Instituto Andaluz de Ciencias de la Instituto Andaluz de Ciencias de la Tierra (IACT), Avenida de las Palmeras 4, 18100 Armilla, Granada, Spain ⁵Istituto Nazionale di Geofisica e Vulcanologia, Via Arcivescovado 8, 67100 L'Aquila, Italy

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #18 Presenting author: María Camila López, https://orcid.org/0009-0003-9140-5336

The Finero Complex (northern Ivrea-Verbano Zone; Southern Alps) has well-exposed contacts between rocks of the upper mantle and lower continental crust. The core of the Complex is composed by the Finero Phlogopite Peridotite (FPP) mantle unit, which is wrapped out by an intercalation of mafic-ultramafic crustal rocks. The first crustal unit in contact with the FPP is the Layered Internal Zone (LIZ), overlaid by the Amphibole Peridotite and the External Gabbro units. In order to characterize the nature of this transition, a detailed investigation has been performed on the outcrop at the confluence between Rio Cannobino and Rio Creves.

The mantle unit mostly comprises secondary coarse-granular harzburgite, including phlogopite and amphibole, showing a pervasive foliation parallel to the mantle/crust contact. This latter is apparently magmatic. As the contact is approached, the size of the olivine grains decreases, and the peridotite composition gets enriched in orthopyroxene, phlogopite and amphibole.

The mantle side of the contact is predominantly characterized by a layer, up to 1 m thick, of weakly-deformed coarse-granular amphibole-biotite-bearing orthopyroxenite, with the strike roughly parallel to the foliation. Subordinately, the phlogopite harzburgite is directly in contact with the LIZ. The crustal side consists of a layered series of garnet-amphibole-bearing gabbroic rocks, which are intruded by concordant to discordant pegmatoidal hornblendite layers and pockets, sometimes with interstitial plagioclase. In places, hornblendite layers reach the contact with the FPP, reacting with the orthopyroxenite layer.

In this area, the FPP is chemically heterogeneous, evidencing the migration of multiple LILE-enriched melts. The amphibole chemistry of the mantle unit meters away from the contact is characterized by large Mg# and Ba, Rb, Cr, Sc and V contents, large and linearly fractionated LREE(PM) values, and low Nb, Ta and HREE. On the other hand, the amphiboles from the orthopyroxenite and the host harzburgite are more enriched in Th, U, Fe, Al and LREE and depleted in Cr, Sc, and V.

The garnet-amphibole-bearing gabbros from the crustal LIZ exhibit markedly different geochemical features, involving a pronounced depletion in highly-incompatible elements, such as LREE, in both amphibole and clinopyroxene, alongside a large positive Eu anomaly in the normalized pattern, which is also shown by the associate garnets.

Hornblendites are composed by titanian pargasites and are richer in Fe, Al, and Ti than in the FPP and garnet-amphibole-bearing gabbro. Furthermore, they show peculiar L-MREE-enriched convex-upward normalized patterns and ubiquitous enrichment in Nb and Ta.

The observation that the mantle harzburgite and the orthopyroxenite at the contact show mineralogical (i.e., segregation of phlogopite, amphibole and orthopyroxene) and geochemical (i.e., LILE and LREE enrichments, Nb, Ta and HREE depletion) features consistent with the rest of FPP (Zanetti et al., 1999), suggests that they formed during the Paleozoic pervasive melt migration events, which assisted the deformation and migration of FPP at mantle depths.

The garnet-amphibole-bearing gabbros show hybrid compositions imparted by multiple injections of mantle-derived LILE/LREE-depleted melts that assimilated previous plagioclase-bearing cumulates. The injection of such melts is not recorded by the FPP, but presumably took place when the mantle sequence was already emplaced at Moho depths.

The hornblendites provide evidence of the relatively late migration of mantle melts with alkaline affinity, which took place in the Southern Alps during the Upper Triassic-Lower Jurassic, being recorded in the IVZ by both mantle and lower continental crust (Ogunyele et al., 2024).

Thus, the combination of the structural and petrochemical features suggests that the rocks at the FPP-LIZ transition have acted as a primary, multistage, lithospheric discontinuity, representing a preferential zone of channeling for uprising mantle melts over a very large time interval, presumably from the end of Variscan orogeny to Lower Jurassic.

Figure 1. Trace element concentrations normalized to the Primitive Mantle (McDonough and Sun, 1995) for the orthopyroxenite and three harzburgite units located at different distances from the contact (A: 3.5 m, B: 2.2 m, C: 50 cm).

References

McDonough, W.F., Sun, S. -s., 1995. The composition of the Earth. Chemical Geology 120, 223–253. https://doi.org/10.1016/0009-2541(94)00140-4

Ogunyele, Abimbola C., et al. «Transition from orogenic-like to anorogenic magmatism in the Southern Alps during the Early Mesozoic: Evidence from elemental and Nd-Sr-Hf-Pb isotope geochemistry of alkali-rich dykes from the Finero Phlogopite Peridotite, Ivrea–Verbano Zone». Gondwana Research, vol. 129, pp. 201-19. ScienceDirect, https://doi.org/10.1016/j.gr.2023.12.011

Zanetti, Alberto, et al. «The Finero Phlogopite-Peridotite Massif: An Example of Subduction-Related Metasomatism». Contributions to Mineralogy and Petrology, vol. 134, n.o 2 pp. 107-22. Springer Link, https://doi.org/10.1007/s004100050472

Tectono-thermal evolution of Luqu ultramafic rocks in the Xigaze ophiolites, Tibetan Plateau: Implications for oceanic lithosphere accretion at slow-to-ultraslow spreading ridges

Yi Cao¹, Jian Ma¹, Junfeng Zhang¹, Chuanzhou Liu^2,3, Qing Xiong¹, Zhaoyi Dai¹, Jinxue Du⁴

¹State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China ²Laoshan Laboratory, Qingdao, 266061, China ³State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China ⁴School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #19 Presenting author: Yi Cao, https://orcid.org/0000-0002-1825-0097

Presumed remnants of the Neo-Tethyan oceanic mantle developed at slow-to-ultraslow spreading ridges, the Luqu ultramafic rocks from the Xigaze ophiolites in the Yarlung-Tsangbo Suture Zone, southern Tibet, provide an excellent example to elucidate the dynamics and evolution of sub-ridge mantle at this tectonic setting. Through a comprehensive investigation on the mineral major- and trace-element compositions and deformation microstructures of representative harzburgites and their pyroxenite veins, we unveil intriguing insights into the genesis and evolution of these rocks.

Our findings reveal extreme depletion of incompatible elements and enrichment of compatible elements in the major constituent minerals of harzburgite hosts, combined with very low water contents in orthopyroxene (~18‒38 wt. ppm) and low oxygen fugacity (ΔFMQ ~ -0.75‒0.26). These results are consistent with high-degree (~13‒18%) anhydrous partial melting in a shallow (spinel-facies) and reduced sub-ridge mantle. Pyroxenite veins, characterized by indistinguishable mineral compositions from their harzburgite hosts, are probably crystallized silica-rich melts derived from the asthenosphere after interaction and equilibration with harzburgite hosts at the lithosphere‒asthenosphere boundary (LAB) beneath a mid-ocean ridge.

