Is the Earth’s transition zone deforming like the upper mantle?

Abstract:

Solid-state convection of the Earth’s mantle involves lithospheric subduction from and the ascent of plumes towards the Earth’s surface. The Earth’s transition zone influences the extent of whole mantle convection by controlling mass transfer between the upper and lower mantle. This depends on the ability of mantle minerals to flow. Flow behavior of the upper mantle relies strongly on the mechanical behavior of its main constituent, Mg₂SiO₄ olivine, in the ductile regime. Because the first ~300 km of the upper mantle is characterized by a significant seismic anisotropy, it is generally believed that the Weertman type of dislocation creep - a deformation mechanism inducing lattice rotation and crystallographic preferred orientations (CPO) in elastically anisotropic minerals as olivine - contributes to its overall deformation. Indeed, recent dislocation dynamics (DD) simulations (Boioli et al. 2015) have shown that intracrystalline plasticity of Mg₂SiO₄ olivine under relevant upper mantle conditions is accommodated by Weertman creep, where climb of dislocations enables the recovery of dislocation junctions, allowing plastic strain to be efficiently produced by dislocation glide.

Entering the mantle transition zone beyond ~410 km depth, olivine transforms first into its high-P polymorph wadsleyite and at ~520 km into ringwoodite. It remains unclear if deformation processes of these more compact structures of the high-P polymorphs of olivine are similar to those of olivine (Ritterbex et al. 2015; Ritterbex et al. 2016). To address this question, we have combined numerical simulations of thermally activated dislocation glide mobilities together with results from experimental diffusion data, and demonstrate that, in contrast to olivine at upper mantle conditions, dislocation climb velocities are exceeding those of glide in the high-P polymorphs of olivine, inducing a transition of deformation mechanism in the dislocation creep regime from Weertman creep to pure climb creep at geologic relevant stresses. Based on theoretical plasticity modeling and constrained by diffusion data from experiments, the current investigation quantifies steady-state deformation of the main transition zone minerals wadsleyite, ringwoodite and majorite garnet as a function of grain size.

In a recently published paper in Earth and Planetary Science Letters (Ritterbex et al. 2020), we show that these modelings are able to explain a number of key features associated with the mantle transition zone (Figure 1). Intracrystalline plasticity of wadsleyite, ringwoodite and majorite garnet by pure climb creep at geological stresses leads to an equiviscous transition zone if the grain size is ~0.1 mm or larger, matching well the available inverted surface geophysical data. Since pure climb creep does not induce lattice rotation and cannot produce CPO, deformation of the transition zone by this mechanism is compatible with its relative seismic isotropy compared to the upper mantle (Figure 1). However, modeling results also predict that CPO is able to develop along with stress concentrations by the activation of Weertman creep for example in corner flows around cold subducting slabs, something that could induce an increase in subduction resistance, explaining why some slabs stall at the base of the transition zone (Figure 1). On the other hand, viscosity reductions are predicted if grains are smaller than ~0.1 mm when the transition zone silicates are deforming in the diffusion creep regime, which is expected to influence flow dynamics in the interior of cold subducting slabs or across phase transitions (Figure 1).