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Grain size reduction associated with the transformation of olivine to its high-pressure polymorphs can result in a reduction in strength of subducting slabs by changing the dominant deformation mechanism from dislocation creep to diffusion creep (see Annual Report 1997). This weakening effect will be transient, depending on the kinetics of grain coarsening of the new phases. Knowledge of grain growth kinetics is therefore required to refine existing models of the rheological structure of subducting slabs. To address this problem, we have undertaken grain growth experiments on (Mg,Fe)₂SiO₄ wadsleyite. Samples of San Carlos olivine (Fo₉₀) were hot-pressed and transformed in the multianvil apparatus at 1150°C, 16 GPa for four hours. Characterization by scanning and transmission electron microscopy shows that this process yields a fully dense polycrystalline aggregate of wadsleyite exhibiting a polygonal microstructure with a uniform grain size of approximately 4 µm. Following hot pressing, the temperature was raised to 1400°, 1500° or 1600°C, for periods up to 30 hours, to induce grain growth. The average grain size of the recovered samples has been measured by analysis of orientation-contrast SEM images acquired using a forward-scattered electron (FSE) detector. Preliminary results obtained at 1400°C are presented in Fig. 3.1-16.
 

Fig. 3.1-16: Orientation-contrast images showing evolution of grain size at 1400°C. The average grain size increases from 4.2 ± 0.7 µm in the starting material to 7.0 ± 0.8 µm and 9.6 ± 1.9 µm after 16 and 24 hours, respectively.