Microstructural characteristics, such as porphyroclastic texture, intracrystalline plasticities (e.g., undulose extinctions, subgrain boundaries, kink bands, and bended exsolution lamellae), curved boundaries between neighboring olivine grains, and correlated crystallographic preferred orientations for olivine (A-/D-type), orthopyroxene (type-AC) and clinopyroxene, are indicative of high-temperature (~1000‒1200 °C) and low-pressure (~3 kbar) plastic deformation via dislocation creep. This deformation event was inferred to occur mainly postdating the silica-rich melt‒rock interaction and accompanying with a relatively slow cooling rate from >1200 °C to ~1000 °C (~8.5×10^-4 °C/y), owing to strong sub-horizontal basal shearing and conductive thermal transfer at the sub-ridge LAB.

In addition, the presence of ubiquitous hydrous minerals (e.g., tremolite, talc and serpentine) within the fractures and along grain boundaries indicates low-temperature (<800 °C) brittle deformation, likely associated with the development of oceanic detachment faults typical of slow-to-ultraslow spreading ridges. Owing to the efficient hydrothermal convection, seawater infiltration or percolation along the detachment faults was responsible for the genesis of hydrous minerals and a fast cooling rate from ~1000 °C to <700 °C (~1.4×10^-1 °C/y) as witnessed by the harzburgite hosts and pyroxenite veins. Our proposed tectono-thermal model illustrates the evolution of the Luqu ultramafic rocks, from their formation in the sub-ridge mantle at a Neo-Tethyan slow-to-ultraslow spreading ridge to exhumation at shallow depths below the seafloor, with oceanic detachment faults played an important role in this process.

Figure 1. Schematic cartoons illustrating the tectono-thermal evolution of oceanic lithosphere at a slow-to-ultraslow spreading center. A cross-section perpendicular to the spreading ridge or parallel to the plate motion direction (yellow arrows) is shown (right panel). The lithosphere‒asthenosphere boundary (LAB) is roughly represented by 1100 °C isotherm (Niu and Green, 2018), where pervasive melt‒rock interaction and intense basal shearing are supposed to occur due to infiltration of ascending melts (black wavy arrows) and asthenospheric mantle flow (black straight arrows), respectively [modified after Cuffaro et al. (2019) and Liu et al. (2019)]. Red stars indicate the localities of the studied samples or their protoliths experienced in sequence: (a) ascending in the asthenosphere, (b) accretion to the LAB, (c) unroofing by detachment faults, and (d) exhumation to the near below seafloor depth. Two intermediate stages (b) and (c) with sketches of the studied samples are shown for details (see inset and left panel). Note that the horizontal and vertical scales are not the same and that the spaces between individual stages (a‒d) are exaggerated horizontally and should not be related to the actual scale. Left panel depicts the details of stage (c), in which normal or detachment faults guided seawater infiltration (blue wavy arrows) and associated hydration (color-shaded domains). The locality of the studied samples before exhumation (red star) and their appearance (see inset) are highlighted [modified after Bickert et al.(2023)].

References

Bickert, M., Cannat, M., Brunelli, D., 2023. Hydrous fluids down to the semi-brittle root zone of detachment faults in nearly amagmatic ultra-slow spreading ridges. Lithos 442-443, 107084. https://doi.org/10.1016/j.lithos.2023.107084

Cuffaro, M., Miglio, E., Penati, M., Viganò, M., 2019. Mantle thermal structure at northern Mid-Atlantic Ridge from improved numerical methods and boundary conditions. Geophysical Journal International 220, 1128-1148. https://doi.org/10.1093/gji/ggz488

Liu, S., Tommasi, A., Vauchez, A., Mazzucchelli, M., 2019. Deformation, Annealing, Melt‐Rock Interaction, and Seismic Properties of an Old Domain of the Equatorial Atlantic Lithospheric Mantle. Tectonics 38, 1164–1188. https://doi.org/10.1029/2018TC005373.

Niu, Y., Green, D.H., 2018. The petrological control on the lithosphere-asthenosphere boundary (LAB) beneath ocean basins. Earth-science Reviews 185, 301-307. https://doi.org/10.1016/j.earscirev.2018.06.011

Diversification in Olivine Crystallographic Preferred Orientation within Mantle Lithosphere: An Example from the Horoman Peridotite Complex

Kazuki Matsuyama¹, Katsuyoshi Michibayashi^1,2

¹Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan ²Volcanoes and Earth’s Interior Research Center, Research Institute for Marine Geodynamics, JAMSTEC, Yokosuka 237-0061, Japan, Address

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #20 Presenting author: Kazuki Matsuyama, https://orcid.org/0009-0006-1614-9151

The primary objective of this study was to examine the development of various olivine crystal-fabrics in the Horoman peridotite complex. The Horoman peridotite complex, located on the southern edge of Hokkaido Island, exhibits various deformation microstructures and crystal-fabrics (e.g., Sawaguchi, 2004; Matsuyama and Michibayashi, 2023). The complex forms a gently east-dipping synformed layering structure, which total thickness is estimated to be 3700 m (Sawaguchi, 2004). The layered structure is mainly composed of peridotites, pyroxenites, and other mafic rocks of various compositions and thicknesses (Niida, 1974). The Pressure-Temperature-deformation-time trajectory and ascent history have been discussed (e.g., Takazawa et al. 2000; Ozawa, 2004). In particular, olivine crystal-fabrics were changed gradually from the southern to the northern (from the geological lower to upper) parts (Matsuyama and Michibayashi, 2023).

In this study, over 50 orientated peridotite samples were collected, and the olivine crystallographic orientations were measured using the SEM-EBSD method. The dataset was statistically analyzed and quantified as crystallographic preferred orientations (CPOs). Then, shear senses preserved within the peridotite were reconstructed using the oblique relationship between the shear plane and the major axis of a strain ellipse in non-coaxial shear.

Observations with a polarized microscope revealed various microstructures in the peridotite samples, including mylonitic, porphyroclastic, and granular textures. These microstructures were linked to mineral assembly, with harzburgite and spinel-bearing lherzolites typically exhibiting mylonitic to porphyroclastic textures, whereas plagioclase-bearing lherzolites showed granular textures. Four olivine CPO types were quantitatively identified by the Vp Flinn diagram method (Michibayashi et al., 2016) and distributed as E, A, D, and AG type in a south-to-north order (from the bottom to the top of the complex) (Matsuyama and Michibayashi, 2023). The E type was commonly found in peridotites showing mylonitic and porphyroclastic textures, whereas the AG type was more commonly associated with granular textures. The reconstructed shear senses were correlated with differences in CPOs, as the E, A and AG types display a north-south orientation, a westward orientation, and a southward orientation, respectively.

Previous experimental studies have demonstrated the E type CPOs under hydrous conditions (e.g., Katayama et al., 2004; Karato et al., 2008). Additionally, magnetotelluric observational surveys in southern Hokkaido revealed the presence of slab-derived fluids along the Hidaka Main Thrust (Ichihara et al., 2016). These findings suggest that the E type CPO in the Horoman peridotite complex is a result of water infiltration within the lithosphere associated with the thrust movement during uplifting. Since A type CPO is commonly found in natural peridotites (e.g., Mainprice, 2007; Michibayashi et al., 2016), A type CPOs in this study is also regarded as a typical CPOs in the upper mantle. The westward shear senses preserved within A type peridotite are consistent with the westward movement of the Paleo-Kuril arc (Kiminami, 2010). Therefore, it suggests that the development of A type CPOs was accommodated with mantle flow during the Late Cretaceous–Palaeogene period.

