The role of sulfides in the fractionation of highly siderophile and chalcophile elements during the formation of martian shergottite meteorites

1Raphael J. Baumgartner, 1Marco L. Fiorentini, 2Jean-Pierre Lorand, 3,4David Baratoux, 5Federica Zaccarini, 6Ludovic Ferrière, 7Marko K. Prašek,8Kerim Sener
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://doi.org/10.1016/j.gca.2017.04.011]
1Centre for Exploration Targeting, School of Earth Sciences, ARC Centre of Excellence for Core to Crust Fluid Systems, University of Western Australia, 6009 Crawley, Australia
2Laboratoire de Planétologie et Géodynamique de Nantes, Université de Nantes and Centre National de la Recherche Scientifique, UMR 6112 44322 Nantes, France
3Géosciences Environnement Toulouse, CNRS, IRD and University of Toulouse, 31400 Toulouse, France
4Institut Fondamental d’Afrique Noire Cheikh Anta Diop, 5005 Dakar, Senegal
5Department of Applied Geological Sciences and Geophysics, University of Leoben, 8700 Leoben, Austria
6Natural History Museum Vienna, 1010 Vienna, Austria
7McGill University, Department of Earth and Planetary Sciences, H3A 0E8, Montreal, Canada
8Matrix Exploration Pty Ltd, 6112 Armadale, Australia
Copyright Elsevier

The shergottite meteorites are ultramafic to mafic igneous rocks whose parental magmas formed from partial melting of the martian mantle. This study reports in-situ laser ablation inductively coupled plasma mass spectrometry analyses for siderophile and chalcophile major and trace elements (i.e., Co, Ni, Cu, As, Se, Ag, Sb, Te, Pb, Bi, and the highly siderophile platinum-group elements, PGE: Os, Ir, Ru, Rh, Pt and Pd) of magmatic Fe-Ni-Cu sulfide assemblages from four shergottite meteorites. They include three geochemically similar incompatible trace element- (ITE-) depleted olivine-phyric shergottites (Yamato-980459, Dar al Gani 476 and Dhofar 019) that presumably formed from similar mantle and magma sources, and one distinctively ITE-enriched basaltic shergottite (Zagami). The sulfides in the shergottites have been variably modified by alteration on Earth and Mars, as well as by impact shock-shock related melting/volatilization during meteorite ejection. However, they inherit and retain their magmatic PGE signatures. The CI chondrite-normalized PGE concentration patterns of sulfides reproduce the whole-rock signatures determined in previous studies. These similarities indicate that sulfides exerted a major control on the PGE during shergottite petrogenesis. However, depletions of Pt (and Ir) in sulfide relative to the other PGE suggest that additional phases such discrete Pt-Fe-Ir alloys have played an important role in the concentration of these elements. These alloys are expected to have enhanced stability in reduced and FeO-rich shergottite magmas, and could be a common feature in martian igneous systems. A Pt-rich PGM was found to occur in a sulfide assemblage in Dhofar 019. However, its origin may be related to impact shock-related sulfide melting and volatilisation during meteorite ejection. In the ITE-depleted olivine-phyric shergottites, positive relationships exist between petrogenetic indicators (e.g., whole-rock Mg-number) and most moderately to strongly siderophile and chalcophile elements in sulfides. These variations extend to incompatible elements like Te and Pd. The whole-rock concentrations of Pd derived from mass-balance calculations decrease by one order of magnitude in the order Y-980459, DaG 476 and Dhofar 019, and broadly overlap the trends in previously published whole-rock analyses. Mantle heterogeneities, and the timing of sulfide saturation as function of mantle melting and/or magma fractionation following ascent from the mantle, may have been the controlling factors of the siderophile and chalcophile element systematics in the analyzed shergottites.

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