Powering Earth’s dynamo with magnesium precipitation from the core

Abstract

Earth's global magnetic field arises from vigorous convection within the liquid outer core. Palaeomagnetic evidence reveals that the geodynamo has operated for at least 3.4 billion years, which places constraints on Earth's formation and evolution. Available power sources in standard models include compositional convection (driven by the solidifying inner core's expulsion of light elements), thermal convection (from slow cooling), and perhaps heat from the decay of radioactive isotopes. However, recent first-principles calculations and diamond-anvil cell experiments indicate that the thermal conductivity of iron is two or three times larger than typically assumed in these models. This presents a problem: a large increase in the conductive heat flux along the adiabat (due to the higher conductivity of iron) implies that the inner core is young (less than one billion years old), but thermal convection and radiogenic heating alone may not have been able to sustain the geodynamo during earlier epochs. Here we show that the precipitation of magnesium-bearing minerals from the core could have served as an alternative power source. Equilibration at high temperatures in the aftermath of giant impacts allows a small amount of magnesium (one or two weight per cent) to partition into the core while still producing the observed abundances of siderophile elements in the mantle and avoiding an excess of silicon and oxygen in the core. The transport of magnesium as oxide or silicate from the cooling core to underneath the mantle is an order of magnitude more efficient per unit mass as a source of buoyancy than inner-core growth. We therefore conclude that Earth's dynamo would survive throughout geologic time (from at least 3.4 billion years ago to the present) even if core radiogenic heating were minimal and core cooling were slow.

Turbulent Kinematic Dynamos in Ellipsoids Driven by Mechanical Forcing

“…As such, it has been the subject of numerous studies since the seminal work of Larmor and Joseph (1919), and it remains today an active and challenging research domain for theoretical, experimental and numerical approaches (see, e.g., Busse & Simitev, 2015;Cardin & Olson, 2015;Christensen & Wicht, 2015;Roberts, 2015). The situation is even more complex before the onset of inner core solidification, where dynamo action can hardly be sustained by thermal convection only and necessitates additional effects (Badro et al, 2016;O'Rourke & Stevenson, 2016). On Earth today, it is generally agreed that the dynamo is driven by turbulent flows sustained by convection coming from the combination of thermal and compositional buoyancy, with a major role played by the solidification of the inner core (Roberts, 2015).…”

The cross-over to magnetostrophic convection in planetary dynamo systems

“…[9]). It is presently unclear as to which buoyancy source, thermal or compositional, is the dominant convective driver [10,11]. This depends on the thermal conductivity of the core fluid [12,13], which determines how efficiently heat is conducted across the core and how much of this heat is available over to drive thermal convection in the fluid [14].…”

Challenges in Solid Earth Sciences

“…It has been proposed that well-organized heat loss along spreading ridges controls the engine of plate tectonics (Sun et al, 2007). The energy that drives the Earth's dynamo is still not well constrained (e.g., Arevalo et al, 2009;Gale et al, 2013;O'Rourke and Stevenson, 2016). The main driving forces so far proposed are slab pull, mantle convection, and ridge push/ slab sliding.…”