Figure 1. (a) The index map of Hokkaido Island (the northernmost island of Jaapan). (b, c, d) Olivine CPO distribution in the Horoman peridotite complex modified after Matsuyama and Michibayashi (2023). Diamond and circle symbols show the data from this study and Sawaguchi (2004), respectively. Colors in symbols referred the differences in olivine CPO patterns. (e) The Vp Flinn diagram for olivine CPO identification modified after Matsuyama and Michibayashi (2023). The method was originally made by Michibayashi et al. (2016). (f) The schematic art representing the structure beneath the central Hokkaido. The basic underground structure was inferred by Kita et al. (2014) using seismic reflect surveys dataset.

References

Ichihara, H., Mogi, T., Tanimoto, K., Yamaya, Y., Hashimoto, T., Uyeshima, M., Ogawa, Y., 2016. Crustal structure and fluid distribution beneath the southern part of the Hidaka collision zone revealed by 3-D electrical resistivity modeling. Geochemistry, Geophysics, Geosystems, 17 (4), 1480–1491. https://doi.org/10.1002/2015GC006222

Karato, S., Jung, H., Katayama, I., Skemer, P., 2008. Geodynamic significance of seis-mic anisotropy of the upper mantle: New insights from laboratory studies. Annual Review of Earth and Planetary Sciences 36, 59–95. https://doi.org/10.1146/annurev.earth.36.031207.124120

Katayama, I., Jung, H., Karato, S.-I., 2004. New type of olivine fabric at modest water content and low stress. Geology, 32 (12), 1045–1048. https://doi.org/10.1130/G20805.1

Kawakami, G., Hidaka Mountain Range Frontier. In The Geological Society of Japan (Eds.) Geological Magazine of Japan (1) Hokkaido Region. Asakura Publishing Co., Ltd. pp.530–532 (in Japanese).

Kiminami, K., 2010. Late Cretaceous-Paleogene convergence zone. In The Geological Society of Japan (Eds.) Geological Magazine of Japan (1) Hokkaido Region. Asakura Publishing Co., Ltd. pp.526–528 (in Japanese).

Mainprice, D., 2007. Seismic anisotropy of the deep Earth from a mineral and rock physics perspective. G. Schubert (Ed.), Treatise in Geophysics, vol. 2, Elsevier, Oxford, UK, 437–492.

Matsuyama, K., Michibayashi, K., 2023. Variation in olivine crystal-fabrics and their seismic anisotropies in the Horoman peridotite complex, Hokkaido, Japan. Journal of Geodynamics, 158, 102006. https://doi.org/10.1016/j.jog.2023.102006

Michibayashi, K., Mainprice, D., Fujii, A., Uehara, S., Shinkai, Y., Kondo, Y., Ohara, Y., Ishii, T., Fryer, P., Bloomer, S.H., Ishiwatari, A., Hawkins, J.W., Ji, S., 2016. Natural olivine crystal-fabrics in the western Pacific convergence region: a new method to identify fabric type. Earth and Planetary Science Letters, 443, 70–80. https://doi.org/10.1016/j.epsl.2016.03.019

Niida, K., 1974. Structure of the Horoman ultramafic massifs of the Hidaka metamorphic belt in Hokkaido, Japan. The Journal of the Geological Society of Japan. 80, 31–44. https://doi.org/10.5575/geosoc.80.31

Ozawa, K., 2004. Thermal history of the Horoman peridotite complex: a record of thermal perturbation in the lithospheric mantle. Journal of Petrology, 45 (2), 253–273. https://doi.org/10.1093/petrology/egg110

Sawaguchi, T., 2004. Deformation history and exhumation process of the Horoman Peridotite Complex, Hokkaido, Japan. Tectonophysics 379 (1–4), 109–126. https://doi.org/10.1016/j.tecto.2003.10.011

Takazawa, E., Frey, F.A., Shimizu, N., Obata, M., 2000. Whole rock compositional variations in an upper mantle peridotite (Horoman, Hokkaido, Japan): are they consistent with a partial melting process? Geochimica et Cosmochimica Acta, 64 (4), 695–716. https://doi.org/10.1016/S0016-7037(99)00346-4

Weak C-type olivine fabric in Nanshanling peridotites from the Dabie ultrahigh-pressure metamorphic belt

Xin-Mao Yang¹, Yong-Feng Wang¹*, Jun-Feng Zhang^1,2, Yang Gao¹, Ke-Qing Zong¹, Yi Cao¹, Qing Xiong², Hai-Jun Xu¹

¹School of Earth Sciences, China University of Geosciences, Wuhan, China ²State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #21 Presenting author: Yong-Feng Wang, https://orcid.org/0000-0003-2236-7893

It is generally believed that the crystallographic preferred orientation (CPO), also known as fabric, generated by the oriented arrangement of olivine during plastic deformation, is the primary cause of seismic anisotropy in the upper mantle. As a result, olivine fabric holds significant geophysical implications. Previous studies, based on extensive experimentation and natural observations, have revealed six main types of olivine fabric: A, B, C, D, E, and AG. Among these, B and C fabric types have garnered particular attention due to their ability to explain specific geophysical phenomena, such as the trench-parallel polarization of the faster shear wave and the rapid attenuation of seismic anisotropy below 250 km in the upper mantle. Currently, there are various hypotheses regarding the formation of B and C type olivine fabrics. The prevailing hypothesis suggests that these fabric types result from the transition of slip systems induced by high water content or high pressure. However, most studies on natural peridotites with developed B or C type olivine fabrics have not extensively explored their deformation mechanisms, leading to uncertainties in their origin explanations.

In this study, we conducted detailed microstructural observations on the garnet-bearing peridotite from Nanshanling in the Dabie ultrahigh-pressure metamorphic belt, aiming to explore its deformation mechanisms and the genesis of olivine fabric. Hand specimen observations revealed that the Nanshanling peridotite exhibits moderate foliation and lineation, primarily manifested by stringer-like distribution of clinopyroxene and/or garnet-rich veins. Optical microstructural observations showed that olivine grains exhibit straight to slightly curved grain boundaries, often featuring triple junctions at approximately 120°. The olivine lacks intragranular plastic deformation features, with only occasional occurrences of subgrain boundaries within individual grains. The results of electron backscatter diffraction (EBSD) reveal that olivine exhibits extremely low levels of intragranular distortion features. Moreover, in the histograms of misorientation angle, the distribution pattern of olivine aligns closely with the theoretical random fabric curve. Olivine demonstrates very weak fabric intensity, with a fabric strength (J-index) ranging between 1.47 and 1.90, exhibiting similarities to a C-type fabric. Additionally, FIB-milled samples from one olivine grain in each of two samples were observed using transmission electron microscopy to assess their dislocation features, revealing very low dislocation densities within the samples. Consequently, our microstructural observations consistently indicate that the dominant deformation mechanism for olivine in the studied samples is non-dislocation creep, and its weak C-type fabric cannot be attributed to slip system transitions induced by high water content or high pressure. As an alternative, we propose another hypothesis: during continental subduction and exhumation processes, the garnet-bearing peridotite from Nanshanling underwent ultrahigh-pressure metamorphism. During this process, olivine experienced oriented metamorphic growth. Previous study (Dilissen et al., 2021) has revealed that during metamorphic growth, olivine predominantly grows along the c-axis, with the slowest growth occurring along the a-axis. Therefore, oriented metamorphic growth can generate a C-type olivine fabric. However, due to the lack of significant intergranular fluids, olivine did not develop prominent shape preferred orientation, which can account for the relatively weak olivine fabric characteristics.

