Title: Thermal and Compositional Evolution of the Core
Abstract: 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 high enough to produce lower-mantle melting and may have dissolved substantial quantities of elements (such as Mg), which later exsolved. 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, with exsolution as a possible additional source term. The core–mantle boundary (CMB) heat flux is estimated at 12 ± 5 TW and is sufficient to drive a dynamo dissipating 0.1–3.5 TW at present. Geodynamo activity started at 3.5 Gy BP at the latest and could have been sustained without an inner core being present. Prior to inner-core formation, some fraction of the lower core was probably stably stratified; a buoyant stable layer may also have gradually developed at the top of the core, whether from inner-core growth, exsolution, or reaction with the mantle. Our baseline model suggests inner-core formation at 0.5 Gy BP. Uncertainty in model parameters could extend the age to 1.7 Ga, but unless a thick, insulating layer exists at the top of the core, the geodynamo must have operated long before inner-core formation. The change in core temperature over 4 Gy was 600–1600 K, implying an early lower mantle that was extensively molten. This molten layer was probably responsible for enhanced CMB heat flows, which helped drive the early dynamo. Several areas require further study. First, the extent and evolution of core stratification, and its effect on the dynamo, are unclear. Second, the evolution of the CMB heat flux over time is currently poorly understood, particularly the effect of lower-mantle melting, and yet has first-order implications for the thermal and chemical histories of both the core and mantle. And finally, future paleomagnetic measurements may help provide further observational constraints on the evolution of the geodynamo and the onset of inner-core growth.
Publication Year: 2015
Publication Date: 2015-01-01
Language: en
Type: book-chapter
Indexed In: ['crossref']
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Cited By Count: 62
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