Sound velocities of iron carbides (Fe3C and Fe7C3) under core conditions

Abstract:

For a carbon-rich core, iron carbides might be the major phase crystallizing to form the Earth’s solid inner core. On basis of high-pressure experiments and theoretical calculations, Fe₃C, Fe₇C₃ and more recently Fe₂C have been considered as the most stable carbide phase under the inner core conditions. The identity of the stable carbide phase in a carbon-containing inner core is still a topic under active debate. It is crucial to determine the elastic and acoustic properties of the relevant carbide phases to core conditions, in order to test the carbon-rich core composition model. In this study, we have performed nuclear resonant inelastic X-ray scattering (NRIXS) measurements of both Fe₇C₃ and Fe₃C up to core pressures at 300 K and determined their shear-wave (V_S) and compressional-wave (V_P) velocities for comparison with seismic observations of the inner core. The high-pressure magnetic properties of both phases have also been investigated by X-ray Emission Spectroscopy (XES) and Synchrotron Mössbauer Spectroscopy (SMS). Our results show that the magnetic transitions from ferromagnetic to paramagnetic and then to nonmagnetic in Fe₇C₃ and Fe₃C significantly affects their V_S and V_P at high pressures. Extrapolating the sound velocities of the nonmagnetic phases to the inner core conditions, we found that sound velocities, particularly V_S, of the iron carbides are markedly low comparing with iron and other iron-rich alloys, making them compelling candidates to explain the seismic observations of the inner core. Our hypothesis of a carbon-rich core may also be consistent with geochemical and petrological evidence on deep carbon inventory in Earth’s interior.