Reference

Dilissen, N., Hidas, K., Garrido, C.J., Sánchez-Vizcaíno, V.L., Kahl, W.-A., 2021. Morphological transition during prograde olivine growth formed by high-pressure dehydration of antigorite-serpentinite to chlorite-harzburgite in a subduction setting. Lithos 382-382, 105949. https://doi.org/10.1016/j.lithos.2020.105949

Experimental investigation of antigorite dehydration fabrics under high pressure and high temperature conditions

Junfeng Zhang¹*, Wenlong Liu¹, Yi Cao¹, Maoshuang Song², Jianfeng Li², David Kohlstedt³

¹School of Earth Sciences, China University of Geosciences, Wuhan, China ²Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China ³Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN,USA

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #22 Presenting author: Junfeng Zhang, https://orcid.org/0000-0002-2834-2833

Antigorite dehydration plays a crucial role at convergent boundaries, influencing mantle wedge partial melting and intermediate-depth earthquakes. Despite its significance, uncertainties persist regarding the crystallographic preferred orientations (CPOs) of prograde minerals from antigorite dehydration and their impact on seismic anisotropy in subducting zones (Nagaya et al., 2014; Padrón-Navarta et al., 2015). In this study, we conducted antigorite dehydration experiments on foliated serpentinized peridotite under both hydrostatic and compression deformation conditions. Hydrostatic experiments were performed at pressures of 0.3–6 GPa and temperatures of 700–900°C (Liu et al., 2021). Our findings reveal an evolution of olivine CPO from a fiber-[100] type in fine grains to a type-C fabric in coarse grains. We propose that the fiber-[100] olivine CPO develops via topotactic growth during initial hydrostatic dehydration, whereas the orthorhombic type-C olivine CPO forms due to anisotropic fluid flow during subsequent dehydration stages. Nonhydrostatic experiments conducted at 300 MPa and 700–750°C demonstrate a progressive evolution of olivine CPO from type-C to type-B with increasing grain size and dehydration extent (Liu et al., 2024). The type-B CPO observed in coarse-grained olivine within fully dehydrated samples predominantly arises from mechanisms involving anisotropic growth, grain rotation, and oriented coalescence of newly formed small olivine grains following antigorite decomposition under compressive stress (Fig. 1). This study provides novel experimental evidence for a low-temperature dehydration mechanism, distinct from high-temperature plastic flow, elucidating the development of type-B/C olivine CPO in forearc regions. Our findings advance understanding of olivine CPO formation and its implications for seismic anisotropy in subduction zone forearcs.

Figure 1. Schematic diagrams illustrating the evolution of olivine CPO during continuous dynamic dehydration.

References

Liu W.L., Qi, H.W., Zhang, J., Wu, X., Li, J., Zhao, R., Wang, Y.F., Jin, Z.M., 2021. Hydrostatic Dehydration Fabrics of Antigorite at High Pressure and High Temperature: Implications for Trench Parallel Seismic Anisotropy at Convergent Plate Boundaries. Journal of Geophysical Research: Solid Earth, 126, e2021JB021671. https://doi.org/10.1029/2023JB01671

Liu, W., Cao, Y., Li, J., Song, M., Xu, H., Wang, Y., Wu, X., Zhang, J., Kohlstedt, D.L., 2024. Type-B Crystallographic Preferred Orientation in Olivine Induced by Dynamic Dehydration of Antigorite in Forearc Region. Journal of Geophysical Research: Solid Earth, 129, e2023JB027929. https://doi.org/10.1029/2023JB027929

Nagaya, T., Wallis, S. R., Kobayashi, H., Michibayashi, K., Mizukami, T., Seto, Y., et al. 2014. Dehydration Breakdown of Antigorite and the Formation of B-type Olivine CPO. Earth and Planetary Science Letters, 387, 67–76. https://doi.org/10.1016/j.epsl.2013.11.025

Padrón-Navarta, J.A., Tommasi, A., Garrido, C.J., Mainprice, D., 2015. On topotaxy and Compaction During Antigorite and Chlorite Dehydration: An Experimental and Natural Study. Contributions to Mineralogy and Petrology, 169(4), 35. https://doi.org/10.1007/s00410-015-1129-4

Microstructures and crystal-fabrics of ultramafic rocks from the Tosa Megamullion, the Shikoku Basin, the Philippine Sea

So Inoue¹*, Katsuyoshi Michibayashi^1,2,* Yumiko Harigane³, Yasuhiko Ohara^1,2,4

¹Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan ²Volcanoes and Earth’s Interior Research Center, Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka 237-0061, Japan ³Research Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan ⁴Hydrographic and Oceanographic Department of Japan, Tokyo 135-0064, Japan

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #23 Presenting author: So Inoue, inoue.so.y5 (at) s.mail.nagoya-u.ac (dot) jp

Megamullions, or oceanic core complexes (OCCs), are dome-like features usually existing in slow-spreading mid-ocean ridge axes, interpreted as exhumed footwalls of low-angle detachment faults (Blackman et al., 2009). Megamullion is characterized by fault rocks forming by ductile deformation in the lower crust up to the uppermost mantle (Cannat et al., 1991). Megamullions have also been discovered in the Shikoku Basin, a back-arc basin in the Philippine Sea, to the south of Japan (Okino et al., 2023). Furthermore, recent research cruises (YK21-06S, YK22-18S, YK23-05S) at the west side of Shikoku Basin identified new megamullions (Sui-shin, Tosa, Sanuki and Awa megamullions) (Ohara, 2023). Ohara (2023) revealed that these megamullions formed during the initial stage of the Shikoku Basin spreading . The very deep water depth (4000-6000 m below sea level) and high mantle Bouguer anomaly (> 150 mGal) associated with these megamullions in this region suggest that this region is magmatically starved (Moriguchi et al., 2024). In this study, we investigate their textural development of ultramafic rocks sampled from the Tosa Megamullion based on microstructural analyses.

We analyzed ultramafic rocks collected from the Tosa Megamullion by the submersible Shinkai 6500 during the research cruise YK23-05S. Rock samples were cut in a plane perpendicular to foliation and parallel to lineation (XZ plane). The foliation and lineation were characterized by elongated structures of plagioclase and orthopyroxene, ranging from 3 mm to a few centimeters by naked eye. All samples were remarkably serpentinized, whereas some retained their primary peridotite minerals: olivine, orthopyroxene, clinopyroxene, plagioclase and spinel. They were dominantly characterized by porphyroclastic textures with coarser (ca. 5 mm) orthopyroxene porphyroclasts. The orthopyroxene porphyroclasts show asymmetric textures and some coarse orthopyroxene grains were remarkably elongated up to several centimeters in length. The matrix composed mainly of relatively finer (ca. 300 µm) olivine grains along with similar fine-grained (ca. 300 µm) clinopyroxene and plagioclase grains. The olivine CPOs showed a weak concentration of (001)[100] pattern with an XZ girdle of [100].

