Melting behavior of Fe-O-S at high pressure: A discussion on the melting depression induced by O and S

Shock compressed to pressures from 106 GPa to 232 GPa, the sound velocities were determined in Fe-O-S (90/8/2 in wt. %), regarded as the candidate compositions of the Earth's outer core. A discontinuity in sound velocity versus pressure relation was observed, which indicated that the shocked sample is initially melted at 149 GPa and completely melted at 167 GPa. On the basis of an energy conservation formulation, the calculated equilibrium melting temperature is about 3880 +/- 500 K at 167 GPa. Taking this point as a reference, a high-pressure melting curve can be inferred through Lindemann Law, which is again confirmed by the release melting temperatures of 3020 +/- 500 K and 3200 +/- 500 K measured at 90 GPa and 106 GPa, respectively. When extrapolating this melting curve to inner core-outer core boundary (330 GPa), the melting temperature of this Fe-O-S composition is about 5400 +/- 500K. Compared with the melting temperature 6000 K for pure iron at 330 GPa, the corresponding melting temperature depression is about 600 K due to the effect of O and S.