Structure of eutectic Fe–FeS melts to pressures up to 17 GPa: Implications for planetary cores

In situ X-ray diffraction experiments have been conducted using the Paris–Edinburgh press to study the Fe–FeS eutectic liquid structure from 3 to 17 GPa near the melting temperature. The eutectic temperature is steadily decreasing over the studied pressure range. The liquid structure evolves discontinuously towards a compact structure, with a non-uniform contraction at pressures between 13 and 17 GPa. We show that this evolution may be correlated to the Fe–S–Si miscibility gap closure (Sanloup, C., Fei, Y., Closure of the Fe–S–Si liquid miscibility gap at high pressure, Phys. Earth Planet. Inter. 147 (2004) 57.) and the magnetic transition observed in Fe3S compound (Lin, J.F., Fei, Y., Sturhahn, W., Zhao, J., Mao, H.K., Hemley, R.J., Magnetic transition and sound velocities of Fe3S at high pressure: implications for Earth and planetary cores, Earth Planet. Sci. Lett. 226 (2004) 33–40.). The occurrence of such structural change in liquids Fe-alloys show that the extrapolation of their physical properties up to Earthʹs core pressure might not be relevant. As Fe–S liquid alloy acquires a compact structure close to that of pure liquid iron at high pressure, sulfur could be considered as one of the possible light element for the Earthʹs core from a pure elastic point of view. In addition, this structural modification could affect the partitioning coefficients of elements between liquid silicates and S-bearing liquid metallic phase.