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 157 GPa 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 5 at.% 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 260–670 K 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 (TCMB) and at the boundary of the inner/outer core (TICB) are estimated to be 3600 ± 200 < TCMB < 4310 ± 350 K and TICB ~ 5630 ± 350 K, respectively. These results provide important constraints on the thermal structure of the Earth's core.

Research Highlights
► The melting temperatures of the Fe75O5S20 alloy are determined up to 157 GPa based on in-situ X-ray diffraction in order to investigate the thermal structure of the Earth's core.
► Due to the alloying effect of S and O, the liquidus temperature is 260–670 K lower than the melting temperature of pure Fe.
► The temperatures at the core/mantle boundary and at the boundary of the inner/outer core are estimated to be 3600–4310 K and 5630 K, respectively.