Shock compression of Fe‐Ni‐Si system to 280 GPa: Implications for the composition of the Earth's outer core

The shock Hugoniot of an Fe‐9 wt %Ni‐10 wt %Si system as a model of the Earth's core has been measured up to ~280 GPa using a two‐stage light‐gas gun. The samples had an average density of 6.853 (±0.036) g/cm 3 . The relationship between shock velocity ( U s ) and particle velocity ( u p ) can be described by U s (km/s) = 3.95 (±0.15) + 1.53 (±0.05) u p (km/s). The calculated Hugoniot temperatures and the melting curve indicate that the model composition melts above a shock pressure of ~168 GPa, which is significantly lower than the shock‐melting pressure of iron (~225 GPa). A comparison of the pressure‐density ( P‐ρ ) profiles between the model composition and the preliminary reference Earth model gives a silicon content close to 10 wt %, necessary to compensate the density deficit in the Earth's outer core from seismological observations, if silicon is present as a major light element in the Fe‐Ni core system.