Dynamic history of the inner core constrained by seismic anisotropy

Daniel A Frost (1) , Marine Lasbleis (2) , Brian Chandler (1) , Barbara Romanowicz (1, 3, 4)

1 EPS - Department of Earth and Planetary Science [UC Berkeley]
2 LPG - Laboratoire de Planétologie et Géodynamique [UMR 6112]
3 CdF (institution) - Collège de France
4 IPGP (UMR_7154) - Institut de Physique du Globe de Paris

Progressive crystallisation of Earth's inner core drives convection in the outer core and magnetic field generation. Determining the rate and pattern of inner core growth is thus crucial to understanding the evolution of the geodynamo. The growth history of the inner core is likely recorded in the distribution and strength of its seismic anisotropy, which arises from deformation texturing constrained by conditions at the inner-core solid-fluid boundary. Here we show from analysis of seismic body wave travel times that the strength of seismic anisotropy increases with depth within the inner core, and the strongest anisotropy is offset from Earth's rotation axis. Then, using geodynamic growth models and mineral physics calculations, we simulate the development of inner core anisotropy in a self-consistent manner. From this we find that an inner core composed of hexagonally close-packed iron-nickel alloy, deformed by a combination of preferential equatorial growth and slow translation, can match the seismic observations without requiring hemispheres with sharp boundaries. Our model of inner core growth history is compatible with external constraints from outer core dynamics, and supports arguments for a relatively young inner core (~0.5-1.5 Ga) and a viscosity >10 18 Pa-s. Main text The presence of seismic anisotropy-the dependence of seismic wavespeed on direction of propagation-in the inner core (IC) was proposed over 30 years ago to explain the early arrival times of IC sensitive seismic body waves (PKPdf) travelling on paths parallel to the Earth's rotation axis 1,2 and anomalous splitting of core-sensitive free oscillations 3. This anisotropy is thought to result from alignment of iron crystals caused by deformation in a flow field induced by the evolution of the core, i.e. deformation texturing. Previously, different geodynamic 4 and plastic deformation mechanisms 5 were explored to explain the variation of PKPdf travel times with angle of the ray path with respect to the rotation axis. Here, we combine geodynamic modelling of flow in the IC, allowing for slow lateral translation, with present knowledge on the mineralogy and deformation mechanisms proposed for the IC to explain spatial patterns of observed seismic travel times in an updated dataset.

Domaines: Sciences de la Terre; Géophysique [physics.geo-ph]

Daniel A Frost, Marine Lasbleis, Brian Chandler, Barbara Romanowicz. Dynamic history of the inner core constrained by seismic anisotropy. Nature Geoscience, 2021, ⟨10.1038/s41561-021-00761-w⟩. ⟨hal-03252777⟩