Phase relations of Fe₃C and Fe₇C₃ up to 185 GPa and 5200 K: Implication for the stability of iron carbide in the Earth's core

We have investigated phase relations and melting behavior of Fe₃C and Fe₇C₃ using X-ray diffraction in a laser-heated diamond cell up to 185 GPa and 5200 K. Our results show that the starting Fe₃C sample decomposes into a mixture of solid orthorhombic Fe₇C₃ and hcp-Fe at above 145 GPa upon laser heating and then transforms into Fe-C liquid and solid Fe₇C₃ at temperatures above 3400 K. Using the intensity of the diffuse scattering as a primary criteria for detecting melting, the experimentally derived liquidus for a bulk composition of Fe₃C fitted with the Simon-Glatzel equation is T_m(K) = 1800 × [1 + (P_m − 5.7)/15.10 ± 2.55]^{1/2.41 ± 0.17} at 24–185 GPa, which is ~500 K higher than the melting curve of iron reported by Anzellini et al. (2013) at Earth's core pressures. The higher melting point and relative stability of Fe₇C₃ in Fe-rich Fe-C system at Earth's core conditions indicate that Fe₇C₃ could solidify out of the early Earth's molten core to become a constituent of the innermost inner core.