Dislocation creep in MgSiO₃ perovskite at conditions of the Earth's uppermost lower mantle

Abstract

Seismic anisotropy provides an important observational constraint on flow in the Earth's deep interior. The quantitative interpretation of anisotropy, however, requires knowledge of the slip geometry of the constitutive minerals that are responsible for producing rock fabrics. The Earth's lower mantle is mostly composed of (Mg, Fe)SiO₃ perovskite¹, but as MgSiO₃ perovskite is not stable at high temperature under ambient pressure, it has not been possible to investigate its mechanical behaviour with conventional laboratory deformation experiments. To overcome this limitation, several attempts were made to infer the mechanical properties of MgSiO₃ perovskite on the basis of analogue materials^2,3,4,5,6,7. But perovskites do not constitute an analogue series for plastic deformation, and therefore the direct investigation of MgSiO₃ perovskite is necessary. Here we have taken advantage of recent advances in experimental high-pressure rheology⁸ to perform deformation experiments on coarse-grained MgSiO₃ polycrystals under pressure and temperature conditions of the uppermost lower mantle. We show that X-ray peak broadening measurements developed in metallurgy can be adapted to low-symmetry minerals to identify the elementary deformation mechanisms activated under these conditions. We conclude that, under uppermost lower-mantle conditions, MgSiO₃ perovskite deforms by dislocation creep and may therefore contribute to producing seismic anisotropy in rocks at such depths.