EFFECT OF CARBON ON THE SEISMIC PROPERTIES SOLID IRON AT HIGH PRESSURES: HIDDEN CARBON RESERVOIR IN EARTH'S CORE

Earth's inner core is known to consist of crystalline iron (Fe) alloyed with a considerable amount of nickel (Ni) and lighter elements, but shear wave (S-wave) travels through the inner core at about half the speed expected for most Fe-rich alloys under relevant pressures. The anomalously low S-wave velocity (v_s) has been attributed to the presence of liquid. The core also exhibits anomalously high Poisson's ratio and those of most Fe-rich alloys are much lower than the observed. Carbon, due to its high cosmic abundance and high solubility in Fe-Ni alloy under the formation and current conditions of outer core, is among the top candidates for the principal light element in the Earth's core. Both iron-carbide phases Fe₃C and Fe₇C₃ have been proposed as components in the Earth's inner core, solidifying from a carbon-containing metallic liquids. Here we report new experimental data from Nuclear Resonant Inelastic X-ray scattering measurements up to core pressures on iron carbide Fe₃C, showing that its sound velocities increase with density at a low rate similar to Fe₇C₃ after going through a pressure-induced spin-pairing transition at 40-50 gigapascals (GPa), as revealed by our X-ray Emission Spectroscopy measurements. Extrapolating to the inner core pressure, we found that the incorporation of carbon in Fe to form Fe₃C and Fe₇C₃ can significantly lower the v_S, but increase the Poisson’s ratio to a level close to the seismologically observed values. We suggest that the alloying of carbon with solid Fe may account for the anomalous elastic properties of the Earth's inner core. Thus, the core may be the largest reservoir of carbon in Earth's deep interiors.