Remarkably elongated orthopyroxene grains along with the asymmetric texture indicates that they resulted from a large amount of shear strain. Furthermore, XZ girdle of olivine [100] in CPOs could be formed by grain boundary slip associated with diffusion creep (Weeler, 2009). We discuss that the ultramafic rocks of the Tosa Megamullion were eventually exposed to the seafloor along the detachment fault after significant amount of ductile shear deformation in the lithosphere.

Figure 1. (A, B, C) major topographic features of the Shikoku Basin, the Philippine Sea and its surrounding areas. The black rectangle indicates the location of the Tosa Megamullion. (D) the mineral phase map of R11 analyzed by SEM-EBSD shows that olivine dominates the matrix, and clinopyroxene and plagioclase are distributed in the foliation.

References

Blackman, D.K., Canales, J.P., Harding, A., 2009. Geophysical signatures of oceanic core complexes. Geophysical Journal International 178, 593–613. https://doi.org/10.1111/j.1365-246X.2009.04184.x

Cannat, M., Mével, C., Stakes, D., 1991. Stretching of the deep crust at the slow-spreading Southwest Indian Ridge. Tectonophysics 190, 73–94. https://doi.org/10.1016/0040-1951(91)90355-V

Moriguchi, T., Okino, K., Ohara, Y., Harigane, Y., Matamura, Y., 2024. Tectonics of the initial stage of the Shikoku Basin spreading. Japan Geoscience Union Meeting 2024, SCG48-P12. https://confit.atlas.jp/guide/event/jpgu2024/subject/SCG48-P12/advanced

Ohara, Y., 2023. Mantle peridotite exposure in the Philippine Sea backarc basins and perspective for future research. The 130th Annual Meeting of the Geological Society of Japan, G1-O-3. https://doi.org/10.14863/geosocabst.2023.0_240

Okino, K., Tani, K., Fujii, M., Zhou, F., Ishizuka, O., Ohara, Y., Hanyu, T., Matamura, Y., 2023. Geophysical investigation of the Mado Megamullion oceanic core complex: implications for the end of back-arc spreading. Progress in Earth Planetary Science 10. https://doi.org/10.1186/s40645-023-00570-2

Wheeler, J., 2009. The preservation of seismic anisotropy in the Earth's mantle during diffusion creep. Geophysical Journal International 178, 1723–1732. https://doi.org/10.1111/j.1365-246X.2009.04241.x

Hydration and Chemical Evolution of Mantle Inclusions from the Colorado Plateau: Insights into the Hydration of the Cold Mantle Wedge

María Ramón-Fernández¹*, José Alberto Padrón-Navarta¹, Françoise Boudier² and Carlos J. Garrido¹

¹Andalusian Institute of Earth Sciences (CSIC), Petrology, Geochemistry and Geochronology, Armilla, Spain ²Géosciences Montpellier, Université de Montpellier & CNRS, Montpellier, France

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #24 Presenting author: María Ramón-Fernández, https://orcid.org/0009-0004-8802-015X

The existence of plate tectonics and its relationship to the deep recycling of volatiles on a global scale is one of the most distinctive features of our planet. The transport and storage of water in the mantle in subduction zones play a crucial role in various geological processes, including magmatism, metasomatism, physical properties and the dynamics of the mantle itself. In typical basalt-hosted mantle xenoliths, low-temperature hydrous minerals such as antigorite and chlorite are unlikely to survive during their ascent. However, mantle inclusions from the Navajo Volcanic Field (NVF) in the Colorado Plateau (CP), exhumed by serpentinized ultramafic microbreccia (SUM) diatremes, represent a unique nearly instantaneous direct sampling of the cold part of the mantle wedge (<800 °C) under high-pressure conditions. These samples belong to the lithospheric mantle hydrated by the fluids related to the low-angle subduction of the ancient Farallon Plate beneath western North America which imposed unusually low mantle heat flux and cooling of the lithosphere.

Colorado Plateau (CP) mantle inclusions from Green Knobs and Moses Rock exhibit significant lithological variability, ranging from lherzolites to dunites and pyroxenites with variable hydration degrees. They contain up to 23% modal proportion of hydrous minerals including chlorite, amphibole, antigorite and humite group minerals, interpreted as being of mantle origin (Smith, 1979) and very rare in mantle xenoliths suites worldwide (Arai & Ishimaru, 2007).

In Green Knobs, the degree of recrystallisation correlates with the modal abundance of hydrous phases. The first stage is limited to the recrystallization along the grain boundaries where pyroxene neoblasts show textural equilibrium contacts with amphibole. Spinel compositional changes are limited to thin rims. In a second stage of hydration, recrystallization zones of amphibole are larger and include in addition chlorite and Fe-chromite. Further hydration results in the complete recrystallization of olivine and pyroxenes and the appearance of antigorite associated with chlorite. This evolution is accompanied by notable chemical changes in ferromagnesian silicates and oxides. Magnesium number in olivine decreases with increasing hydration level, from 91 to 88 and oxides evolve from spinel to Fe-chromite. Aluminum and chromium content significantly decreases in both ortho- and clinopyroxenes from coarse-grained less hydrated textures to neoblasts and remains low and constant in fully recrystallized rocks. Amphibole composition evolves from pargasitic to tremolitic whereas chlorite changes from clinochlore to penninite. The Moses Rock locality also includes a chlorite-bearing orthopyroxenite with amphibole and Fe-chromite and a clinopyroxenite with chlorite, amphibole and magnetite.

These unique mantle inclusions are interpreted as sub-Moho high-pressure (> 1 GPa) hydration of peridotites and pyroxenites from the lithospheric mantle (Smith, 1979, 1995) or as fragments of a tectonic mélange (Smith, 2010), documenting the hydration of the mantle wedge from slab-derived fluids during the Farallon low-angle subduction before diatreme formation.

Figure 1. Backscattered electron images showing the texture of Green Knobs peridotites with recrystallization zones. a) Spinel harzburgite exhibiting incipient hydration. Olivine, orthopyroxene, clinopyroxene and amphibole neoblasts are limited to coarse-grain boundaries. b) Chlorite-harzburgite exhibiting larger recrystallization zones including amphibole, chlorite and Fe-chromite. Ol=Olivine, Opx=Orthopyroxene, Cpx=Clinopyroxene, Amph=Amphibole, Chl=Chlorite, Spl=Spinel.

