Metal-saturated sulfide assemblages in NWA 2737: Evidence for impact-related sulfur devolatilization in Martian meteorites

Authors

  • Jean-Pierre LORAND,

    Corresponding author
    1. Laboratoire de Planétologie et Géodynamique de Nantes, Université de Nantes and Centre National de la Recherche Scientifique (UMR 6112), 2 Rue La Houssinière, BP 92208, 44322 Nantes, Cédex 3, France
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  • Jean-Alix BARRAT,

    1. Université de Bretagne Occidentale, Institut Universitaire Européen de la Mer, Place Nicolas Copernic, 29280 Plouzané Cedex, France
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  • Vincent CHEVRIER,

    1. W.M. Keck Laboratory for Space and Planetary Simulation, Arkansas Center for Space and Planetary Science, MUSE 202, University of Arkansas, Fayetteville, Arkansas 72701, USA
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  • Violaine SAUTTER,

    1. Muséum National d’Histoire Naturelle, Laboratoire de Minéralogie et Cosmochimie, CNRS UMR 7202, 61 Rue Buffon, 75005, Paris, France
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  • Sylvain PONT

    1. Muséum National d’Histoire Naturelle, Laboratoire de Minéralogie et Cosmochimie, CNRS UMR 7202, 61 Rue Buffon, 75005, Paris, France
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Corresponding author. E-mail: jean-pierre.lorand@univ-nantes.fr

Abstract–

NWA 2737, a Martian meteorite from the Chassignite subclass, contains minute amounts (0.010 ± 0.005 vol%) of metal-saturated Fe-Ni sulfides. These latter bear evidence of the strong shock effects documented by abundant Fe nanoparticles and planar defects in Northwest Africa (NWA) 2737 olivine. A Ni-poor troilite (Fe/S = 1.0 ± 0.01), sometimes Cr-bearing (up to 1 wt%), coexists with micrometer-sized taenite/tetrataenite-type native Ni-Fe alloys (Ni/Fe = 1) and Fe-Os-Ir-(Ru) alloys a few hundreds of nanometers across. The troilite has exsolved flame-like pentlandite (Fe/Fe + Ni = 0.5–0.6). Chalcopyrite is almost lacking, and no pyrite has been found. As a hot desert find, NWA 2737 shows astonishingly fresh sulfides. The composition of troilite coexisting with Ni-Fe alloys is completely at odds with Chassigny and Nahkla sulfides (pyrite + metal-deficient monoclinic-type pyrrhotite). It indicates strongly reducing crystallization conditions (close to IW), several log units below the fO2 conditions inferred from chromites compositions and accepted for Chassignites (FMQ-1 log unit). It is proposed that reduction in sulfides into base and precious metal alloys is operated via sulfur degassing, which is supported by the highly resorbed and denticulated shape of sulfide blebs and their spongy textures. Shock-related S degassing may be responsible for considerable damages in magmatic sulfide structures and sulfide assemblages, with concomitant loss of magnetic properties as documented in some other Martian meteorites.

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