Partial melting in the iron–sulfur system at high pressure: A synchrotron X-ray diffraction study

Partial melting in the Fe–S system was investigated at high pressures because of its importance to understanding the formation, composition, and thermal structure of the Earth's core. Earlier studies at very high pressure (>25 GPa) took place before the discovery of Fe3S, which compromised the interpretation of those results. Furthermore, they relied on textural criteria for melting that are difficult to apply at high pressure. In this study synchrotron X-ray diffraction was used to monitor coexisting metal and sulfide at high pressures and temperatures, during laser heating in a diamond anvil cell. The criterion for melting was the disappearance of one of the two coexisting phases, and reappearance upon quench. Temperatures of eutectic melting between Fe and Fe3S were bracketed in this way up to 60 GPa, and a lower bound was established at 80 GPa. The accuracy of the melting point measured in these studies was improved through modelling of the axial temperature distribution through the thickness of the sample; this indicated a ∼6% correction to the spectroradiometrically determined temperature. The Fe–Fe3S eutectic composition remains close to 15 wt% S up to 60 GPa.