Fe-FeH Eutectic Melting Curve and the Estimates of Earth's Core Temperature and Composition

Abstract

Fe and FeH form a binary eutectic system above ∼40 GPa. Here we performed melting experiments in a laser-heated diamond-anvil cell and obtained the Fe-FeH eutectic melting curve between 52 and 175 GPa. Its extrapolation shows the eutectic temperature to be 4,350 K at the inner core boundary (ICB), which is lower than that in Fe-FeSi but is higher than those in the Fe-S, Fe-O, and Fe-C systems. In addition, its dT/dP slope is comparable to those of the melting curves of Fe and FeH endmembers, suggesting that the eutectic liquid composition changes little with increasing pressure and is about FeH_0.6 at the ICB pressure. We also estimated the effect of each light element on depressing the liquidus temperature at 330 GPa based on a combination of binary eutectic temperature and composition and found that the effect is large for C and S and small for H, O, and Si when considering the amount of each element that reduces a certain percentage of a liquid iron density. Furthermore, we searched for a set of possible outer core liquid composition and ICB temperature (the liquidus temperature of the former at 330 GPa should match the latter), which explains the outer core density deficit that depends on core temperature. The results demonstrate that relatively low core temperatures, lower than the solidus temperature of a pyrolitic lowermost mantle at the core-mantle boundary (CMB), are possible.