Watt's going on with the quartz recrystallized grain size?? Application of the wattmeter for predicting quartz recrystallized grain size

Due to the abundance of quartz in the continental crust and the piezometric relationship between recrystallized (rxl) grain size and stress, a practical method for estimating stress in the crust is measuring quartz rxl grain size. Published rxl grain size piezometers for quartz were calibrated from deformation experiments conducted in axial compression (AC). Recent analyses of quartz aggregates deformed in the shear geometry do not match the piezometric relationship found in AC experiments; for a given grain size, the majority of shear experiments show systematically larger stresses than AC experiments. This difference in rxl grain size piezometry has important implications for our understanding of stress in the crust because most naturally deforming rocks deform in the shear geometry. To explore this difference in piezometric relationships, we applied the wattmeter, a grain size evolution model based on the dynamic balance between rates of grain growth and grain size reduction. The wattmeter is sensitive to grain growth kinetics; as part of our analysis, we recalculated a quartz grain growth law, removing data from previous published studies in which samples show abnormal grain growth and/or pinning from secondary phases. We determined an activation enthalpy of 155 kJ/mol, which is significantly smaller than the previous estimate of 215 kJ/mol and similar in magnitude to the activation enthalpy determined for dislocation creep flow laws for quartz (~115-140 kJ/mol), suggesting there is only a small temperature dependence between stress and grain size. Finally, we use two flow laws calibrated for wet quartzite, where the flow laws are distinguished based on the c-axis fabric of the sample, either prism or basal fabrics (Tokle et al. 2019) to plot two wattmeters in stress-grain size space. The two wattmeters not only accurately predict the stress-rxl grain size relationships of the experimental samples, they show a strong correlation between the c-axis fabric of the samples and the two wattmeter relationships. This observation suggests there are two distinct piezometric relationships between rxl grain size and stress in quartz which are a function of the c-axis fabric. Our analyses suggest the current rxl grain size piezometer may significantly underestimate the stress for natural samples with basal fabrics.