The rate of water penetration in experimentally deformed quartzite: implications for hydrolytic weakening

Abstract

In order to determine the rate at which water penetrates the grains of a quartzite we performed several types of experiments, including water-added deformation and hydrostatic experiments on vacuum-dried samples, and hydrostatic annealing experiments on previously deformed quartzites. Samples of Heavitree quartzite with an average grain size of 200 μm and intragranular water concentration of 1750±420 H/10 6 Si (as measured by FTIR) were vacuum-dried at 800°C, ∼6 Pa for 12 h. After vacuum-drying the grains showed thermal cracks with an average spacing of 50 μm and an intragranular water concentration of 240±60 H/10 6 Si. One constraint on the rate of water penetration into quartz was obtained by deforming vacuum-dried samples with 0.3 wt% (∼20,000 H/10 6 Si) water added at 800°C, 1500 MPa and a strain rate of 10 −6/s, conditions at which as-is quartzite undergoes climb-accommodated dislocation creep. The time between reaching experimental conditions and yielding of the sample was between 2 and 9 h. The samples had yield stresses of 100–250 MPa, values well below the yield stress of 1500 MPa for a vacuum-dried sample. Deformation of the grains was homogeneous on both optical and TEM scales, indicating that the grains had fully equilibrated with the water-related defect that is responsible for water weakening. FTIR measurements are consistent with this interpretation; one of the deformed samples has an intragranular water concentration of 820±160 H/10 6 Si, and vacuum-dried samples hydrostatically annealed for 3 or 10 h at the same conditions as the deformed samples have an average intragranular water concentration of 730±180 H/10 6 Si. To obtain a second constraint on the rate of water penetration, hydrothermal annealing experiments were performed on portions of a sample of Heavitree quartzite which were previously deformed without water added at 700°C, 1200 MPa and 10 −5/s and have a high (∼10 16/m 2) and homogeneous dislocation density. Two samples were annealed at 700°C, 1500 MPa for 24 h: one with 0.2 wt% water and one with no water added. The grains in the water-added sample have a rim approximately 15 μm wide with dislocation recovery microstructures, indicating equilibration with the water-related defect; the sample with no water added has a homogeneous dislocation density throughout the grains which is similar to that of the original deformed sample. The 15-μm-wide recovered rim suggests a penetration rate approximately an order of magnitude faster than the rate of oxygen diffusion under hydrothermal conditions. The observed rate of penetration of the water-related species is consistent with theoretical considerations and calculations which predict a diffusion rate for H 2O that is an order of magnitude faster than the rate of hydrothermal oxygen diffusion. We infer from our experiments that the rate of water penetration into quartz is consistent with diffusion of H 2O.