Melting experiments on Fe–Fe₃S system to 254 GPa

Melting experiments were performed on the Fe–Fe₃S system at high pressures between 34 and 254 GPa in a laser-heated diamond-anvil cell (DAC), using starting materials of fine-grained homogeneous mixtures of Fe and FeS (<500 nm) prepared by induction melting and rapid quenching techniques. Melting phase relations including the liquid/solid partitioning of sulfur were examined on the basis of textural and chemical characterizations of recovered samples using a focused ion beam (FIB) and electron microprobes. The results demonstrate that the sulfur content in eutectic liquid decreases substantially with increasing pressure. The eutectic liquid Fe with 5.7(±0.3) wt.% S coexisted with both solid Fe₃S and Fe containing 3.9(±0.4) wt.% S at 254 GPa and 3550 K. The eutectic liquid at inner core boundary (ICB) pressure includes less sulfur than is required to account for the density deficit of the outer core (≥10 wt.% S). Furthermore, the difference in sulfur concentration between coexisting liquid and solid is not sufficient to account for the observed density jump across the ICB. These indicate that sulfur cannot be a predominant light element in the core.