• Andrault, Denis
• Fiquet, G.
• Morard, Guillaume
• Sanloup, C.
• Mezouar, M.
• Petitgirard, S.
• Guignot, N.

Abstract

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.

Topics

• density
• impedance spectroscopy
• phase
• x-ray diffraction
• simulation
• iron
• interstitial
• phase diagram
• Sulphur