The eutectic liquid composition in the Fe-Fe3S binary system at the core pressure range

Abstract:

Sulfur is considered to be an important component in the Earth's core because it is depleted in the crust and mantle compared to other volatile elements and often included in iron meteorites. Here we examine the liquidus phase relations in the Fe-Fe₃S binary system between 38 and 138 GPa based on characterization of a sample recovered from a melting experiment at high pressure and temperature in a laser-heated diamond-anvil cell. Both Fe-8wt.%S and Fe-14wt.%S samples were employed as starting materials, which were homogeneous mixtures of fine-grain Fe and FeS (<1 µm) prepared by induction melting and rapid quenching technique [Morard et al., 2011 PCM]. We used a focused ion beam (FIB) equipped with energy dispersive X-ray spectrometry for textural and chemical characterizations of recovered samples. The samples exhibited a melting texture with quenched liquid alloy at the hottest part and solid Fe or Fe₃S at its outside. In a couple of samples, the quenched liquid was in direct contact with solid Fe₃S, and solid Fe was also present right next to Fe₃S, suggesting that the composition of such liquid is close to a eutectic composition (~10 wt.% S at 66 GPa and ~12 wt.% S at 138 GPa). Indeed, this interpretation is consistent with the results of other experiments obtained in this study. Our data demonstrate that the eutectic liquid composition in the Fe-Fe₃S binary system decreases its sulfur concentration with increasing pressure, which is in agreement with previous studies [Morard et al., 2008 EPSL; Kamada et al., 2012 EPSL]. The sulfur content in the eutectic liquid composition may be less than 10 wt.% at the inner core boundary pressure. The recent shock-wave study by Huang et al.[2013 GRL] suggested 10 wt.% S in the outer core, but the present study indicates that such liquid alloy with 10 wt.% S crystalizes the B2 phase of Fe-S alloy and thus does not support the sulfur-rich outer core.