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

Jean-Pierre Lorand, R.H. Hewins, M. Humayun, L. Remusat, … S. Pont

Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 5 September 2018

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“Unlike other martian meteorites studied so far, Martian regolith breccia NWA 7533 and paired meteorites that have sampled 4.4 Ga-old impact lithologies show only sulfides of hydrothermal origin (mostly pyrite (<1 vol.%) and scarce pyrrhotite). NWA 7533 pyrite has been analyzed for 25 chalcophile-siderophile trace elements with laser ablation-inductively coupled plasma mass spectrometer (LA-ICPMS). Micronuggets of highly siderophile elements-HSE (Os, Ir, Pt, Ru, Rh) along with occasional detection of Mo and Re were observed in half of the 52 analyzed crystals as random concentration spikes in time-resolved LA-ICPMS data. These nuggets are interpreted as variably altered remnants from repeated meteorite bombardment of the early martian crust, as are chondritic Ni/Co ratios of pyrite (10-20). Pyrite displays superchondritic S/Se (54,000 to 3,300) and Te/Se (0.3 - >1). The reasonably good positive correlation (R2=0.72) between Se and Ni reflects a temperature control on the solubility of both elements. Apart from the chalcogens S, Se and Te, pyrite appears to be a minor contributor (<20%) to the whole-rock budget for both HSE (including Ni and Co) and chalcophile metals Ag, As, Au, Cu, Hg, Pb, Sb, Tl and Zn. This deficit can result from i) high (>400°C) temperature crystallization for NWA 7533 pyrite, as deduced from its Se and Ni contents, ii) magmatic sulfide-depletion of brecciated early martian crust, iii) precipitation from near neutral H2S-HS-H2O-rich hydrothermal fluids that did not provide halogen ligands for extensive transport of chalcophile-siderophile metals. It is suggested that the 1.4 Ga lithification event that precipitated hydrothermal pyrite left the chalcophile-siderophile element budget of the early martian crust nearly unmodified, except for S, Se and Te.”