Caleb W. Holyoke

Dislocation creep of dry quartz

Small-scale shear zones within the Permian Truzzo meta-granite developed during the Alpine orogeny at amphibolite facies conditions. In these shear zones magmatic quartz deformed by dislocation creep and recrystallized dynamically by grain boundary migration with minor subgrain rotation recrystallization to a grain size of around 250–750 μm, consistent with flow at low differential stresses. Fourier transform infrared (FTIR) spectroscopy reveals very low water contents in the interior of recrystallized grains (in the form of discrete OH peaks, ~20 H/10 6 Si and very little broad band absorption, <100 H/10 6 Si). The spectral characteristics are comparable to those of dry Brazil quartz. In FTIR spectra, magmatic quartz grains show a broad absorption band related with high water concentrations only in those areas where fluid inclusions are present while other areas are dry. Drainage of fluid inclusions and synkinematic growth of hydrous minerals indicates that a hydrous fluid has been available during deformation. Loss of intragranular water during grain boundary migration recrystallization did not result in a microstructure indicative of hardening. These FTIR measurements provide the first evidence that quartz with extremely low intragranular water contents can deform in nature by dislocation creep at low differential stresses. Low intragranular water contents in naturally deformed quartz may not be necessarily indicative of a high strength, and the results are contrary to implications taken from deformation experiments where very high water contents are required to allow dislocation creep in quartz. It is suggested that dislocation creep of quartz in the Truzzo meta-granite is possible to occur at low differential stresses because sufficient amounts of intergranular water ensure a high recovery rate by grain boundary migration while the absence of significant amounts of intragranular water is not crucial at natural conditions.

Deformation of Fine‐Grained Quartz Aggregates by Mixed Diffusion and Dislocation Creep

Hot-pressed polycrystalline quartz samples with grain sizes of 1.7–12.0 μm and water contents of 3,500 ppm H/Si were deformed in a Griggs apparatus at temperatures of 600 to 950 °C, confining pressures of 0.9 to 1.5 GPa and strain rates of 10 3.3 to 10 /s. Two different flow regimes are distinguished at low and high temperatures. The stress exponent determined at low temperatures (600–750 °C) increased from 2.9 to 5.2 with an activation energy of 129 ± 33 kJ/mol, consistent with previous quartz dislocation creep laws indicating operation of dislocation creep. In contrast, the stress exponent determined at high temperatures (800–950 °C) is 1.7 ± 0.2 with an activation energy of 183 ± 25 kJ/mol. A fugacity exponent determined at 800 °C was 1.0 ± 0.2. All samples show evidence of basal slip. However, flow strengths at high temperatures also depend on grain size with a small grain size exponent of 0.51 ± 0.13. Mechanical, microstructural, and textural results suggest that deformation occurs by a combination of intracrystalline and grain boundary processes. The flow law determined from the high-temperature data can be fit by _ ε 1⁄4 10 σd f 1:0±0:2 H2O exp 183±25 kJ=mol RTk with stress, σ in MPa, grain size, d in μm, fH2O in MPa, and Tk in Kelvin. At conditions of the middle crust and tectonic strain rates, deformation depends on grain size where the strength is weaker than for pure dislocation creep even for grain sizes >10 μm.