Subgrain-size piezometry as a tool for measuring stress in polymineralic rocks

• Main Author: Goddard, CRM
• Other Authors: Hansen, L, Wallis, D, Kumamoto, K
• Format: Thesis
• Language: English
• Published: 2021
• Subjects: Rheology; Great Slave Lake
• Online Access: https://ora.ox.ac.uk/objects/uuid:70527c6f-bfae-4718-946c-4961da369a51

We present a new subgrain-size piezometer calibrated using electron backscatter diffraction data from experimentally deformed quartz and olivine. Subgrain size is found to be inversely proportional to the applied stress. We tested a variety of different critical misorientations to define a subgrain boundary and concluded that a 1- or 2-degree threshold provides the optimum piezometric relationship. The efficacy of subgrains to record the stress within individual phases in a polymineralic rock was tested using mixtures of olivine and orthopyroxene in a Deformation-DIA apparatus situated on a synchrotron X-ray beamline. For samples with grain sizes large enough to contain subgrains, piezometric measurements in olivine within the original calibration range displayed a good agreement with stresses measured through X-ray diffraction. Results indicated that a fine grain size severely limits the application of subgrain-size piezometry. For orthopyroxene, subgrains were too sparse to conclusively state whether the Goddard et al. (2020) piezometer can be applied to this mineral. Comparison of stresses measured through X-ray diffraction in each phase indicated that orthopyroxene supported a greater proportion of the load than olivine. To explore stress partitioning further, we deformed olivine-orthopyroxene mixtures of various phase proportions in a gas-medium apparatus. The sample with intermediate composition was found to be weaker than would be expected simply from the grain-size sensitivity of traditional deformation mechanisms. Results from subgrain-size piezometry indicated that orthopyroxene is solely responsible for the weakening. We apply the subgrain-size piezometer to quartz in polymineralic aggregates from the Great Slave Lake shear zone. To calculate the depth associated with each stress measurement, we conducted geothermometry. These stress measurements from subgrains are higher than those measured in other equivalent shear zones using grain-size piezometry, suggesting that the two microstructural features ...