References

Arai, S., Ishimaru, S., 2007. Insights into petrological characteristics of the lithosphere of mantle wedge beneath arcs through peridotite xenoliths: a review. Journal of Petrology 49, 665–695. https://doi.org/10.1093/petrology/egm069

Smith, D., 2010. Antigorite Peridotite, Metaserpentinite, and other Inclusions within Diatremes on the Colorado Plateau, SW USA: Implications for the Mantle Wedge during Low-angle Subduction. Journal of Petrology 51, 1355–1379. https://doi.org/10.1093/petrology/egq022

Smith, D., 1995. Chlorite-rich ultramafic reaction zones in Colorado Plateau xenoliths: recorders of sub-Moho hydration. Contrib Mineral Petrol 121, 185–200. https://doi.org/10.1007/s004100050098

Smith, D., 1979. Hydrous minerals and carbonates in peridotite inclusions from the green knobs and bull park kimberlitic diatremes on the Colorado Plateau, in: Boyd, F.R., Meyer, H.O.A. (Eds.), The Mantle Sample: Inclusion in Kimberlites and Other Volcanics. American Geophysical Union, Washington, D. C., pp. 345–356. https://doi.org/10.1029/SP016p0345

High-pressure metaserpentinites sequester sediment-derived methane and CO₂: a proxy for mantle wedge carbonation

Michał Bukała¹*, José A. Padrón-Navarta¹, Manuel D. Menzel¹, Vicente López Sánchez-Vizcaíno², Carlos J. Garrido¹

¹Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC, Armilla, Spain, ²Departamento de Geología (Unidad Asociada al IACT, CSIC–UGR), Universidad de Jaén, Escuela Politécnica Superior, Linares, Spain

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #25 Presenting author: Michał Bukała, https://orcid.org/0000-0001-7045-3150

High-pressure (HP) interactions between aqueous fluids and subducting crustal and mantle rocks alter the deep volatile recycling and redox capacity of fluids migrating toward the overlying mantle wedge. Recent findings (Padrón-Navarta et al., 2023) show that reduced fluids derived from graphite-bearing metasediments can interact with serpentinites, (1) promoting antigorite breakdown and (2) modifying the redox capacity of the released deserpentinization fluids.

The Cerro Pingano ultramafic body (Sierra de Baza, Spain) preserves a deserpentinization front oblique to the foliation, similar to the type locality of Cerro del Almirez massif (Padrón-Navarta et al., 2023) outcropping in Nevado-Filabride Complex (NFC) of Sierra Nevada. The ultramafics are hosted within a sequence of graphite-bearing metasediments and calcite-dolomite marbles of the Tahal formation (NFC) and together underwent Alpine HP metamorphism at ~16 kbar and ~660°C. A transition from Atg-serpentinite through Atg-Chl-Opx-Ol rock to Chl-harzburgite can be traced across a ~50 cm wide reaction front. The influx of reducing, C-bearing fluids along the reaction front is inferred from the decrease in the bulk Fe³⁺/ΣFe ratio and the appearance of magnesite in Chl-harzburgites.

Thermodynamic models confirm that infiltration of less than 10 moles (<0.2 weight f/r ratio) of a metasediment-derived fluid into the serpentinite effectively reduces the Fe³⁺/ΣFe ratio and, by decreasing the bulk #Mg, destabilizes antigorite and promotes complete deserpentinization. Graphite-bearing metasediments can release reducing fluids over a broad range of fO₂, but only fluids with fO₂ (FMQ) > –1.0 contain a sufficiently high CO₂ concentration and CO₂/CH₄ ratio to trigger coupled reduction and simultaneous carbonation of serpentinite.

Our new reduction-carbonation model shows that serpentinite can capture carbon from CH₄ and CO₂ via redox-driven carbonate precipitation under high-pressure conditions along a hot subduction geotherm. While the reduction-carbonation model explains carbon capture within the downgoing slab, we propose that it can also be used as a proxy for mantle wedge processes at subarc depths.

Funding: Research funded by RUSTED project PID2022-136471N-B-C21 & C22 funded by MICIN/ AEI/10.13039/501100011033 and FEDER program, and “Juan de la Cierva” Fellowship JFJC2021-047505-I by MCIN/AEI/10.13039/501100011033 and CSIC (M. Bukała).

References

Padrón-Navarta, J.A., López Sánchez-Vizcaíno, V., Menzel, M.D. et al., 2023. Mantle wedge oxidation from deserpentinization modulated by sediment-derived fluids. Nature Geoscience. 16, 268–275. https://doi.org/10.1038/s41561-023-01127-0

A new method for calculating olivine crystal orientation using polarized FTIR spectroscopy

Marco A. Lopez-Sanchez¹, José A. Padrón-Navarta²

¹Departamento de Geología, Universidad de Oviedo, 33005, Oviedo, Spain ²Instituto Andaluz de Ciencias de la Tierra, CSIC, Granada, Spain.

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #26 Presenting author: Marco A. Lopez-Sanchez, http://orcid.org/0000-0002-0261-9267

FTIR spectroscopy, widely used for measuring trace OH in nominally anhydrous minerals (NAMs), requires accurate crystal orientation for precise OH content determination. Current strategies—pre-orienting crystals or taking multiple measurements on random orientations—are time-consuming, sometimes unfeasible, or assume random crystal orientation, which is often untrue for important NAMs like olivine in deformed mantle peridotites. We present a novel method to determine olivine crystallographic orientation using FTIR. Unlike previous approaches (e.g. Asimov et al. 2006), our method uses a single frequency within the silicate overtone region, measured at various angles to the polarizer. Our preliminary tests, considering factors such as the number of measurements at different angles, minimization algorithms, and accuracy, reveal that this method can significantly enhance the precision and efficiency of crystal orientation determination using FTIR. This advancement holds potential for broader application in mineralogical studies, improving the precision of OH content measurements.

Funding: Government of the Principality of Asturias and the Foundation for the Promotion of Applied Research in Asturias (FICYT) (grant SV-PA-21-AYUD/2021/57163) under the Asturias Plan for Science, Technology and Innovation 2018-2022 (PCTI-Asturias).

References

Asimow, P.D., Stein, L.C., Mosenfelder, J.L., Rossman, G.R., 2006. Quantitative polarized infrared analysis of trace OH in populations of randomly oriented mineral grains. American Mineralogist 91, 278–284. https://doi.org/10.2138/am.2006.1937

PyRockWave: a new open-source Python tool for reading elasticity databases and modeling the elastic properties of Earth materials

Marco A. Lopez-Sanchez¹

¹Departamento de Geología, Universidad de Oviedo, 33005, Oviedo, Spain

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #27 Presenting author: Marco A. Lopez-Sanchez, http://orcid.org/0000-0002-0261-9267

PyRockWave (https://github.com/marcoalopez/PyRockWave) is a new open-source Python tool designed for reading single-crystal elastic databases and modeling various elastic properties of Earth materials. Current functionalities include (1) calculation of seismic velocities and anisotropy based on the Christoffel equation and various polar and orthorhombic models, (2) seismic properties of layered media including anisotropic reflectivity (Ruger, 1998) or effective elasticity estimates (Schoenberg and Muir, 1989), (3) elastic average property estimation and seismic averaging schemes, (4) a suite of tools for elastic tensor manipulation, such as rotation, decomposition (Browaeys and Chevrot, 2004) or estimation of anisotropy indexes. Additionally, it provides general tools for coordinate transformation, rotations, vector generation, and visualization. Its modular design allows for easy integration of new modules, functionalities, and compatibility with other codes. The tool relies only on standard and well-tested scientific Python libraries: Numpy (Harris et al., 2020), Scipy (Virtanen et al., 2020), Pandas (McKinney, 2010) and Matplotlib (Hunter, 2007). Application examples, created using Jupyter notebooks, promote ease of use and reproducible workflows.

