The unusual capability of solid crystalline materials to deform plastically, known as superplasticity, has been found in metals and even in ceramics1. Such superplastic behaviour has been speculated for decades to take place in geological materials, ranging from surface ice sheets to the Earth’s lower mantle2,3,4,5,6,7,8. 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.
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Technical support from S. Sano, N. Ohashi, S. Ohtsuka, K. Ibe, M. Uchida, H. Yoshida and A. Yasuda is appreciated. Scientific discussions with T. Kubo, S. Honda, T. Takei and D. L. Kohlstedt were valuable. Part of the synthesis of the specimens was supported by S. Sano, Ube Materials. Scientific and editorial comments from C. McCarthy were valuable. This research was supported by the JSPS through a Grant-in-Aid for Young Scientists (A 20684024), by the Earthquake Research Institute’s cooperative research programme (to T.H.) and by a Grant-in-Aid for Young Scientists (A 19686042), a Grant-in-Aid for Scientific Research (B 21360328) and a Grant-in-Aid for Scientific Research on Priority Areas (474-19053008) (to H.Y.).
This file contains Supplementary Figures 1-6 with legends, Supplementary Table 1, Supplementary Results, a Supplementary Discussion and additional references.