ELEMENT PARTITIONING BETWEEN METALLIC LIQUID, SILICATE LIQUID, AND LOWER-MANTLE MINERALS - IMPLICATIONS FOR CORE FORMATION OF THE EARTH

We determined the partition coefficients of 19 elements between metall ic liquid and silicate liquid at 20 GPa and 2500 degrees C, and betwee n metallic liquid and silicate perovskite at 27 GPa and 2200 degrees C . Remarkable differences were observed in the partitioning behaviors o f Si, P, W, Re, and Pb among the silicate liquid, perovskite, and magn esiowustite coexisting with metallic liquid, reflecting incompatibilit y of the elements in the silicate or oxide phase. We could not observe any significant difference in the partitioning behaviors of V, Cr, Mn , Co, Ni, and Cu among the phases coexisting with metallic liquid. Com parison of the present partitioning data with those obtained previousl y at lower pressure and temperature suggests that the exchange partiti on coefficients, K-met/sil, of Co, Ni, Mo, and W decrease, whereas tho se of V, Cr, and Mn increase and tend to approach unity with increasin g pressure and temperature. We also made preliminary experiments to cl arify the effect of sulfur on the partitioning behaviors. Sulfur lower s the exchange partition coefficients, K-met/sil of Mo and W between m etallic liquid and silicate liquid significantly at 20 GPa and 2300 de grees C. The mantle abundances of Co, Ni, Cu, Mo, and W calculated for the metal-silicate equilibrium model are lower than those of the real mantle, whereas P, K, and Mn are overabundant in the calculated mantl e. The discrepancies in the abundances of Co and Ni could be explained by the chemical equilibrium at higher pressure and temperature. Large discrepancies in Mo and W between the calculated and real mantles cou ld be accounted for by the effect of sulfur combined with the effects of pressure and temperature on the chemical equilibrium. The mantle ab undances of P, K, and Cu could be accounted for by volatile loss in th e nebula, perhaps before accretion of the Earth, combined with the che mical equilibrium at higher pressure and temperature. Thus the observe d mantle abundances of P, K, Co, Ni, Cu, Mo, and W may be consistent w ith a model of sulfur-bearing metal-silicate equilibrium in lower-mant le conditions. (C) 1997 Elsevier Science B.V.