Chalcophile-siderophile element systematics of hydrothermal pyrite from martian regolith breccia NWA 7533

Abstract : Unlike other martian meteorites studied so far, Martian regolith breccia NWA 7533 and 56 paired meteorites that have sampled 4.4 Ga-old impact lithologies show only sulfides of 57 hydrothermal origin (mostly pyrite (<1 vol.%) and scarce pyrrhotite). NWA 7533 pyrite has been 58 analyzed for 25 chalcophile-siderophile trace elements with laser ablation-inductively coupled 59 plasma mass spectrometer (LA-ICPMS). Micronuggets of highly siderophile elements-HSE (Os, Ir, 60 Pt, Ru, Rh) along with occasional detection of Mo and Re were observed in half of the 52 analyzed 61 crystals as random concentration spikes in time-resolved LA-ICPMS data. These nuggets are 62 interpreted as variably altered remnants from repeated meteorite bombardment of the early martian 63 crust, as are chondritic Ni/Co ratios of pyrite (10-20). Pyrite displays superchondritic S/Se (54,000 64 to 3,300) and Te/Se (0.3->1). The reasonably good positive correlation (R 2 =0.72) between Se and 65 Ni reflects a temperature control on the solubility of both elements. Apart from the chalcogens S, Se 66 and Te, pyrite appears to be a minor contributor (<20%) to the whole-rock budget for both HSE 67 (including Ni and Co) and chalcophile metals Ag, As, Au, Cu, Hg, Pb, Sb, Tl and Zn. This deficit 68 can result from i) high (>400°C) temperature crystallization for NWA 7533 pyrite, as deduced from 69 its Se and Ni contents, ii) magmatic sulfide-depletion of brecciated early martian crust, iii) 70 precipitation from near neutral H 2 S-HS-H 2 O-rich hydrothermal fluids that did not provide halogen 71 ligands for extensive transport of chalcophile-siderophile metals. It is suggested that the 1.4 Ga 72 lithification event that precipitated hydrothermal pyrite left the chalcophile-siderophile element 73 budget of the early martian crust nearly unmodified, except for S, Se and Te. 74 75 76
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Jean-Pierre Lorand, R.H. Hewins, M. Humayun, L. Remusat, B. Zanda, et al.. Chalcophile-siderophile element systematics of hydrothermal pyrite from martian regolith breccia NWA 7533. Geochimica et Cosmochimica Acta, Elsevier, 2018, 241, pp.134-149. ⟨10.1016/j.gca.2018.08.041⟩. ⟨hal-02182381⟩

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