The role of diffusion-driven pure climb creep on the rheology of bridgmanite under lower mantle conditions

Riccardo Reali,Jeffrey S Pigott,James A Van Orman,Patrick Cordier,Francesca Boioli,Philippe Carrez,Jennifer M Jackson

doi:10.1038/s41598-018-38449-8

Abstract

The viscosity of Earth’s lower mantle is poorly constrained due to the lack of knowledge on some fundamental variables that affect the deformation behaviour of its main mineral phases. This study focuses on bridgmanite, the main lower mantle constituent, and assesses its rheology by developing an approach based on mineral physics. Following and revising the recent advances in this field, pure climb creep controlled by diffusion is identified as the key mechanism driving deformation in bridgmanite. The strain rates of this phase under lower mantle pressures, temperatures and stresses are thus calculated by constraining diffusion and implementing a creep theoretical model. The viscosity of MgSiO3 bridgmanite resulting from pure climb creep is consequently evaluated and compared with the viscosity profiles available from the literature. We show that the inferred variability of viscosity in these profiles can be fully accounted for with the chosen variables of our calculation, e.g., diffusion coefficients, vacancy concentrations and applied stresses. A refinement of these variables is advocated in order to further constrain viscosity and match the observables.

Highlights

Significant insight into the major features that affect the surface of the Earth has been gained through the understanding that global convection animates the mantle to dissipate the internal heat of our planet
Another strong constraint on deep mantle convection is linked to seismic anisotropy, which can provide clues on the active deformation mechanisms
In a thick layer consisting of elastically anisotropic phases, such as the lower mantle, activation of dislocation creep is expected to generate detectable seismic anisotropy, which is inconsistent with observations[20]

Summary

Introduction

Significant insight into the major features that affect the surface of the Earth (seismicity, volcanism, mountain building) has been gained through the understanding that global convection animates the mantle to dissipate the internal heat of our planet. Suggest a rheology contrast located around 1000 km depth[16,17] Another strong constraint on deep mantle convection is linked to seismic anisotropy, which can provide clues on the active deformation mechanisms. Since this parameter is extremely poorly constrained in the mantle, the conditions imposed by the mean grain size, the grain size distribution and its possible evolution have attracted attention recently[23,24]. (Mg,Fe,Al)(Si,Fe,Al)O3 with the orthorhombic perovskite structure, is thought to be elastically anisotropic[25] and is considered to be the main constituent of the bulk lower mantle, along with (Mg,Fe)O ferropericlase and CaSiO3 perovskite[26] The rheology of this mineral is of primary importance to understand and model convection in the mantle and the dynamics of Earth’s interior. Some studies have demonstrated the possibility for diffusion creep and even superplasticity in CaTiO3, a mechanism which could be compatible with seismic observations[39,40,41]

Results

Discussion

Conclusion