Thermal and Compositional Evolution of the Core

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This chapter examines the thermal and compositional evolution of the Earth’s core, from shortly after its formation to the present day. The initial state and composition of the core were determined mainly by the manner in which the Earth accreted. The initial core temperature was probably high enough to produce lower-mantle melting. The light element composition of the core remains uncertain; O and S are the most likely culprits, though Si, H, or C might also be present. The present-day core geodynamo is maintained primarily by compositional convection as the inner core solidifies. The core–mantle boundary (CMB) heat flux is estimated at 10 ± 4 TW and is sufficient to drive a dynamo dissipating 1–5 TW. The evolution of the core to its present-day state involved two events of importance: the initiation of the geodynamo and the onset of inner core formation. Geodynamo activity started at 3.5 Gyr BP at the latest, and could have been sustained without an inner core being present. Theoretical estimates suggest that the inner core probably formed at ∼1 Gyr BP, unless either significant quantities of potassium were present in the core, or both the ohmic dissipation (<0.25 TW) and the CMB heat flow (<4 TW) were very low. The Re–Os–Pt isotopic system has been used to infer that inner-core solidification started by 3.5 Gyr BP, but this hypothesis remains highly controversial. For a moderately dissipative dynamo, the change in core temperature over 4 Gy was probably 200–800 K, implying an early lower mantle that was extensively molten. The CMB heat flux probably evolved in two stages: an early, high heat flux stage, due to the melting of the lower mantle, and potentially generating very strong magnetic fields; and a later, lower heat flux stage. Several areas require further study. There is a discrepancy between estimates of the inner core age based on Os isotope systematics and those based on thermal evolution models. Neither cosmochemical nor geophysical arguments have so far resolved whether the core contains significant potassium. The evolution of the CMB heat flux over time is currently poorly understood, particularly the effect of lower-mantle melting. Several material parameters, particularly the thermal conductivity of iron under core conditions, need to be better known. Finally, future paleomagnetic measurements may help to provide further observational constraints on the evolution of the geodynamo, and thus the thermal evolution of the core.