THE INFLUENCE OF SILICATE MELT COMPOSITION ON DISTRIBUTION OF SIDEROPHILE ELEMENTS AMONG METAL AND SILICATE LIQUIDS

Liquid metal-liquid silicate partitioning of Fe, Ni, Co, P, Ge, W and Mo among a carbon-saturated metal and a variety of silicate melts (mag nesian-tholeiitic-siliceous-aluminosiliceous basalts) depends modestly to strongly upon silicate melt structure and composition. Low valency siderophile elements, Fe, Ni and Co, show a modest influence of silic ate melt composition on partitioning. Germanium shows a moderate but c onsistent preference for the depolymerized magnesian melt. High valenc y siderophile elements, P, Mo, and W, show more than an order of magni tude decrease in metal-silicate partition coefficients as the silicate melt becomes more depolymerized. Detailed inspection of our and other published W data shows that polymerization state, temperature and pre ssure are more important controls on W partitioning than oxidation sta te. For this to be true for a high and variable valence element implie s a secondary role in general for oxidation state, even though some ro le must be present. Equilibrium core segregation through a magma ocean of 'ultrabasic' composition can provide a resolution to the 'excess' abundances of Ge, P, W and Mo in the mantle, but the mantle compositio n alone cannot explain the excess abundances of nickel and cobalt in c hondritic proportions. (C) 1997 Elsevier Science B.V.