Coupled Precipitation of Light Elements from the Core into the Mantle: Importance for Powering Earth's Dynamo

Earth has had a global magnetic field for at least 3.5 billion years, but recent high conductivity measurements make it difficult to explain this duration with energy from cooling and inner-core growth alone. Chemical interactions between the core and mantle allow for precipitation of light elements (e.g., magnesium, silicon, and oxygen) from Earth's core, which releases significant gravitational energy for powering the dynamo. The rate of precipitation is controlled by the temperature and compositional evolution of the core and mantle. We develop a new framework of coupled thermo-chemical evolution of the Earth by combining models for Earth's thermal evolution and Earth's core energetics with a chemical model for the coupled light element precipitation from the core and its interaction with the overlying mantle layer. The precipitated material changes the composition of an interaction layer at the base of the mantle, which in turn is continuously being swept away by the background mantle convection. In addition, we allow the precipitation rate of all three species (MgO, FeO, and SiO₂), thus providing a key interaction not considered by most previous studies.

We find that, although MgO, SiO₂, and FeO precipitation can each dominate entropy production for a different set of reported equilibrium constants and initial state, the three species together can explain the duration of Earth's magnetic field across a range of plausible scenarios. We conclude that the core loses ~1-2 wt% silicon and oxygen over Earth's history, indicating the core must have formed in relatively oxidizing conditions to explain present-day seismic observations. Additionally, our results show that precipitation does not always have a systematic influence on the timing or size of paleomagnetic signal from inner-core nucleation. However, the onset of precipitation produces jumps in paleomagnetic intensity at various points through time that could be used to constrain Earth's thermochemical evolution.