1Jean-Pierre Lorand, 2,3R.H.Hewins, 4M.Humayun,2L.Remusat, 2B.Zanda, 1C.La, 2S.Pont
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.08.041]
1Laboratoire de Planétologie et Géodynamique à Nantes, CNRS UMR 6112, Université de Nantes, 2 Rue de la Houssinère, BP 92208, 44322 Nantes Cédex 3, France
2Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC) – Sorbonne Université- Muséum National d’Histoire Naturelle, UPMC Université Paris 06, UMR CNRS 7590, IRD UMR 206, 61 rue Buffon, 75005 Paris, France
3Department of Earth & Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USA
4Department of Earth, Ocean & Atmospheric Science and National High Magnetic, Field Laboratory, Florida State University, Tallahassee, FL 32310, USA
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.