PARTITIONING OF MODERATELY SIDEROPHILE ELEMENTS AMONG OLIVINE, SILICATE MELT, AND SULFIDE MELT - CONSTRAINTS ON CORE FORMATION IN THE EARTHAND MARS

This study investigates the effects of variations in the fugacities of oxygen and sulfur on the partitioning of first series transition meta ls (V, Cr, Mn, Fe, Co, Ni, and cu) and W among coexisting sulfide melt , silicate melt, and olivine. Experiments were performed at 1 atm pres sure, 1350 degrees C, with the fugacities of oxygen and sulfur control led by mixing CO2, CO, and SO2 gases. Starting compositions consisted of a caO-MgO-Al2O3-SiO2-FeO-Na2O analog for a barred olivine chondrule from an ordinary chondrite and a synthetic komatiite. The fo(2)/fs(2) conditions ranged from log fo(2) = -7.9 to 10.6, with log fs(2) value s ranging from -1.0 to -2.5. Our experimental results demonstrate that the fo(2)/fs(2) dependencies fo sulfide melt/silicate melt partition coefficients for the first series transition metals are proportional t o their valence states. The fo(2)/fs(2), dependencies for the partitio ning of Fe, Co, Ni, and Cu are weaker than predicted on the basis of t heir valence states. Variations in fo(2)/fs(2), conditions have no sig nificant effect on olivine/melt partitioning other than those resultin g from fo(2)-induced changes in the valence state of a given element. The strong fo(2)/fs(2) dependence for the olivine/silicate melt partit ioning of V is attributable to a change of valence state, from 4+ to 3 +, with decreasing fo(2). Our experimentally determined partition coef ficients are used to develop models fro the segregation of sulfide and metal from the silicate portion of the early Earth and the Shergottit e parent body (Mars). We find that the influenced of S is not sufficie nt to explain the overabundance of siderophile and chalcophile element s that remained in the mantle of the Earth following core formation. I mportant constraints on core formation in mars are provided by our exp erimental determination of the partitioning of Cu between silicate and sulfide melts. When balance constraint from Fe, the experiments allow a determination of the mass of the Martian core (similar to 0.4 wt%). These modeling results indicate that Mars is depleted in S, and that its core is solid. (C) 1997 Elsevier Science Ltd.