Effect of water on the dislocation creep microstructure and flow stress of quartz and implications for the recrystallized grain size piezometer

Deformation experiments on Black Hills quartzite with three different initial water contents (as-is, water-added, and vacuum-dried) were carried out in the dislocation creep regime in order to evaluate the effect of water on the recrystallized grain size/flow stress piezometer. Samples were deformed in axial compression at temperatures of 750°-1100°C, strain rates between 2 × 10^-7 s^-1 and 2 × 10^-4 s^-1 and strains up to 46% using a molten salt assembly in a Griggs apparatus. An increase of the initial water content at otherwise constant deformation conditions caused a decrease in flow stress, an effect known as hydrolytic weakening. The total water content of the starting material was analyzed by Karl Fischer titration (KFT) and Fourier transform infrared (IR) spectroscopy, and quenched samples were analyzed microstructurally and by IR. Changes in the dynamic recrystallization microstructure correlate with changes in flow stress, but there is no independent effect of temperature, strain rate or water content. IR absorption spectra of the deformed spectra indicate that different water contents were maintained in the three sample sets throughout the experiments. However, the amounts of water measured within the vacuum-dried (∼260 ± 40 ppm H₂O), the as-is (∼340 ± 50 ppm H₂O), and the water-added (∼430 ± 110 ppm H₂O) samples are significantly smaller than the initial content of the quartzite (∼640 ± 50 ppm H₂O). Water from the inclusions in the starting material adds to the free fluid phase along the grain boundaries, which probably controls the water fugacity and the flow strength, but this water is largely lost during IR sample preparation. Vacuum-dried as well as water-added samples have the same recrystallized grain size/flow stress relationship as the piezometer determined for as-is samples. No independent effect of water on the piezometric relationship has been detected.