Grain Size Piezometry is Affected by Increasing Stress during Cooling

Many, if not most, exposed middle- and lower-crustal shear zones experienced deformation while cooling. We investigate the effect of the strengthening associated with such cooling on microstructures and recrystallized grain size. Typical geologic ratios of strain-per-temperature-change were applied in Griggs Rig general shear experiments on quartzite at rates of 2˚C/hr, 4˚C/hr, and 10˚C/hr from 900 to 800˚C, a shear strain rate of ~ 2×10^-5 sec^-1, and confining pressure ~ 1.1 GPa. Comparisons between these "ramp" experiments and control experiments at constant temperatures of 800 and 900˚C indicate that continuous cooling during deformation produces wider and more skewed distributions of grain sizes. Statistical measures of spread and skew increase for samples that experienced higher cooling rates. Recrystallization does not "keep pace" with evolving stress and the mean recrystallized grain sizes of the cooling-ramp experiments are roughly twice as large as would be expected from the final stresses of the experiments. These results suggest that, in general, samples from exhumational settings can be expected to have grain size distributions that are significantly influenced by cooling during recrystallization, and are microstructurally out-of-equilibrium. Natural rocks that experienced fast cooling rates or slow strain rates will show the most significant effects. If applied directly to natural shear zones, the results suggest that stress during deformation and recrystallization could be 30% larger than indicated by the recrystallized grain size.