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The partitioning of carbon between the core and mantle during the formation of terrestrial planets may have controlled the distribution of carbon in terrestrial planets. However, the abundance of carbon in the Earth's mantle is higher than a prediction based on previous metal-silicate partitioning experiments of carbon at carbon-saturated conditions by more than an order of magnitude. Here, we report new metal-silicate partitioning experiments of carbon at carbon contents of 0.25-0.5 wt%. We show that the metal-silicate partition coefficient of carbon (DCmet/sil) strongly correlates with nonbridging oxygen per tetrahedral cations (nbo/t) of silicate melts at f( O2 )conditions where C-H species are stable. Moreover, the results suggest that (DCmet/sil) at carbon-undersaturated conditions may be lower than that at carbon-saturated condition. Thus, (DCmet/sil) at low carbon concentrations is essentially important to investigate the distribution of carbon in the Earth during the core formation.

Plain Language Summary Carbon, one of the most abundant volatile elements in the Earth's mantle, affects planetary climate and volcanism. Therefore, elucidating when and how carbon in the Earth was distributed is important for understanding the habitability and chemical evolution of the Earth. Core-mantle partitioning of carbon is thought to be a key process for determining the distribution of carbon in terrestrial planets. Here, we experimentally investigated carbon partitioning between metal droplets and silicate melts under high-pressure conditions (4-12 GPa) and 0.25-0.5 wt% carbon concentration. Our results suggest that carbon may not be highly siderophile than previously thought. Therefore, metal-silicate partitioning experiments on carbon at low concentrations are necessary to constrain the distribution of carbon in the Earth during its formation.