Equation of State for Shocked Fe-8.6 wt% Si up to 240 GPa and 4,670 K

Using dynamic compression technique, the equation of state for Fe-8.6 wt% Si was measured up to 240 GPa and 4,670 K. A least squares fit to the experimental data yields the Hugoniot parameters C-0 = 4.6030.101 km/s and lambda = 1.5050.037 with initial density rho(0)=7.3860.021 g/cm(3). Based on the Hugoniot data, the calculated isothermal equation of state is consistent with static compression data when the lattice Gruneisen parameter gamma(l) =1.65(7.578/rho) and electronic Gruneisen parameter gamma(e)=1.83. The calculated pressure-density data at 300 K were fitted to a third-order Birch-Murnaghan equation of state with zero pressure the parameters K-0=192.16.3 GPa, K0'=4.710.27 with fixed rho(0 epsilon) =7.578 +/- 0.050 g/cm(3). Under the conditions of Earth's core, the densities of Fe-8.6 +/- 2.0 wt% Si and Fe-3.8 +/- 2.9 wt% Si agree with preliminary reference Earth mode (PREM) data of the outer and the inner core, respectively. These are the upper limits for Si in the core assuming Si is the only light element. Simultaneously considering the geophysical and geochemical constraints for a Si-S-bearing core, the outer core may contain 3.8 +/- 2.9 wt% Si and 5.6 +/- 3.0 wt% S. Plain Language Summary Silicon (Si) may be a dominant light element in the core, and precise evaluation of its amount requires accurate thermal equation of state of Fe-Si alloy. Here we present the Hugoniot data for Fe-8.6 wt% Si, measured up to 240 GPa with two-stage light gas gun. The experimental data allow us to establish reliable thermal equation of state of Fe-Si alloy over a wide pressure-temperature range and to constrain the composition of the core. The results indicate that the densities of Fe-8.6 wt% Si and Fe-3.8 wt% Si agree with the observed density profiles of the outer core and the inner core, respectively. Our model prefers an Fe-Si-S core that contains about 3.8 +/- 2.9 wt% Si and 5.6 +/- 3.0 wt% S in the outer core.