Carbon partitioning between metal and silicate melts during Earth accretion

In the accreting Earth and planetesimals, carbon was distributed between a core forming metallic melt, a silicate melt, and a hot, potentially dense atmosphere. Metal melt droplets segregating gravitationally from the magma ocean equilibrated near its base. To understand the distribution of carbon, its partitioning between the two melts is experimentally investigated at 1.5-6.0 GPa, 1300-2000 degrees C at oxygen fugacities of -0.9 to -1.9 log units below the iron-wuestite reference buffer (IW). One set of experiments was performed in San Carlos olivine capsules to investigate the effect of melt depolymerization (NBO/T), a second set in graphite capsules to expand the data set to higher pressures and temperatures. Carbon concentrations were analyzed by secondary ionization mass spectrometry (SIMS) and Raman spectra were collected to identify C-species in the silicate melt. Partition coefficients are governed by the solubility of C in the silicate melt, which varies from 0.01 to 0.6 wt%, while metal melts contain similar to 7 wt% C in most samples. C solubility in the silicate melt correlates strongly with NBO/T, which, in olivine capsules, is mostly a function of temperature. Carbon partition coefficients D-C(metal/silicate) at 1.5 GPa, 1300-1750 degrees C decrease from 640(49) to 14(3) with NBO/T increasing from 1.04 to 3.11. For the NBO/T of the silicate Earth of 2.6, D(C)(metal/silicate )is 34(9). Pressure and oxygen fugacity show no clear effect on carbon partitioning. The present results differ from those of most previous studies in that carbon concentrations in the silicate melt are comparatively higher, rendering C to be about an order of magnitude less siderophile, and the discrepancies may be attributed to differences in the experimental protocols. Applying the new data to a magma ocean scenario, and assuming present day mantle carbon mantle concentrations from 120 to 795 ppm, implies that the core may contain 0.4-2.6 wt% carbon, resulting in 0.14-0.9 wt% of this element for the bulk Earth. These values are upper limits, considering that some of the carbon in the modern silicate Earth has very likely been delivered by the late veneer. (C) 2020 Published by Elsevier B.V.

Automated summary

Carbon distribution in accreting Earth and planetesimals was studied experimentally, revealing differences in carbon concentrations between silicate and metal melts. Solubility of carbon in the silicate melt depends on NBO/T and decreases with increasing temperature. Pressure and oxygen fugacity do not significantly affect carbon partitioning.