Low viscosity and high attenuation in MgSiO3 post-perovskite inferred from atomic-scale calculations

Weak cubic CaSiO3 perovskite in the Earth's mantle

Cubic CaSiO₃ perovskite is a major phase in subducted oceanic crust, where it forms at a depth of about 550 kilometres from majoritic garnet^1,2,28. However, its rheological properties at temperatures and pressures typical of the lower mantle are poorly known. Here we measured the plastic strength of cubic CaSiO₃ perovskite at pressure and temperature conditions typical for a subducting slab up to a depth of about 1,200 kilometres. In contrast to tetragonal CaSiO₃, previously investigated at room temperature^3,4, we find that cubic CaSiO₃ perovskite is a comparably weak phase at the temperatures of the lower mantle. We find that its strength and viscosity are substantially lower than that of bridgmanite and ferropericlase, possibly making cubic CaSiO₃ perovskite the weakest lower-mantle phase. Our findings suggest that cubic CaSiO₃ perovskite governs the dynamics of subducting slabs. Weak CaSiO₃ perovskite further provides a mechanism to separate subducted oceanic crust from the underlying mantle. Depending on the depth of the separation, basaltic crust could accumulate at the boundary between the upper and lower mantle, where cubic CaSiO₃ perovskite may contribute to the seismically observed regions of low shear-wave velocities in the uppermost lower mantle^5,6, or sink to the core-mantle boundary and explain the seismic anomalies associated with large low-shear-velocity provinces beneath Africa and the Pacific^7-9.

Prediction of Mechanical Twinning in Magnesium Silicate Post-Perovskite

The plastic properties of MgSiO3 post-perovskite are considered to be one of the key issues necessary for understanding the seismic anisotropy at the bottom of the mantle in the so-called D" layer. Although plastic slip in MgSiO3 post-perovskite has attracted considerable attention, the twinning mechanism has not been addressed, despite some experimental evidence from low-pressure analogues. On the basis of a numerical mechanical model, we present a twin nucleation model for post-perovskite involving the emission of 1/6 <110> partial dislocations. Relying on first-principles calculations with no adjustable parameters, we show that {110} twin wall formation resulting from the interaction of multiple twin dislocations occurs at a twinning stress comparable in magnitude to the most readily occurring slip system in post-perovskite. Because dislocation activities and twinning are competitive strain-producing mechanisms, twinning should be considered in future models of crystallographic preferred orientations in post-perovskite to better interpret seismic anisotropy in the lowermost lower mantle.

Seismic anisotropy of the D″ layer induced by (001) deformation of post-perovskite

Crystallographic preferred orientation (CPO) of post-perovskite (Mg,Fe)SiO₃ (pPv) has been believed to be one potential source of the seismic anisotropic layer at the bottom of the lower mantle (D″ layer). However, the natural CPO of pPv remains ambiguous in the D″ layer. Here we have carried out the deformation experiments of pPv-(Mg_0.75,Fe_0.25)SiO₃ using synchrotron radial X-ray diffraction in a membrane-driven laser-heated diamond anvil cell from 135 GPa and 2,500 K to 154 GPa and 3,000 K. Our results show that the intrinsic texture of pPv-(Mg_0.75,Fe_0.25)SiO₃ should be (001) at realistic P–T conditions of the D″ layer, which can produce a shear wave splitting anisotropy of ∼3.7% with V_SH>V_SV. Considering the combined effect of both pPv and ferropericlase, we suggest that 50% or less of deformation is sufficient to explain the origin of the shear wave anisotropy observed seismically in the D″ layer beneath the circum-Pacific rim.

The source of the anisotropic layer (D'' layer) at the bottom of the lower mantle remains unclear. Here, using high pressure and temperature experiments, the authors find that seismic anisotropy observed at the D'' layer is caused by 50% deformation of the minerals post-perovskite and ferropericlase.

Modeling defects and plasticity in MgSiO3 post-perovskite: Part 3-Screw and edge [001] dislocations

In this study, we investigate the complex structure of [001] screw and edge dislocation cores in MgSiO₃ post-perovskite at the atomic scale. Both [001] screw and edge dislocations exhibit spontaneous dissociation in (010) into two symmetric partials characterized by the presence of <100> component. In case of edge dislocations, dissociation occurs into ½<101> partials, while for the screw dislocations the <100> component reaches only 15%. Under applied stress, both [001](010) screw and edge dislocations behave similarly. Above the Peierls stress, the two partials glide together while keeping their stacking-fault widths (~11 and ~42 Å for the screw and edge dislocations, respectively) constant. The Peierls stress opposed to the glide of [001](010) screw dislocations is 3 GPa, while that of edge dislocations is 33% lower. Relying on the observed characteristics of the dislocation cores, we estimate the efficiency of [001](010) dislocation glide under the P-T conditions relevant to the lowermost mantle and demonstrate that dislocation creep for this slip system would occur in the so-called athermal regime where lattice friction for the considered slip system vanishes when the temperature rises above the critical T _a value of ~2,000 K.