X-ray diffuse scattering of Fe-C alloys shock compressed to 3 Mbar

The density of Earth’s outer core is 8-10% lower than that of liquid iron at the relevant pressure and temperature conditions (137 - 330 GPa, 4500 - 6500 K). Candidate light alloying elements – Si, O, S, C and H – are expected to be dissolved in liquid iron to account for the discrepancy, but the proportions present in the core remain unknown. High quality measurements of the density of liquid iron alloys at multi-megabar (1 Mbar = 100 GPa) pressures are virtually non-existent due to experimental challenges at these pressures in the diamond anvil cell. We have recently made XRD measurements along the Hugoniot of Fe₃C (25 at% C) and Fe-10at%C alloy by laser shock compression at the C-Hutch at the Dynamic Compression Sector (DCS) at APS, Argonne National Lab. Fe-10at%C samples were synthesized by physical vapor deposition directly onto LiF windows. We determine the density of Fe-10at%C liquids directly from the liquid structure factor. These novel equation of state measurements will be discussed in the context of the carbon content of Earth’s liquid outer core. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.