Viscosity of Fe-Ni-C Liquids up to Core Pressures and Implications for Dynamics of Planetary Cores

Abstract

The viscosity of iron alloy liquids is the key for the core dynamo and core-mantle differentiation of terrestrial bodies. Here we measured the viscosity of Fe-Ni-C liquids up to 7 GPa using the floating sphere viscometry method and up to 330 GPa using first-principles calculations. We found a viscosity increase at ∼3–5 GPa, coincident with a structural transition in the liquids. After the transition, the viscosity reaches ∼14–27 mPa·s, a factor of 2–4 higher than that of Fe and Fe-S liquids. Our computational results from 5 to 330 GPa also indicate a high viscosity of the Fe-Ni-C liquids. For a carbon-rich core in large terrestrial body, the level of turbulence in the outer core would be lessened approaching the inner core boundary. It is also anticipated that Fe-Ni-C liquids would percolate in Earth's deep silicate mantle at a much slower speed than Fe and Fe-S liquids.