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

Wednesday, 17 December 2014
Bin Chen1, Zeyu Li2, Dongzhou Zhang3, Jiachao Liu2, Wenli Bi4, Jiyong Zhao4, Esen E Alp4, Michael Y. Hu4 and Jie Li2, (1)University of Hawaii at Manoa, Hawaii Institute of Geophysics and Planetology, Honolulu, HI, United States, (2)University of Michigan Ann Arbor, Department of Earth and Environmental Sciences, Ann Arbor, MI, United States, (3)California Institute of Technology, Seismological Laboratory, Pasadena, CA, United States, (4)Argonne National Laboratory, Advanced Photon Source, Argonne, IL, United States
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 (VS) and compressional-wave (VP) 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 VS and VP at high pressures. Extrapolating the sound velocities of the nonmagnetic phases to the inner core conditions, we found that sound velocities, particularly VS, 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.