Melting experiments on the Fe-C binary system up to 255 GPa: Constraints on the carbon content in the Earth's core

Abstract

Phase relations, including the eutectic liquid composition in the Fe–C binary system, remain unclear under the core pressure range, which makes estimating the carbon budget in the Earth's core difficult. To explore this issue, we have conducted melting and subsolidus experiments on Fe–C alloys in a diamond-anvil cell up to 255 GPa. Textural and compositional characterizations of quenched samples show that carbon concentration in the eutectic liquid slightly decreases with increasing pressure and is about 3 wt.% at the inner core boundary (ICB) pressure. The solubility of carbon in solid Fe is found to be almost constant at ∼1.0 wt.%. In situ X-ray diffraction data indicate that Fe forms eutectic melting with Fe3C to 203 GPa and with Fe7C3 at 255 GPa. Previous studies on liquid Fe–C alloys suggested that the density of the outer core is explained by liquid Fe containing 1.8 to 4.2 wt.% C. If the liquid core includes <3 wt.% C as a single light element, hexagonal close-packed (hcp) Fe crystallizes at the ICB. However, the carbon content in such solid Fe is ≤1 wt.%, less than that required to account for the inner core density deficit from pure iron. When the outer core includes ≥3 wt.% C, it forms Fe7C3 at the ICB, whose density is too small for the inner core. Carbon is therefore not a primary light element in the core. Nevertheless, the outer core liquid can be Fe–C–Si, Fe–C–S, or Fe–C–H. Such core liquid crystallizes solid Fe with light elements including less than 1 wt.% C, which may explain the density and the sound velocities observed in the inner core.