Constraining the Si composition and thermal history of Earth's liquid core from ab initio calculations

Earth's core has sustained a global magnetic field for much of the last 4 billion years which is, at present, sustained by the power associated with inner core growth. High thermal conductivity of the core suggests the solid inner core is young and models of this predict that there is insufficient power from secular cooling to sustain a geodynamo prior to inner core formation. Precipitation of light elements dissolved into the liquid core offers an alternative power source for the magnetic field in the absence of inner core growth.We present the first ab initio calculations of the silicon partition coefficient at core-mantle boundary conditions and a thermodynamic partitioning model based on interaction parameters which captures previous experimental results. We report our model and its implications for the past and present core composition as well as the effect of silicon precipitation on the early geodynamo. Oxygen competes with silicon in the liquid metal, meaning for one to be abundant, the other must be sparse. We calculate precipitation rates of ~10-4 to 10^-6} wt. % K^-1 for oxygen concentrations of 0.6 to 3.1 wt. %.Incorporating our partitioning model into a classic thermal evolution model of the core coupled to a parameterised model of the solid mantle, we show that precipitation of Si can satisfy constraints of the present inner core size, convective heat flux of the mantle and mantle temperature, all whilst sustaining a magnetic field until inner core formation, but requires that the initial oxygen content of the core was < 3 wt. %. We find that the core inner age is between 840 and 940 Myrs and that the ancient core was hot, with a core mantle boundary temperature of ~4700 K, 3.5 Ga.