High‐Pressure Melting Experiments of Fe3C and a Thermodynamic Model of Fe‐C Liquids for the Earth's Core.

Melting experiments of Fe3C were conducted to 85 GPa in laser‐heated diamond anvil cells with in situ X‐ray diffraction and post‐experiment textural observation. From the determined pressure‐temperature conditions of the melting curve for Fe3C, together with literature data on the melting point of diamond and eutectic point of the system Fe‐Fe3C/Fe7C3 under high pressures, we established a self‐consistent thermodynamic model for high‐pressure melting of the system Fe‐C including the mixing parameters for liquids. The results show that mixing of Fe and C liquids is negatively nonideal from 1 bar to the pressure at the center of the Earth. The departure from ideal mixing becomes progressively larger with increasing pressure, which leads to greatly stabilized liquids under core pressures. The modeled carbon content in eutectic melts under core pressures is 3.3–4.4 wt%. From the Gibbs free energy, we derived an internally consistent parameters for Fe‐C outer cores which included the crystallizing points at their bottoms, isentropic thermal profiles, and densities and longitudinal seismic wave speeds (Vp). While the addition of carbon in excess of the eutectic melt composition effectively reduces the density of iron liquid, the Vp of iron liquid is not greatly changed. Therefore, the low density and high Vp of PREM relative to pure iron cannot be reconciled by an Fe‐C liquid. Therefore, the Earth's core cannot be approximated by the system Fe‐C and should include another light element. Plain Language Summary: The presence or absence of carbon in the Earth's central core has long been discussed in order to account for the observed core density lower than iron because the identification of such light elements in the core will provide critical information about the origin and evolution of the solid Earth. The relevance of carbon in the Earth's core is testable by comparing the density and seismic wave velocities of candidate Fe‐C materials with observations. Here we present a new experiment‐based thermodynamic model for the system Fe‐C. We first determined the melting points of Fe3C to 85 GPa. We then assessed thermodynamic parameters of Fe‐C liquids, from the constrained pressure‐temperature conditions of the melting curve for Fe3C, together with literature data. From the established model, we consistently computed the properties of hypothetical Fe‐C liquid outer cores including the density and longitudinal seismic wave velocity. While a carbon concentration greater than 4 wt% effectively reduces the density of iron liquid, the seismic velocity is not greatly changed, and therefore carbon cannot reconcile the low density and high velocity nature of the Earth's outer core. Therefore, the Earth's core cannot be approximated by the system Fe‐C and should include another light element. Key Points: Melting experiments of Fe3C were conducted to 85 GPa in diamond anvil cells with in situ X‐ray diffraction and textural observationA thermodynamic model for melting of the system Fe‐C which is applicable to the center of the Earth was establishedThe Earth's core cannot be approximated by the system Fe‐C and should include another light element [ABSTRACT FROM AUTHOR]