Zaicong Wang, Harry Becker
Earth and Planetary Science Letters 463, 56-68 Link to Article [http://dx.doi.org/10.1016/j.epsl.2017.01.023]
Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstrasse 74-100, 12249 Berlin, Germany
It is generally believed that the Martian mantle and core are rich in sulfur and that shergottites originated from sulfide-saturated magma. However, recent work suggests that the high FeO contents would require very high S concentrations in shergottite parent magmas at sulfide saturation. Here we combine new and published data on chalcophile elements in shergottites, nakhlites and ALH84001 to constrain the sulfide saturation state of the parent magmas and the chalcophile element concentrations in their mantle sources.
Regardless of the MgO content and the long-term depletion history of incompatible lithophile elements as indicated by initial ε143Nd, different groups of shergottites display limited variations in ratios of Pt, Pd, Re, Cu, S, Se and Te. The emplacement of most shergottites within the crust and limited variations of ratios of chalcophile elements with substantial differences in volatility during eruption (e.g., Cu/S, Cu/Se and Pt/Re) indicate little degassing losses of S, Se, Te and Re from shergottites. Limited variations in ratios of elements with very different sulfide–silicate melt partition coefficients and negative correlations of chalcophile elements with MgO require a sulfide-undersaturated evolution of the parent magmas from mantle source to emplacement in the crust, consistent with the FeO-based argument. Sulfide petrography and the komatiite-like fractionation of platinum group elements (PGE) in shergottites also support this conclusion. The absence of accumulated sulfides in the ancient Martian cumulate ALH84001 results in very low contents of PGE, Re, Cu, Se and Te in this meteorite, hinting that sulfide-undersaturated magmas may have occurred throughout the Martian geological history. The negative correlation of Cu and MgO contents in shergottites suggests approximately Cu in the Martian mantle. The ratios of Cu, S, Se and Te indicate 360±120 μg/g (1s) S, 100±27 ng/g (1s) Se and 0.50±0.25 ng/g (1s) Te in the Martian mantle. At such low S concentrations, all S in Martian mantle sources may dissolve in basaltic melts that form at >5 % partial melting.
Assuming equilibrium metal–silicate partitioning, and provided that the compositional model of the Martian mantle based on SNC meteorites is correct, Martian mantle inventories of Cu, S and Se were mostly established by core formation and the Martian core should contain <5–10 wt.% S only (depending on the choice of metal–silicate partition coefficients). The low S content in the Martian interior is consistent with the low Zn content in the Martian mantle, which indicates about 5 wt.% S in the core. In contrast, the highly siderophile PGE, Re and Te were added to the mantle by late accreted material after the Martian core formed. The near chondritic PGE ratios and the very low ratio of volatile Te to refractory PGE reflect a strongly volatile element-depleted late veneer and imply that the delivery of Martian water, presumably from carbonaceous chondrite like materials, must have occurred before accretion of the late veneer, likely within 2–3 million years after formation of the solar system.