DENSITY AND SOUND VELOCITIES OF Fe3C AND IMPLICATIONS FOR THE EARTH'S INNER CORE

Carbon is one of the candidate light elements for the Earth's core. Wood [1993] proposed that for most conceivable sulfur to carbon ratios, Fe₃C could be the major inner core component rather than iron-nickel alloy. A critical test for this hypothesis is to compare the density and sound velocities of Fe₃C with those of the inner core under corresponding pressure and temperature (P-T) conditions.

To date, density of Fe₃C have been experimentally measured using X-ray diffraction at high pressure and ambient temperature [Scott et al., 2001, Li et al., 2002] or at high temperature and ambient pressure using neutron diffraction technique [Wood et al., 2004]. In our study, we carried out X-ray diffraction experiments of Fe₃C at simultaneous high pressure and high temperature (up to 18 GPa and 1523 K), using a T-cup device and synchrotron X-ray diffraction setup at beamline 13-ID of the Advanced Photon Source. Based on these results, we have developed a thermal equation of state of Fe₃C. Our results indicate that addition of carbon to iron can possibly provide a match in density to the Earth's inner core.

We have also carried out nuclear resonant scattering measurements on ⁵⁷Fe-enriched Fe₃C up to 50 GPa at 300 K [Gao et al., 2008]. On the basis of our nuclear resonant inelastic X-ray scattering spectra and existing equation of state [Scott et al., 2001], we have derived the compressional wave velocity V_P(km/s)=-3.99+1.29ρ (density, g/cm³) and the shear wave velocity V_S(km/s)=1.45+0.24ρ(g/cm³) for the high-pressure nonmagnetic phase. The addition of carbon to iron-nickel alloy brings V_P and V_S closer to seismic observations, supporting carbon as a principal light element in the Earth's inner core.