Carbon solubility in core melts in a shallow magma ocean environment and distribution of carbon between the Earth's core and the mantle

The solubility of carbon in Fe and Fe-5.2 wt.% Ni melts, saturated with graphite, determined by electron microprobe analysis of quenched metal melts was 5.8 +/- 0.1 wt.% at 2000 degrees C, 6.7 +/- 0.2 wt.% at 2200 degrees C, and 7.4 +/- 0.2 wt.% at 2410 degrees C at 2 GPa, conditions relevant for core/mantle differentiation in a shallow magma ocean. These solubilities are slightly lower than low-pressure literature values and significantly beneath calculated values for even higher pressures [e.g., Wood B. J. (1993) Carbon in the core. Earth Planet. Sci. Lett. 117, 593-607]. The trend of C solubility versus temperature for Fe-5.2 wt.'% Ni melt, within analytical uncertainties, is similar to or slightly lower (similar to 0.2-0.4 wt.%) than that of pure Fe. Carbon content of core melts and residual mantle silicates derived from equilibrium batch or fractional segregation of core liquids and their comparison with our solubility data and carbon content estimate of the present day mantle, respectively, constrain the partition coefficient of carbon between silicate and metallic melts, D-C(silicate/metal) in a magma ocean. For the entire range of possible bulk Earth carbon content from chondritic to subchondritic values, D-C(silicate/metal) of 10(-4) to 1 is derived. But for similar to 1000 ppm bulk Earth carbon, D-C(silicate/metal) is between 10(-2) and 1. Using the complete range of possible D-C(silicate/metal) for a magma ocean at 2200 C, we predict maximum carbon content of the Earth's core to be similar to 6-7 wt.% and a preferred value of 0.25 +/- 0.15 wt.% for a bulk Earth carbon concentration of similar to 1000 ppm. (C) 2008 Elsevier Ltd. All rights reserved.