High-pressure study of the system Fe-Fe3S and its implications for sulphur in Earth’s core

Abstract

The composition of the Earth’s core is unknown, beyond being composed of mostly iron and nickel. Seismological observations revealed that the core is less dense than Fe by 7% in the outer core and 4% in the inner core. Cosmochemical evidence from meteorites and the Earth’s mantle suggests that sulphur could be a source of this core density deficit. From high-pressure and -temperature experiment and thermodynamic modelling of the stability of Fe3S, the most Fe rich sulphide at core conditions, I here assess the suitability of sulphur as a component of the core. First, the thermal equation of state for solid Fe3S was newly determined through in-situ high-pressure and -temperature X-ray diffraction experiments in diamond anvil cells (DAC). The results show that existing models overestimated the thermal expansivity. Second, the melting curve of Fe3S was experimentally determined in the DAC. Utilising these experimental findings, I have constructed a thermodynamic model of the system Fe-Fe3S which includes the thermal equation of state for liquid Fe3S. The model accurately reproduces the changes in eutectic composition with pressure from existing literature. The isentropic temperature profile for Fe-6wt%S liquid which represents the maximum sulphur content across the outer core was calculated from the constructed model, constraining a temperature range of 4272-3446 K. The Fe-6wt%S liquid seismic velocity and density profiles derived from the model do not match the seismic model, suggesting sulphur cannot be a major component of the Earth’s core.