THE RATE OF WATER PENETRATION IN EXPERIMENTALLY DEFORMED QUARTZITE, IMPLICATIONS FOR HYDROLYTIC WEAKENING

In order to determine the rate at which water penetrates the grains of a quartzite we performed several types of experiments, including wate r-added deformation and hydrostatic experiments on vacuum-dried sample s, and hydrostatic annealing experiments on previously deformed quartz ites. Samples of Heavitree quartzite with an average grain size of 200 mu m and intragranular water concentration of 1750 +/- 420 H/10(6) Si (as measured by FTIR) were vacuum-dried at 800 degrees C, similar to 6 Pa for 12 h. After vacuum-drying the grains showed thermal cracks wi th an average spacing of 50 mu m and an intragranular water concentrat ion of 240 +/- 60 H/10(6) Si. One constraint on the rare of water pene tration into quartz was obtained by deforming vacuum-dried samples wit h 0.3 wt% (similar to 20,000 H/10(6) Si) water added at 800 degrees C, 1500 MPa and a strain rate of 10(-6)/s, conditions at which as-is qua rtzite undergoes climb-accommodated dislocation creep. The time betwee n reaching experimental conditions and yielding of the sample was betw een 2 and 9 h. The samples had yield stresses of 100-250 MPa, values w ell below the yield stress of 1500 MPa for a vacuum-dried sample. Defo rmation of the grains was homogeneous on both optical and TEM scales, indicating that the grains had fully equilibrated with the water-relat ed defect that is responsible for water weakening. FTIR measurements a re consistent with this interpretation; one of the deformed samples ha s an intragranular water concentration of 820 +/- 160 H/10(6) Si, and vacuum-dried samples hydrostatically annealed for 3 or 10 h at the sam e conditions as the deformed samples have an average intragranular wat er concentration of 730 +/- 180 H/10(6) Si. To obtain a second constra int on the rate of water penetration, hydrothermal annealing experimen ts were performed on portions of a sample of Heavitree quartzite which were previously deformed without water added at 700 degrees C, 1200 M Pa and 10(-5)/s and have a high (similar to 10(16)/m(2)) and homogeneo us dislocation density. Two samples were annealed at 700 degrees C, 15 00 MPa for 24 h: one with 0.2 wt% water and one with no water added. T he grains in the water-added sample have a rim approximately 15 mu m w ide with dislocation recovery microstructures, indicating equilibratio n 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-mu m-wide recovered r im suggests a penetration rate approximately an order of magnitude fas ter than the rate of oxygen diffusion under hydrothermal conditions. T he observed rate of penetration of the water-related species is consis tent with theoretical considerations and calculations which predict a diffusion rate for H2O that is an order of magnitude faster than the r ate of hydrothermal oxygen diffusion. We infer from our experiments th at the rate of water penetration into quartz is consistent with diffus ion of H2O. (C) 1998 Elsevier Science B.V. All rights reserved.