Earth’s Early Magnetic Field Powered by Exsolution of Silicates from Liquid Iron

Abstract:

The Earth’s magnetic field is generated by the geodynamo: the process in which the rotation and convection of liquid iron in the outer core generates a magnetic field. Convection in the liquid outer core is driven by a combination of thermal convection and crystallization of the inner core. Paleomagnetic observations show that the Earth’s magnetic field dates back at least 3.45 billion years. However, thermal evolution models suggest that the Earth’s inner core began to crystallize only one billion years ago. While we have a convincing explanation for what has powered the magnetosphere for the last billion years, it is not clear what powered it prior to the solidification of the inner core.

Recent studies suggest the mixture of mantle and core material from the Moon-forming giant impact. Mantle material dissolves in the core at very high post-impact temperatures, forming a homogeneous solution. The mixture becomes thermodynamically unstable as the system cools, creating a heterogeneous system of silicate and metallic compositions. The compositional differences result in convection, where buoyant mantle material rises and denser liquid metal sinks. This movement of liquid iron may be sufficient in creating the Earth’s early magnetic field.

Here, we focus on understanding silicate exsolution from liquid iron by running first-principles molecular dynamics (MD) simulations. The calculations implement density functional theory (DFT) through the Vienna ab initioSimulation Package (VASP), allowing us to observe how the system behaves at an atomic scale. We start with a simple model of the core-mantle boundary that initially consists of a silicate on one side of the simulation cell and an iron-rich liquid on the other side to represent the mantle and core, respectively. The system is then set to target temperature/pressure conditions to that of early Earth. We compute chemical compositions, reaction rates, reaction mechanisms, solubility, and the energy released by exsolution of silicate components in liquid iron.