Funding: Government of the Principality of Asturias and the Foundation for the Promotion of Applied Research in Asturias (FICYT) (grant SV-PA-21-AYUD/2021/57163) under the Asturias Plan for Science, Technology and Innovation 2018-2022 (PCTI-Asturias).

References

Browaeys, J.T., Chevrot, S., 2004. Decomposition of the elastic tensor and geophysical applications. Geophysical Journal International 159, 667–678. https://doi.org/10.1111/j.1365-246X.2004.02415.x

Harris, C.R., et al., 2020. Array programming with NumPy. Nature 585, 357–362. https://doi.org/10.1038/s41586-020-2649-2

Hunter, J.D., 2007. Matplotlib: A 2D Graphics Environment. Comput. Sci. Eng. 9, 90–95. https://doi.org/10.1109/MCSE.2007.55

McKinney, W., 2010. Data Structures for Statistical Computing in Python. Austin, Texas, pp. 56–61. https://doi.org/10.25080/Majora-92bf1922-00a

Rüger, A., 1998. Variation of P -wave reflectivity with offset and azimuth in anisotropic media. GEOPHYSICS 63, 935–947. https://doi.org/10.1190/1.1444405

Schoenberg, M., Muir, F., 1989. A calculus for finely layered anisotropic media. GEOPHYSICS 54, 581–589. https://doi.org/10.1190/1.1442685

Virtanen, P., et al., 2020. SciPy 1.0: fundamental algorithms for scientific computing in Python. Nat Methods 17, 261–272. https://doi.org/10.1038/s41592-019-0686-2

Advance in geological knowledge of the Cabo Ortegal Complex through its geological maps

Sergio Llana-Fúnez¹, Marco Antonio Lopez-Sanchez¹

¹Departamento de Geología, Universidad de Oviedo, 33005, Oviedo, Spain

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #28 Presenting author: Sergio Llana-Fúnez, https://orcid.org/0000-0002-8748-5623

Geological maps are built from two major geological elements: the identification of different geological materials, and their structure or spatial arrangement, which determines the cartographic pattern of the different geological units. Early studies of the Cabo Ortegal Complex faced the challenge of identifying and describing an extensive variety of uncommon deep rocks at the surface. Through many MSc and PhD thesis the group of Leiden did an extensive work in the northwest of Iberia from the 50s to the 70s. This group produced the first complete geological map of the high pressure and high temperature unit in the Cabo Ortegal complex (Vogel et al., 1967). The current understanding about the allochthonous position of the complex as a collection of thrust sheets was established during the early years of plate tectonics by, among others, Ries and Shackleton (1971). In the late 70s, Bayer and Matte (1979) provided geophysical evidence demonstrating the allochthonous nature of the Cabo Ortegal Complex.

A more detailed complete geological map of the complex was first produced by the Spanish geological survey in two sheets at 1:50.000 scale (Fernández Pompa and Fernández Martínez, 1976 and Fernández Pompa and Monteserín López, 1976). Bastida et al. (1984) later integrated these into a 1:200.000 scale geological map covering a larger area, which included a complete section through the complex. The fundamental structural elements established in this work remain to this day. From the 70-80s, three major Spanish research groups have focused on different areas and units of the Cabo Ortegal Complex. Their work has resulted in slightly different geological maps, which will be present in the accompanying poster to this contribution at the same scale.

The group from the University of the Basque Country in Bilbao, together with French colleagues, focused on the ultramafic rocks as well as the eclogites and gneisses, all in the Upper Tectonic Unit, the target of the pre-conference excursion. The geological map that better collects their overall cartographic contributions can be found in Ábalos et al. 2004, 2011.

The group from the Complutense University of Madrid worked mainly in the Lower Tectonic Unit, which includes the mélange, basal and ophiolitic units. More recently, they have extended their work to the quartz-feldspathic gneisses of the Upper Tectonic Unit. The latest map that compiles their general cartographic contributions was published in Arenas et al. (2009).

The group from the University of Oviedo produced the first cartographic synthesis of all units at a 1:200.000 scale mentioned above (Bastida et al., 1984). This group later focused their structural studies on the metabasites, eclogites and quartzo-feldspathic gneisses of the Upper Tectonic Unit, as well as the structure of the Lower Tectonic Unit. The map that compiles this work was published in Marcos et al. (2002). One major outcome that stems from their cartographic work and the relations between the different cartographic units was to regard the different lithological units in the Upper Tectonic Unit as an organized rock sequence made of mantle rocks, metabasites, eclogites and quartzo-feldspathic gneisses. This rock sequence was interpreted to represent a condensed section from the upper mantle to the lower crust, which was later studied as an analog to a mantle-crust transition (e.g. Brown et al., 2009).

The poster accompanying this contribution presents the main geological maps of the complex from different research groups at the same scale and highlights areas where there is overall agreement and identifies regions where there is still discussion of rocks and structures.

References

Ábalos, B., Grassi, K.G., Fountain, D.M., Gil Ibarguchi, J.I., 2004. Geología Estructural, petrofábrica y petrofísica de las eclogitas de Cabo Ortegal (NO de España). Serie Nova Terra 24, 86p.

Ábalos, B., Fountain, D.M., Ibarguchi, J.I.G., Puelles, P., 2011. Eclogite as a seismic marker in subduction channels: Seismic velocities, anisotropy, and petrofabric of Cabo Ortegal eclogite tectonites (Spain). Geological Society of America Bulletin 123, 439–456. https://doi.org/10.1130/B30226.1

Arenas, R., Sánchez-Martínez, S., Castiñeiras, P., Jeffries, T.E., Díez Fernández, R., Andonaegui, P., 2009. The basal tectonic mélange of the Cabo Ortegal Complex (NW Iberian Massif): a key unit in the suture of Pangea. Journal of Iberian Geology 35(2), 85-125. http://hdl.handle.net/10261/26764

Bastida, F., Marcos, A., Marquínez, J., Martínez Catalán, J.R., Pérez-Estaún, A., Pulgar, J.A., 1984. Geological map of Spain, La Coruña and report: Instituto Geológico Minero España, Madrid, p. 1–155, scale 1:200,000, sheet 1.

Bayer, A., Matte, Ph., 1979. Is the mafic/ultramafic massif of Cabo-Ortegal (Northwest Spain) a nappe emplaced during Variscan obduction?: A new gravity interpretation. Tectonophysics 57 (2-4), T9-T18. https://doi.org/10.1016/0040-1951(79)90138-0

Brown, D., Llana-Fúnez, S., Carbonell, R., Alvarez-Marron, J., Marti, D., Salisbury, M.H., 2009. Laboratory measurements of P-wave and S-wave velocities across a surface analog of the continental crust-mantle boundary: Cabo Ortegal, Spain. Earth and Planetary Science Letters 285, 27–38, https://doi.org/10.1016/j.epsl.2009.05.032

Fernández Pompa, F., Fernández Martínez, V., 1974. Geological map of Spain, scale 1:50,000: sheet no. 1 (Cariño) and report. Instituto Geológico Minero España, Madrid, 34 p.

Fernández Pompa, F., Monteserín López, V., 1972. Geological map of Spain, scale 1:50,000: sheet no. 7 (6–3) (Cedeira) and report. Instituto Geológico Minero España, Madrid, 73 p.

