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

Abstract

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 °C, 6.7 ± 0.2 wt.% at 2200 °C, and 7.4 ± 0.2 wt.% at 2410 °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 (∼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 ∼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 ∼6–7 wt.% and a preferred value of 0.25 ± 0.15 wt.% for a bulk Earth carbon concentration of ∼1000 ppm.