Phase relations of Fe 3 C and Fe 7 C 3 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 Fe 3 C and Fe 7 C 3 using X‐ray diffraction in a laser‐heated diamond cell up to 185 GPa and 5200 K. Our results show that the starting Fe 3 C sample decomposes into a mixture of solid orthorhombic Fe 7 C 3 and hcp‐Fe at above 145 GPa upon laser heating and then transforms into Fe‐C liquid and solid Fe 7 C 3 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 Fe 3 C 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 Fe 7 C 3 in Fe‐rich Fe‐C system at Earth's core conditions indicate that Fe 7 C 3 could solidify out of the early Earth's molten core to become a constituent of the innermost inner core.