THE IMPACT OF CARBON ON ELEMENT DISTRIBUTION DURING CORE FORMATION

Abstract

The distribution of Fe, Ni, Co, P, Ge, W, Mo, and O between molten silicate and liquid metal was determined at pressures and temperatures relevant to core formation (50-80 kb, 2000-2300°C) under both carbon-free and graphite-saturated conditions. The effect of carbon is very pronounced in reducing the siderophile tendencies of P and Ge. Germanium showed a significant reduction in metal-silicate partition coefficients (Dmet/sil) from a value too high to be determined by the electron microprobe in the carbon-free metal/ultrabasic silicate system down to a value of # 33 in the carbon-bearing metal/ultrabasic silicate system. Phosphorus becomes lithophile at carbon saturation. Nickel and cobalt show a modest reduction, and W and Mo show a modest increase in siderophility at graphite saturation. These effects are seen both in basic, aluminous, and ultrabasic, magnesian silicate liquids. Carbon-bearing liquids, in combination with their sulfurous cousins, would be less effective at depleting silicate liquids in many siderophile elements than C- and S-free liquids. Limits upon any geochemical role for carbon in the core forming process, however, are provided by P which becomes lithophile at carbon saturation. As P is depleted rather than enriched in the mantle, core formation probably did not occur at carbon saturation.