Sound Velocities and Densities of Liquid Fe-8.6wt% Si under Earth’s Outer Core Conditions: Implications for a Si-rich Core

Using a two-stage light gas gun, the sound velocities of Fe-8.6 Si (weight percent) were measured up to 298 GPa and 5500 K. A discontinuity in sound velocities along Hugoniot indicates the onset of melting at 216.4 GPa (4200 K) and complete melting at 260.5 GPa (4530 K). When extrapolated the melting temperature via Lindemann Law, the melting temperature of Fe-8.6Si is 5000 K at the inner core boundary pressure of 330 GPa. Combing the measurements of Hugoniot data and sound velocities, we established a thermodynamic model to calculate the equation of state (EOS) along solidus, liquidus and liquid Fe-Si alloys. The calculated EOSs and sound velocities for Fe and Fe-8.6 Si are consistent with measurements by static and dynamic compression. Under the outer condition, densities and sound velocities of Fe-8.6Si simultaneously match the seismic data of the outer core. The maximum amount of Si in the outer core is 7.6~8.6 wt % for a pure Fe-Si core.