Melting experiments on Fe-C-O to 200 GPa; liquidus phase constraints on core composition

Recent theoretical calculations suggested that carbon and oxygen are important light elements in the Earth ’ s inner and outer core, respectively. We performed melting experiments on the Fe-C-O system and obtained ternary liquidus phase relations at ∼ 50, ∼ 136, and ∼ 200 GPa based on textural and compositional characterisations of recovered samples. Considering the previously reported Fe-C binary eutectic liquid composition, these results are extrapolated to 330 GPa, which constrains C and O concentrations in the liquid outer core that crystallises Fe at the inner core. Theory has predicted a possible range of the solid inner core composition in Fe-C-S-Si that explains seismological observations. The compositions of liquids Fe-C-O-S-Si in equilibrium with such solid Fe-C-S-Si alloys are calculated with the solid-liquid partition coefficient of C obtained in this study along with those of S and Si in the literature. These liquid compositions, however, do not satisfy constraints from both outer core observations and the liquidus phase relations examined in this study, suggesting that the inner core is not Fe-C-S-Si alloy but may include H as an important impurity element.