Silicon and Carbon content in the Earth’s inner core constrained from sound velocities at extreme conditions

The inner core is comprised of iron and nickel as the main components. However, the composition of the Earth’s core remains enigmatic to date. The comparison of densities and seismic velocities between pure Fe-Ni alloys and seismological models reveals a density deficit and reduced velocities in the core. These observations can be explained by the addition of 3-7 wt% light elements to Fe-Ni alloys. Possible candidates are H, C, O, Si, and S. To understand the formation, evolution, and dynamics of the core of the Earth and other terrestrial planets, constraining the light element budget is crucial. Silicon could be the dominant light element because of its partitioning behaviour, isotope fractionation, and chemical properties. The low shear velocity (Vs) and high Poisson ratio in the inner core could be explained by the addition of carbon. Indeed, recent ab initio calculations proposed that simultaneous incorporation of Si and C can provide a good match both for density and sound velocities.

In search of the light elements and their amounts, we carried out nuclear inelastic scattering (NIS) and x-ray diffraction experiments in diamond anvil cells at pressures up to 160 GPa. We determined the sound velocities of the Fe-2wt%Si-0.4wt%C-alloy at high pressures and used thermoelastic parameters extracted from the NIS data to extrapolate them to core conditions. Here, we will present the results and discuss the availability of silicon and carbon in the inner core, as well as the possible amount of each element to match seismic observations.