Velocity-Density Systematics of Fe-5wt%Si: Constraints on Si Content in the Earth's Inner Core

The elasticity of hcp-Fe-5wt%Si has been investigated by synchrotron X-ray diffraction up to 110GPa and 2,100K and by picosecond acoustics measurements at ambient temperature up to 115GPa. The established Pressure-Volume-Temperature equation of state shows that the density of the Earth's inner core can be matched by an Fe-Si alloy with 5wt% Si for all reasonable core temperatures, but that its compressional and shear velocities remain too high with respect to seismological observations. On the other hand, Fe-Si alloys whose velocities are expected to get close to seismological observations are too dense at relevant temperatures. Thus, based on these combined velocity-density measurements, silicon is not likely to be the sole light element in the inner core. Plain Language Summary Combining measurements of sound velocity and density at high pressures and temperatures shows that while Fe-5wt% Si can match the density profile of the Earth's inner core, its sound velocity is too high with respect to seismological observations.