12 Application of Modern Techniques to Address Fundamental Problems in Planetary Geology

Thursday, 2 April 2009
1:00 PM - 5:00 PM

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

Paper number: 12-6
Program: Symposia Sessions
Time: 2:40 PM – 3:00 PM

Authors

  • Lili Gao Presenting Author
    University of Illinois at Urbana-Champaign, Urbana, IL
  • Bin Chen
    University of Illinois at Urbana-Champaign, Urbana, IL
  • Esen Alp
    Argonne National Laboratory, Lemont, IL
  • Wolfgang Sturhahn
    Argonne National Laboratory, Lemont, IL
  • Jie Li
    University of Illinois at Urbana-Champaign, Urbana, IL

Abstract

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 VP(km/s)=-3.99+1.29� (density, g/cm3) and the shear wave velocity VS(km/s)=1.45+0.24�(g/cm3) for the high-pressure nonmagnetic phase. The addition of carbon to iron-nickel alloy brings VP and VS closer to seismic observations, supporting carbon as a principal light element in the Earth's inner core.