Assessment of Quartz Grain Growth and the Application of the Wattmeter to Predict Quartz Recrystallized Grain Sizes

Abstract

Relationships between the recrystallized grain size and stress are investigated for experimentally deformed water-added quartz aggregates. For stresses ≥100 MPa there is a variation in the measured recrystallized grain size for a given stress. This variation correlates with a change in the c-axis fabric in general shear experiments, where samples with larger recrystallized grain sizes for a given stress have dominantly prism <a> c-axis fabrics and samples with smaller recrystallized grain sizes for a given stress have dominantly basal <a> c-axis fabrics. The dislocation creep flow law also changes at conditions where these two c-axis fabrics form (Tokle et al., 2019, https://doi.org/10.1016/j.epsl.2018.10.017). Using the wattmeter model (Austin & Evans, 2007, https://doi.org/10.1130/G23244A.1), different piezometric relationships are quantified for samples that develop prism <a> and basal <a> c-axis fabrics, respectively. The wattmeter model is sensitive to grain growth kinetics; a new grain growth law for quartz is formulated based on reanalysis of microstructures in samples from previous work. The activation enthalpies and water fugacity exponents for our grain growth law and dislocation creep flow laws are the same within error, suggesting the recrystallized grain size versus stress relationships are nearly independent of temperature and water fugacity, consistent with laboratory observations. The wattmeters successfully predict the recrystallized grain size versus stress relationships of all quartzite samples from experiments with added water. These results support the use and extrapolation of the wattmeter model for both experimental and geologic conditions to investigate the stress state and grain size evolution of quartz rich rocks.