Magnetic phase diagrams and thermal equations of state of Fe7C3 and Fe3C up to the pressure-temperature conditions of Earth’s core

Abstract:

Iron carbides Fe₇C₃ and Fe₃C have been proposed as candidates for the dominant components of the Earth's inner core to explain its density deficit and velocity discrepancy with respect to those of pure iron at relevant pressure-temperature conditions (e.g. Chen et al., 2012; Chen et al., 2014; Gao et al., 2011). Testing the hypothesis of carbon-rich inner core requires knowledge of the thermal equations of state of the iron carbides to megabar pressures. Existing experimental data, however, are restricted to either ambient temperature or the pressures near the top of the lower mantle. In particular, the thermal expansion coefficients of the carbides under high pressures remain poorly constrained. Previous studies showed that both Fe₇C₃ an Fe₃C are Invar-type alloys with extremely low thermal expansions in the ferromagnetic phases, and their thermal expansion coefficients more than triple across the ferromagnetic–paramagnetic transition at 1 bar (Litasov et al., 2015). On the other hand, both iron carbides experience pressure-induced spin transitions under high pressures (e.g. Chen et al., 2012; Prescher et al., 2012), but their effects on the thermal expansion are still unknown.

We conducted synchrotron X-ray diffraction measurements on Fe₇C₃ and Fe₃C up to the core’s pressure and temperature conditions. High pressures up to ~140 GPa were generated by using diamond anvil cells (DAC), while high temperatures were generated using the whole-cell resistive heating device up to 700 K, and using the double-sided laser-heating system up to ~ 3500 K. The new data allow us to construct the magnetic phase diagrams of Fe₇C₃ and Fe₃C and to assess the influences of magnetic transitions on their thermal expansion coefficients. The density profiles of appropriate magnetic phases are calculated and compared with that of the Earth’s inner core to estimate its carbon content.