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 (T-CMB) and at the boundary of the inner/outer core (T-ICB) are estimated to be 3600 +/- 200 < T-CMB < 4310 +/- 350 K and T-ICB similar to 5630 +/- 350 K, respectively. These results provide important constraints on the thermal structure of the Earth's core. (C) 2011 Elsevier B.V. All rights reserved.