Effect of temperature on sound velocities of compressed Fe3C, a candidate component of the Earth's inner core

The global carbon cycle may involve iron carbide as a component of the Earth's inner core. In this study, we report the first experimental measurements of phonon density of states, sound velocities, and Debye temperatures of Fe3C up to 47 GPa and 1450 K using the nuclear resonant inelastic X-ray scattering (NRIXS) method in a laser-heated diamond anvil cell. Within the investigated pressure and temperature range, the effect of temperature on compressional velocity v(p) falls within the measurement uncertainties. The shear velocity v(s) decreases with increasing temperature in a non-linear fashion: at similar to 15 GPa, v(s) decreases by similar to 12% between 300 K and 740 K; At similar to 46 GPa, temperature produces a similar to 5% reduction between 300 and 960 K and a similar to 6% further reduction between 960 K and 1370-1450 K. The high-temperature v(s) of Fe3C deviates from Birch's law behavior towards smaller values, potentially matching the anomalously low v(s) in the inner core under the relevant pressure and temperature conditions. Our results support Fe3C as a major component of the inner core. (C) 2011 Elsevier B.V. All rights reserved.