Liquidus and solidus temperatures of a Fe-O-S alloy up to the pressures of the outer core: Implication for the thermal structure of the Earth's core

The solidus and liquidus temperatures of the Fe₇₅O₅S₂₀ alloy are determined up to 157GPa using a laser-heated diamond anvil cell combined with in situ X-ray diffraction technique. Fe (fcc/hcp), Fe₃S₂/Fe₃S, and FeO B1/B8/rhombohedral phases are stable under subsolidus conditions. First, Fe₃S₂ or Fe₃S phase melts at a temperature close to the eutectic point of the Fe-Fe₃S system, suggesting that the alloying effect of 5at.% oxygen on the eutectic temperature in the Fe-Fe₃S system is minor. Then FeO melts at several hundreds of degrees Kelvin higher than the solidus, and Fe is a liquidus phase in this system. The liquidus temperature is 260-670K lower than the melting temperature of pure Fe because of the alloying effect of S and O on the melting temperature of Fe. Based on our results, the temperatures at the core/mantle boundary (T_CMB) and at the boundary of the inner/outer core (T_ICB) are estimated to be 3600±200<T_CMB<4310±350K and T_ICB~5630±350K, respectively. These results provide important constraints on the thermal structure of the Earth's core.