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