[S-IT17] TRANSPORT PROPERTIES AND PROCESSES IN THE EARTH

[SIT17-07] Sustaining the geodynamo through mantle “conveyor belt” of core precipitates

*Jie Li¹ (1.University of Michigan Ann Arbor)

キーワード：geodynamo, oxide precipitate, thermo-chemical convection, mantle convection, inner core nucleation, paleomagnetic record

Earth’s magnetic field originates in the iron-rich core through convection of molten metallic fluid, a process known as the geodynamo. Paleomagnetic records suggest that the field started at least 3.5 Ga years ago. The geodynamo was likely driven by thermal convection early in the Earth’s history. As the core cooled, compositional buoyancy became the principal source. The present-day dynamo power mainly comes from the gravitational energy released during inner core growth. Recent mineral physics data and dynamic models suggest that thermal convection alone can only sustain the dynamo for the first 0.5-1.0 Ga of the Earth’s history and that the inner core is less than 1 Ga old. Precipitation of light elements such as magnesium, silicon, and oxygen from the core has been proposed as a potential mechanism to power the geodynamo through the Archean and Proterozoic eons, but the mechanism and consequences of oxide-driven dynamo remains poorly understood. Here I present a cross-disciplinary study aimed at testing the hypothesis of sustaining the geodynamo through precipitation of oxides from the core (Mittal et al. 2020 Earth and Planetary Science Letters). We developed a framework of coupled thermo-chemical evolution of the Earth to consider precipitation of multiple light components from the core and their interaction with the overlying mantle layer. The precipitated material accumulates in a layer at the base of the mantle, which is then continuously eroded by mantle convection through a “conveyor belt” like mechanism. We allow the precipitation of three species (MgO, FeO, and SiO2) and consider their interactions. We find that MgO, SiO2, and FeO precipitation may each dominate entropy production depending on the choice of equilibrium constants and initial model states and that the three species together can explain the duration of Earth’s magnetic field across a range of plausible scenarios. Over the Earth’s history, we find that the core can lose ∼1–2 wt.% silicon and oxygen, suggesting that light element precipitation is potentially an important process for the core compositional evolution and core-mantle chemical exchange. Additionally, our results show that, in most cases, precipitation does not, have a systematic influence on the timing of inner-core nucleation or magnitude of the resulting paleomagnetic signal. However, the onset of precipitation of individual species could produce additional sharp increases in paleomagnetic intensity at various points through Earth’s history besides the inner-core nucleation event.