Physical properties of Fe-Ni-C liquids at high pressures: A combined experimental and theoretical approach

Abstract:

The Earth’s outer core is believed to be constituted of an alloy of liquid iron and light elements, in order to account for its densities and velocities inferred from seismology. Compared with experimental investigations on physical properties of crystalline iron alloys as candidates for the inner core, there is a remarkable lack of data on properties of liquid iron-rich alloys due to experimental challenges. Liquid properties are usually investigated at pressures under 10 GPa, far below the conditions of the outer core. This lack of data on liquid iron-rich alloys has prompted us to employ a combination of experiments at low-pressures and computations at relevant core conditions. Among the lighter elements considered for the Earth’s core, carbon is considered a leading candidate for the principal light element alloying with iron in the core. In this study, we have determined the structure, velocity, and viscosity of Fe-Ni-C liquids under high pressures in a Paris-Edinburgh Cell by means of X-ray diffraction, ultrasonic interferometry, and sinking/floating sphere viscometry, respectively. A short-range atomic structure transition was observed in the Fe-Ni-C liquids between 4 and 6 GPa. First-principles calculations based on the Density Functional Theory were performed to provide insights into the experimentally-determined structural and viscoelastic properties. We used the low-pressure experimental data to benchmark and validate the theoretical results at similar conditions, and then explored the physical properties of Fe-Ni-C liquids under core conditions by theoretical calculations. The results were used to provide new insights into the carbon-rich outer core composition models for the Earth and other terrestrial planets such as Mercury.