THE IMPACT OF CARBON ON ELEMENT DISTRIBUTION DURING CORE FORMATION

The distribution of Fe, Ni, Co, P, Ge, W, Mo, and O between molten sil icate and liquid metal was determined at pressures and temperatures re levant to core formation (50-80 kb, 2000-2300 degrees C) under both ca rbon-free and graphite-saturated conditions. The effect of carbon is v ery pronounced in reducing the siderophile tendencies of P and Ge. Ger manium showed a significant reduction in metal-silicate partition coef ficients (D-met/sil) from a value too high to be determined by the ele ctron microprobe in the carbon-free metal/ultrabasic silicate system d own to a value of similar to 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 se en both in basic, aluminous, and ultrabasic, magnesian silicate liquid s. Carbon-bearing liquids, in combination with their sulfurous cousins , would be less effective at depleting silicate liquids in many sidero phile 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 rat her than enriched in the mantle, core formation probably did not occur at carbon saturation. Copyright (C) 1997 Elsevier Science Ltd.