Geodynamo origin to inner core growth: Core chemical evolution traced by paleomagnetism

Paleomagnetic, radiometric age, and geochemical data support a geodynamo at least as old as 4.2 Ga. The Hadean geomagnetic field may have been driven by core precipitation of MgO and/or SiO₂; changes in the importance of this mechanism may be preserved in the magnetic record. The strength of the field progressively weakened in Archean to Proterozoic times, suggesting a long-term decrease in power for the geodynamo. A critical point was reached at 565 Ma in the Ediacaran period, when the field strength fell to a level 10 times less than that of present-day (Bono et al., Nat. Geosc, 2019). Other indicators of the state of the geodynamo reveal a rapid frequency of reversals and high non-dipole fields. These observations suggest the Ediacaran geodynamo neared the "weak-field" state, where core kinetic energy approached that of the magnetic energy. Together with predictions from numerical simulations (Driscoll, GRL, 2016; Landeau et al., EPSL 2017), these data support an Ediacaran age for the onset of inner core nucleation. Collapse of the geodynamo was averted by the new energy source; light elements are rejected during inner core solidification driving compositional convection. The transition to a geodynamo powered by inner core growth was a critical stage in the chemical evolution of the core, enabling continuity of magnetic shielding of the atmosphere from erosion by the solar wind. We will discuss new paleomagnetic (paleointensity), geochemical and electron microscope data from key sites in Western Australia, southern and northern Africa, and North America supporting these conclusions.