Marcos, A., Farias, P., Galán, G., Fernández, F.J., Llana-Fúnez, S., 2002. Tectonic framework of the Cabo Ortegal Complex: A slab of lower crust exhumed in the Variscan orogen (northwestern Iberian Peninsula). In: Martinez Catalán, J.R., Hatcher Jr., R.D., Arenas, R., Díaz García, F. (eds) Variscan-Appalachian dynamics: the building of the late Paleozoic basement. GSA Special Paper 364, 143-162. https://doi.org/10.1130/0-8137-2364-7.143

Ries, A.C., Shackleton, R.M., 1971. Catazonal Complexes of North-West Spain and North Portugal, Remnants of a Hercynian Thrust Plate. Nature Physical Science 234, 65–68. https://doi.org/10.1038/physci234065a0

Vogel, D.E., 1967. Petrology of an eclogite- and pyrigarnite- bearing polymetamorphic rock complex at Cabo Ortegal: NW Spain. Leidse Geologische Mededelingen 40, 121–213. 1:50000 scale map by Vogel, D.E., Engels, J.P., Ho Len Fat, A.G.. Geological Institute, Leiden.

Crystalline massifs in the upper crust: their role favoring stress concentration offshore the North Cantabrian margin

Sergio Llana-Fúnez¹, Marco Antonio Lopez-Sanchez¹

¹Departamento de Geología, Universidad de Oviedo, 33005, Oviedo, Spain

When (1 min. presentation): Thursday 3rd October at 12:20 Poster location: Panel #29 Presenting author: Sergio Llana-Fúnez, https://orcid.org/0000-0002-8748-5623

The Cabo Ortegal Complex is currently sitting very close to the continental margin of the Iberian plate. What became the current northern edge of the Iberian Peninsula, was subjected to rifting and lithospheric extension that produced the Atlantic between the late Jurassic and early Cretaceous (e.g. Tugend et al., 2015). The record and amount of extension varied along the margin, from very limited at the western end to hyperextension in the Basque-Cantabrian region, towards the East (Cadenas et al., 2018).

The Paleogene convergence of Iberia against Europe was resolved differently along such heterogeneous North Iberian margin. A double verging orogenic wedge formed to produce the Pyrenees at the eastern end (e.g. Teixell et al., 2018). The adjacent hyperextended crust, in the middle, the Basque-Cantabrian region, resulted in thickening of the previously thinned crust, but insufficient to produce an orogenic root (Fernández-Viejo et al., 2021). The western end, dominated by the crystalline core of the Variscan orogen and recording previous limited extension onshore, developed at the continental margin a modest accretionary wedge during the alpine convergence and the initiation of the subduction of oceanic crust from the Bay of Biscay, which was very short-lived (Álvarez-Marrón et al., 1997; Ayarza et al., 2004; Fernández-Viejo et al., 2012).

Since its emplacement during the Variscan orogeny, the Cabo Ortegal Complex has been in upper crustal conditions subject to several episodes of brittle deformation. Given the nature of its rock formations it is unlikely the healing of subsequent brittle events that followed its emplacement, and has therefore the potential to preserve some of the recent (brittle) tectonic history of the Iberian plate. Two of the main cartographic faults, trending northeasterly and northwesterly, and relatively steep, are likely to be related or have been active during the major extension episode. Some of the best preserved lower crustal and upper mantle rocks have also an intense microfracturing, which relates to meso-scale joints and fractures that are also likely to register brittle post-emplacement history. The microfracturing reduces significantly seismic velocities measured directly at room pressure (Brown et al., 2009).

The instrumental seismic record in northwestern Iberia shows low magnitude by persistent activity with a distribution of events responding to the presence of crustal-scale geological structures under some stress. Intersecting alpine faults, subvertical and gently dipping, nucleate some clusters (Fernández-Viejo et al., 2021b). Bodies of rigid crystalline basement rocks in the upper crust may also be potential candidates for stress accumulation in the vicinities of alpine structures. Offshore the North Iberian margin we have identified two clusters, the first one is located to the north of the Cabo Ortegal Complex.

As part of an ongoing research project, we will precise the location of seismicity by using two seismic networks onshore and offshore and will collect samples at the edge of the continental platform ahead of the shores of “Cabo Ortegal”.

References

Alvarez-Marrón, J., Rubio, E., Torné, M., 1997. Subduction-related structures in the North Iberian Margin. Journal of Geophysical Research 102 (B10), 22497–22511.

Ayarza, P., Martínez-Catalán, J.R., Alvarez-Marrón, J., Zeyen, H., Juhlin, C., 2004. Geophysical constraints on the deep structure of a limited ocean-continent subduction zone at the North Iberian Margin. Tectonics 23, http://dx.doi.org/10.1029/2002TC001487

Brown, D., Llana-Fúnez, S., Carbonell, R., Alvarez-Marron, J., Marti, D., Salisbury, M.H., 2009. Laboratory measurements of P-wave and S-wave velocities across a surface analog of the continental crust-mantle boundary: Cabo Ortegal, Spain. Earth and Planetary Science Letters 285, 27–38, https://doi.org/10.1016/j.epsl.2009.05.032

Cadenas, P., Fernández-Viejo, G., Pulgar, J.A., Tugend, J., Manatschal, G., Minshull, T., 2018. Constraints imposed by rift inheritance on the compressional reactivation of a hyperextend- ed margin: Mapping rift domains in the north Iberian margin and in the Cantabrian Mountains. Tectonics 37, 758–785, https://doi.org/10.1002/2016TC004454

Fernández-Viejo, G., Pulgar, J. A., Gallastegui, J., and Quintana, L. (2012). The fossil accretionary wedge of the Bay of Biscay: critical wedge analysis on depth migrated seismic sections and geodynamical implications. J. Geology 120, 315–331. https://doi.org/10.1086/664789

Fernández-Viejo, G., Cadenas, P., Acevedo, J., Llana-Fúnez, S., 2021a. The unevenness of the north Iberian crustal root, a snapshot of an elusive stage in margin reactivation. Geology 49, 1426–1430. https://doi.org/10.1130/G49341.1

Fernández-Viejo, G., Llana-Fúnez, S., Acevedo, J., López-Fernández, C. 2021b. The Cantabrian Fault at Sea. Low Magnitude Seismicity and Its Significance Within a Stable Setting. Frontiers Earth Sciences 9:645061. https://doi.org/10.3389/feart.2021.645061

Teixell, A., Labaume, P., Ayarza, P., Espurt, N., de Saint Blanquat, M., Lagabrielle, Y. 2018. Crustal structure and evolution of the Pyrenean- Cantabrian belt: A review and new interpretations from recent concepts and data: Tectonophysics 724, 146–170, https://doi.org/10.1016/j.tecto.2018.01.009

Tugend, J., Manatschal, G., Kusznir, N.J., 2015. Spatial and temporal evolution of hyperextended rift systems: Implication for the nature, kinematics and timing of the Iberian-European plate boundary. Geology 43(1), 15-18. https://doi.org/10.1130/G36072.1

List of participants in alphabetical order

Go to https://lherzolite2024.github.io/

Last update 2024-10-01