Melting Experiments on Liquidus Phase Relations in the Fe-S-O Ternary System Under Core Pressures

Melting experiments on the Fe-S-O ternary system were performed to 208 GPa in a laser-heated diamond-anvil cell. Compositions of liquids and coexisting solids in recovered samples were examined using a field-emission-type electron microprobe. The results demonstrate that the ternary eutectic point shifts toward the oxygen-rich, sulfur-poor side with increasing pressure, in accordance with changes in eutectic liquid compositions in the Fe-O and Fe-S binary systems. We also found that solid Fe crystallizing from liquid Fe-S-O does not include oxygen, while the partitioning of sulfur into solid Fe is enhanced with increasing pressure. These indicate that oxygen-rich, sulfur-poor liquid crystallizes Fe at the inner core boundary; however, it makes a large density difference between the liquid and solid core, which is inconsistent with observations. Alternatively, we found that a range of C-bearing, S-poor/O-rich liquids account for the density and velocity in the outer core and the density in the inner core.