The discontinuous effect of pressure on twin boundary strength in MgO

J Van Driel,G Schusteritsch,C J Pickard,D P Dobson,J P Brodholt

doi:10.1007/s00269-019-01079-1

J Van Driel, G Schusteritsch + Show 3 more

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https://doi.org/10.1007/s00269-019-01079-1

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Abstract

MgO makes up about 20% of the Earth’s lower mantle; hence, its rheological behaviour is important for the dynamics and evolution of the Earth. Here, we investigate the strength of twin boundaries from 0 to 120 GPa using DFT calculations together with structure prediction methods. As expected, we find that the energy barrier and critical stress for shear-coupled migration of the 310/[001] interface vary strongly with pressure. However, what is surprising is that the twin boundary also exhibits sudden strong discontinuities in strength which can both weaken and strengthen the boundary with increasing pressure. Since twin boundary migration is a proposed mechanism for both deformation and seismic attenuation in MgO, these results may suggest that MgO can undergo sudden changes in rheology due to transitions in grain boundary structure. The multiplicity of interfaces, however, necessitates the need for further studies to examine the role that phase changes in grain boundary structure play in mediating polycrystalline plasticity in the Earth.

Highlights

The Earth’s lower mantle is thought to be composed of three main phases: (Mg,Fe)SiO3 bridgmanite (Mg-pv), calcium perovskite (Ca-pv) and (Fe,Mg)O ferropericlase (Ringwood 1966)
Twin boundaries are important as their mobility is a mechanism for anelasticity and seismic attenuation, and their structure, strength and mobility are commonly used as an analogue for more general grain boundaries
We find that MgO exhibits strong discontinuities in the strength of the 310/[001] twin boundary, and over parts of the lower mantle this twin boundary is competitive with dislocation mobility, potentially allowing for fast grain growth, high attenuation and low viscosity

Summary

Introduction

The Earth’s lower mantle is thought to be composed of three main phases: (Mg,Fe)SiO3 bridgmanite (Mg-pv), calcium perovskite (Ca-pv) and (Fe,Mg)O ferropericlase (Ringwood 1966). Volumetrically inferior to the combined silicate phases, MgO is relatively ductile and so an interconnected network may control the bulk rheology of the Earth’s mantle (Girard et al 2016; Miyagi and Wenk 2016). This study investigates the mechanical strength of twin boundaries in MgO in relation to their potential significance as a deformation mechanism within Earth’s interior. Work by Harris and co-workers, as well as more recent ab initio studies on a handful of {n10} interfaces have illustrated the influence of pressure on fundamental properties such as interfacial volume VGB and energy EGB (McKenna and Shluger 2009; Verma and Karki 2010; Harris et al 1996; Yokoi and Yoshiya 2018). These, previous studies have shown that similar to single crystals, interfaces may, at certain P-T conditions, undergo a series of structural phase transitions

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