Shear deformation of polycrystalline wadsleyite up to 2100 K at 14-17 GPa using a rotational Drickamer apparatus (RDA)

Shear deformation experiments on polycrystalline wadsleyite (water content, similar to 200-2200 H/10(6) Si) have been conducted at 14.4-17.0 GPa, 1690-2100 K, and strain rates of 2.6-16 x 10(-5) s(-1) using a rotational Drickamer apparatus (RDA) at a synchrotron facility. The stress was measured from the orientation dependence of lattice spacing for the (013), (211), (141), (240) and (244) planes. On the basis of the mechanical and microstructural observations, we infer that deformation occurs by exponential creep through the Peierls mechanism at relatively low temperatures of 1690-2030 K. However, a sample deformed at the temperature of 2100 K showed significant grain-size reduction, and most of small grains are dislocation-free, although sub-boundaries were observed in some grains in the sample. We interpret these observations as evidence for dynamic recrystallization and that diffusion creep (and grain boundary sliding) plays an important role after dynamic recrystallization caused by power law creep. Consequently, the strength observed in the high-temperature conditions determined by the present study provides an important constraint on strength of diffusion creep and a lower limit for that of the power law dislocation creep. We conclude that the strength of wadsleyite in the power law dislocation creep is higher than or comparable to that of olivine and the strength of wadsleyite in the Peierls regime is similar to that of olivine.