Olivine crystals align during diffusion creep of Earth's upper mantle.

The crystallographic preferred orientation (CPO) of olivine produced during dislocation creep is considered to be the primary cause of elastic anisotropy in Earth's upper mantle and is often used to determine the direction of mantle flow. A fundamental question remains, however, as to whether the alignment of olivine crystals is uniquely produced by dislocation creep. Here we report the development of CPO in iron-free olivine (that is, forsterite) during diffusion creep; the intensity and pattern of CPO depend on temperature and the presence of melt, which control the appearance of crystallographic planes on grain boundaries. Grain boundary sliding on these crystallography-controlled boundaries accommodated by diffusion contributes to grain rotation, resulting in a CPO. We show that strong radial anisotropy is anticipated at temperatures corresponding to depths where melting initiates to depths where strongly anisotropic and low seismic velocities are detected. Conversely, weak anisotropy is anticipated at temperatures corresponding to depths where almost isotropic mantle is found. We propose diffusion creep to be the primary means of mantle flow.

Mantle superplasticity and its self-made demise.

The unusual capability of solid crystalline materials to deform plastically, known as superplasticity, has been found in metals and even in ceramics. Such superplastic behaviour has been speculated for decades to take place in geological materials, ranging from surface ice sheets to the Earth's lower mantle. In materials science, superplasticity is confirmed when the material deforms with large tensile strain without failure; however, no experimental studies have yet shown this characteristic in geomaterials. Here we show that polycrystalline forsterite + periclase (9:1) and forsterite + enstatite + diopside (7:2.5:0.5), which are good analogues for Earth's mantle, undergo homogeneous elongation of up to 500 per cent under subsolidus conditions. Such superplastic deformation is accompanied by strain hardening, which is well explained by the grain size sensitivity of superplasticity and grain growth under grain switching conditions (that is, grain boundary sliding); grain boundary sliding is the main deformation mechanism for superplasticity. We apply the observed strain-grain size-viscosity relationship to portions of the mantle where superplasticity has been presumed to take place, such as localized shear zones in the upper mantle and within subducting slabs penetrating into the transition zone and lower mantle after a phase transformation. Calculations show that superplastic flow in the mantle is inevitably accompanied by significant grain growth that can bring fine grained (≤1 µm) rocks to coarse-grained (1-10 mm) aggregates, resulting in increasing mantle viscosity and finally termination of superplastic flow.