Revisiting Earth’s Thermal Evolution: A Solution to the Mantle and Core Paradoxes

English Abstract:

Recent discoveries relating to the thermal and magnetic history of the Earth have revealed that a solid surface and a dynamo generated magnetic field have persisted over almost all of its history. However, the geochemically inferred radioactivity and geodynamically inferred heat loss efficiency predict wide spread mantle melting, a “thermal catastrophe”, only 2−3 Ga. Similarly, paleomagnetic observations that indicate a geodynamo as old as 3.4 Gyr are at odds with the “new core paradox”, which claims insufficient energy to drive the ancient geodynamo prior to inner core nucleation ~ 1 Ga in light of recent revisions to the thermal conductivity of the core. We present a single solution to both paradoxes that invokes a high core heat flow of ~13 TW, a low mantle secular cooling rate of ~13 TW, and an inner core age of ~800 Myr. By including light element depression of the core solidus as the inner core grows and enriches the outer core, core radioactivity is inferred to be ~1 TW or less. This new model has important implications for the history of the geodynamo, including a switch from top driven thermal convection to mixed thermo-chemical convection following inner core nucleation. We propose that this switch is evident in existing paleomagnetic observations.