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The rheology of subducting lithosphere in the mantle transition zone controls the dynamics of subduction, the ability of a slab to penetrate the 660 km discontinuity, and the occurrence and distribution of deep-focus earthquakes. It has been proposed that the transformation of olivine to the high-pressure polymorphs wadsleyite and ringwoodite can result in a transient decrease in strength of several orders of magnitude. This occurs by a reduction in grain size during the phase transformation that causes the deformation mechanism to change from a dislocation glide regime to grain size sensitive diffusion creep. This process is likely to operate in the interior of a subducting slab where temperatures are relatively low and would result in the development of a complex rheological structure. The strength reduction is transient because grain growth of the high-pressure phase results in a return to a dislocation glide regime. A recent attempt to model the evolution of grain size and rheology during subduction has been based on a simple kinetic model involving grain boundary nucleation and interface-controlled growth (Riedel and Karato, 1997, EPSL 148, 27). Recent high-pressure experiments on San Carlos olivine at 15-20 GPa and 600-1400°C demonstrate several complexities that would reduce the grain size significantly more than predicted by this study (see also Sections 3.1a and 3.1e). First, the development of reaction rims of wadsleyite or ringwoodite on olivine grain boundaries results in the development of elastic strain energy because of the 6-8% volume decrease. At low temperatures, where viscous relaxation is slow, this strain energy inhibits the extent of growth, thus contributing to a small product grain size. In addition, the stored strain energy can drive further grain size reduction through dynamic recrystallization of the reaction rim. Second, intracrystalline nucleation occurs readily on stacking faults in olivine at pressures >= 17 GPa and can lead to the replacement of coarse-grained olivine by fine-grained high-pressure phases. If the stacking faults are created by non-hydrostatic stress, the nucleation rate and therefore the final grain size will be stress dependent. Thus, in addition to temperature and pressure, the evolution of grain size in subducting lithosphere during phase transformations is likely to be controlled by both self-induced and externally-applied stresses.