Equation of State and Magnetic Properties of Fe3C to Multi-Megabar Pressure with Implications for the Earth's Inner Core

Carbon is one of the major candidates for the principal light element in the Earth’s core. Wood (1993) proposed Fe3C as the dominant inner core component rather than iron-nickel alloy based on thermodynamic calculations. However, equation of states (EoS) of Fe3C derived from experimental data up to 73 GPa (Scott et al., 2001; Li et al., 2002; Lin et al., 2004) and the EoS from a first-principle calculation study (Vočadlo et al., 2002) indicate that Fe3C alone is too light to match the density of the inner core. In this study, we carried out single crystal X-ray diffraction measurements on Fe3C in neon pressure medium up to 200 GPa, well into the pressures of Earth’s core. In addition, we collected synchrotron Mössbauer spectra of Fe3C up to 90 GPa. No structural phase transition was observed upon compression to 200 GPa at 300 K or upon annealing at > 1000 K and 30 GPa. Compression curves and f-F relationship show discontinuities at ~ 6 GPa and ~ 65 GPa. The discontinuity at ~ 6 GPa coincides with the pressure-induced magnetic phase transition (Gao et al., 2008). The discontinuity at 65 GPa may be explained by another magnetic transition between 50 and 70 GPa as indicated by our synchrotron Mössbauer data. The compression data above and below 65 GPa can not be described using the same equation of state (EoS) and must be fitted separately. A smaller derivative of bulk modulus is obtained at pressures above 65 GPa, leading to a good match between the density of the inner core and that of Fe3C at corresponding conditions (e.g., Dziewonski and Anderson, 1981). Our results support carbon as a potential major light element in the inner core. This work is supported by NSF grant EAR0337612 and EAR0609639. References: Dziewonski, A.M., and Anderson, Phys. Earth Planet. Inter., 25:297-356, 1981. Gao et al. (2008), Geophys. Res. Lett., doi:10.1029/2008GL034817. Li, J. et al. (2002), Phys. Chem. Miner., 29(3), 166- 169. Lin, J.F. et al., Phys. Rev. B, 70(21):212405-4, 2004. Scott, H. P. et al. (2001), Geophys. Res. Lett., 28, 1875- 1878 Vočadlo, L., et al. (2002), Earth Planet. Sci. Lett., 203(1), 567-575. 347. Wood, B. J. (1993), Earth Planet. Sci. Lett., 117(3- 4), 593- 607.