Eutectic melting in the system Fe–S to 44 GPa

Melting relations in the Fe–S system were measured in the laser-heated diamond cell by analyzing recovered samples using scanning electron microscopy. The eutectic melting temperatures are lower than in previous diamond-cell studies using in situ melt detection and are in good agreement with recent multi-anvil measurements at lower pressures. The new measurements, ranging from 22 to 44 GPa, show that the eutectic composition becomes increasingly iron-rich with increasing pressure. Thus, if sulfur is the main light component in the Earth's core, the sulfur content would have to be below 10 wt.%, because the density contrast between the inner and outer cores implies that the core composition is on the iron-rich side of the eutectic. At 44 GPa, we find eutectic melting temperatures that are about 1000 K below that of pure iron. Those eutectic temperatures suggest even lower temperatures in the Earth's core than previous measurements on pure iron and, for Mars, they imply a liquid core.