Melting-induced stratification above the Earth’s inner core due to convective translation

Abstract

In addition to its global North-South anisotropy, there are two other enigmatic seismological observations related to the Earth's inner core: asymmetry between its eastern and western hemispheres and the presence of a layer of reduced seismic velocity at the base of the outer core. This 250-km-thick layer has been interpreted as a stably stratified region of reduced composition in light elements. Here we show that this layer can be generated by simultaneous crystallization and melting at the surface of the inner core, and that a translational mode of thermal convection in the inner core can produce enough melting and crystallization on each hemisphere respectively for the dense layer to develop. The dynamical model we propose introduces a clear asymmetry between a melting and a crystallizing hemisphere which forms a basis for also explaining the East-West asymmetry. The present translation rate is found to be typically 100 million years for the inner core to be entirely renewed, which is one to two orders of magnitude faster than the growth rate of the inner core's radius. The resulting strong asymmetry of buoyancy flux caused by light elements is anticipated to have an impact on the dynamics of the outer core and on the geodynamo.

Grain growth and loss of texture during annealing of alloys, and the translation of Earth’s inner core

“…Rather, the grains that grow are likely to be newly nucleated grains that are relatively free of the secondary phase, pushing the secondary phase to the grain boundaries. These newly nucleated, growing grains have a random texture, as seen in Figures 2c and 3b. [17] This suggests that if one of the light elements in the core has low solubility in iron [Sherman, 1995], and if the inner core is convectively translating [Alboussiere et al, 2010], then the inner core texture should weaken from west to east, with length of annealing time after solidification. It is difficult to estimate precisely the time for the texture to weaken because of the uncertainty regarding the grain growth exponent.…”

“…[16] As the directionally solidified iron alloy in the inner core moves eastward as a result of enhanced solidification in the westward hemisphere [Alboussiere et al, 2010;Monnereau et al, 2010], it is annealing. This results in grain growth, though a classical model may not apply because the intergranular phase volume is fixed.…”

“…Another possibility, explored here, is that the inner core is translating eastward due to convection [Alboussiere et al, 2010]. As the recently solidified iron alloy crystals move eastward they anneal and undergo grain growth, sufficient to explain the hemispherical differences in seismic P wave velocity and overall attenuation (by means of scattering [Monnereau et al, 2010]).…”

Constraints from material properties on the dynamics and evolution of Earth’s core

“…There is also evidence for an anomalously dense layer in the lowermost 150 km of the outer core 37 , which probably has a chemical origin 38 . Two explanations have been proposed: the layer could be a stable density-stratified zone of partial melt through which light elements pass by progressive melting and freezing 38 , or parts of the inner core could be melting, releasing excess heavy liquid into the outer core 39 . In either case normal modes would measure the density difference between the inner core and main part of the outer core while body waves would measure the smaller difference between the solid inner core and the heavy liquid in the anomalous layer.…”

The feasibility of thermal and compositional convection in Earth's inner core

“…The thermal and compositional structure of the inner core resulting from its gradual solidification may lead to internal convection (Jeanloz & Wenk 1988;Gubbins et al 2013). Different modes of convection have been proposed to explain some of the seismically observed features of the inner core (Jeanloz & Wenk 1988;Buffett 2009;Alboussière et al 2010;Monnereau et al 2010).…”