Melting Experiments on Fe-O-H: Evidence for Eutectic Melting in Fe-FeH
and Implications for Hydrogen in the Core
We examined liquidus phase relations in Fe-O+/-H at ~40
and ~150 GPa, and subsolidus phase equilibria in Fe-FeH.
While it has been speculated that Fe and FeH form continuous solid
solution to core pressures, our experiments show the coexistence of the
H-poor hcp and H-rich fcc phases in the Fe-FeH system. Considering
higher melting temperature of stoichiometric FeH than that of Fe-FeH, it
indicates eutectic melting between Fe and FeH. It is consistent with the
liquidus phase diagram in Fe-O-H, which implies the Fe-FeH binary
eutectic liquid composition of FeH0.42 at ~40 GPa. We
estimated the outer core liquid composition to be Fe + 2.9-5.2% O +
0.03-0.32% H + 0-3.4% Si + 1.7% S by weight, based on the liquidus
phase relations, solid-liquid partitioning, and outer/inner core
densities and velocities, indicating that O and either H or Si are
important core light elements.