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Deformation experiments on olivine aggregates were performed under hydrous conditions using a deformation-DIA apparatus combined with synchrotron in situ X-ray observations at pressures of 1.5-9.8 GPa, temperatures of 1223-1800 K, and strain rates ranging from 0.8 x 10(-5) to 7.5 x 10(-5) s(-1). The pressure and strain rate dependencies of the plasticity of hydrous olivine may be described by an activation volume of 17 +/- 6 cm(3) mol(-1) and a stress exponent of 3.2 +/- 0.6 at temperatures of 1323-1423 K. A comparison between previous data sets and our results at a normalized temperature and a strain rate showed that the creep strength of hydrous olivine deformed at 1323-1423 K is much weaker than that for the dislocation creep of water-saturated olivine and is similar to that for diffusional creep and dislocation-accommodated grain boundary sliding, while dislocation microstructures showing the  slip or the (100) slip system were developed. At temperatures of 1633-1800 K, a much stronger pressure effect on creep strength was observed for olivine with an activation volume of 27 +/- 7 cm(3) mol(-1) assuming a stress exponent of 3.5, water fugacity exponent of 1.2, and activation energy of 520 kJ mol(-1) (i.e., power-law dislocation creep of hydrous olivine). Because of the weak pressure dependence of the rheology of hydrous olivine at lower temperatures, water weakening of olivine could be effective in the deeper and colder part of Earth's upper mantle.
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