Sound velocity in iron carbide (Fe3C) at high pressure: Implications for the carbon content of the Earth's inner core

We measured compressional sound velocities of Fe3C cohenite at high pressure by inelastic X-ray scattering (IXS). We show that Fe3C follows Birch's law for the longitudinal acoustic velocity VP, namely a linear dependence between velocity and density. This dataset completes the previous sets recently established by Badro et al. (2007) for FeO, FeSi, FeS, and FeS2, and provides new mineralogical constraints on the composition of Earth's core. Our results, combined with data already obtained for other iron alloys, are compared with seismic data. This suggests that a reduced carbon amount in the inner core could reasonably explain density and velocity differences between measurements made on pure iron and seismic models. This conclusion, however, depends on the remaining uncertainty on magnetic structure for a very low carbon content in the iron alloy. It does not preclude the incorporation of another light element in the inner core, such as silicon.