In situ determination of Fe-Fe3S phase diagram and liquid structural properties up to 65 GPa
Résumé
Lighter elements than iron such as sulphur are required in the Earth's core to account of the core density deficit. Accurate determination of the evolution of the Fe-FeS phase diagram at high pressure is essential to determine sulphur amount in the Earth's core. Ab initio calculations predict extensive solubility of S in solid Fe at core pressures of 330 GPa, whereas multi anvil quench analysis exhibits deep eutectic system at moderate pressure of 21 GPa. In this study, we investigated the Fe-rich part of Fe-FeS phase diagram up to 65 GPa and 2200 K using in situ angle dispersive X-ray diffraction. We report a uniform increase with pressure of the eutectic temperatures (TEut), of about 15 K/GPa. Above 50 GPa, we evidence a decrease of S content in eutectic liquid with increasing pressure. Extrapolating this trend to inner core boundary pressures suggests that S cannot account for the 10 wt.% outer core density deficit and that other light elements, such as Si and O, are needed. Diffraction pattern recorded at 42 GPa and 2150 K was selected for structural investigations of the Fe-S liquid. By applying liquid structure simulation based on Gaussian distribution of atoms around crystalline positions, a good agreement has been found with hcp Fe model-structure, rather than with Fe3S structure. It suggests that S acts as an interstitial impurity in the liquid state. Therefore, S could have a relatively minor effect on sound velocities in liquid outer core.