1. Zoisite in cratonic eclogite xenoliths - Implications for water in the upper mantle
- Author
-
Radu, I.B., Moine, B.N., Bolfan-Casanova, N., Ionov, D.A., Devidal, J.L., Deloule, E., Korsakov, A.V., Golovin, A.V., Oleinikov, O.B., Cottin, J.Y., Laboratoire Magmas et Volcans (LMV), Institut national des sciences de l'Univers (INSU - CNRS)-Institut de Recherche pour le Développement et la société-Centre National de la Recherche Scientifique (CNRS)-Université Clermont Auvergne (UCA)-Observatoire de Physique du Globe de Clermont-Ferrand (OPGC), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Clermont Auvergne (UCA)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Clermont Auvergne (UCA), Géosciences Montpellier, Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université des Antilles (UA)-Université de Montpellier (UM), Centre de Recherches Pétrographiques et Géochimiques (CRPG), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS), Sobolev Institute of Geology and Mineralogy [Novosibirsk], Siberian Branch of the Russian Academy of Sciences (SB RAS), Diamond and Precious Metal Geology Institute, Laboratory of Excellence ClerVolc, Centre National de la Recherche Scientifique (CNRS), Russian Foundation for Basic Research (RFBR), Russian Federation state assignment project of IGM SB RAS, Jean Monnet University-Saint-Etienne, Russian Federation state assignment project of DPMGI SB RAS, ANR-10-LABX-0006,CLERVOLC,Clermont-Ferrand centre for research on volcanism(2010), and ANR-16-IDEX-0001,CAP 20-25,CAP 20-25(2016)
- Subjects
Eclogite xenoliths Zoisite Exsolution Omphacite Water Nominally anhydrous minerals NAMs ,Eclogite xenoliths ,Water ,[SDU.STU]Sciences of the Universe [physics]/Earth Sciences ,Geology ,Geokemi ,Omphacite ,Geochemistry ,[SDU]Sciences of the Universe [physics] ,Geochemistry and Petrology ,Zoisite ,Geologi ,Exsolution ,Nominally anhydrous minerals NAMs - Abstract
co-auteur étranger; International audience; A significant portion of the Earth's water is found at trace levels in nominally anhydrous rock forming minerals in the crust and mantle. Its distribution has far-reaching implications in melt-generating processes, mantle rheology and the stability of the lithosphere. We report petrographic, major and trace element, Fourier transform infrared spectroscopy and secondary ion mass spectrometry data on four eclogite xenoliths from the Obnazhennaya kimberlite, Siberia that contain hydrous minerals at upper mantle conditions. Al-rich (>9.9 wt% Al2O3) omphacitic clinopyroxene, a major mineral of eclogites, contains crystallographically controlled exsolutions of CaO-rich (up to 23.6 wt%) garnet (coronae, lenses), garnet-zoisite (lamellae) or amphiboles (needle/blade-like exsolutions). The omphacite hosting the exsolved lamellae shows enrichment or depletion in major (Al, Si, Mg) and trace (e.g., HREE) elements compatible with garnet and zoisite, proportional to the abundance of exsolutions. We argue that zoisite likely formed concomitantly with garnet exsolution from a water- and Al-rich precursory omphacite, without any fluid addition. The compositional gradients are consistent with diffusion-controlled partitioning in a closed system, and their preservation indicates the exsolution likely took place shortly prior to eclogite entrapment by the kimberlite. Large, non-exsolved, omphacite grains contain minimum ~870-1500 ppm wt. H2O (determined as OH). We infer that intertwined lamellae of hydrous garnet (600-960 ppm wt. H2O) and minor (2O) formed at mantle conditions due to progressive exsolution, without external fluids. Recalculated whole rock water contents range from ~320 to 970 ppm wt. H2O. These estimates exceed by far those for the surrounding peridotitic mantle and suggest that, although a minor component, eclogites may locally be an important water reservoir in the cratonic mantle.
- Published
- 2022