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

Hot-pressed polycrystalline quartz samples with grain sizes of 1.7-12.0m and water contents of 3,500ppm H/Si were deformed in a solid-pressure-medium (Griggs-type) deformation apparatus at temperatures of 600 to 950 degrees C, confining pressures of 0.9 to 1.5GPa and strain rates of 10(-3.3) to 10(-5.9)/s. Two different flow regimes are distinguished at low and high temperatures. The stress exponent determined at low temperatures (600-750 degrees C) increased from 2.9 to 5.2 with an activation energy of 12933kJ/mol, consistent with previous quartz dislocation creep laws indicating operation of dislocation creep. In contrast, the stress exponent determined at high temperatures (800-950 degrees C) is 1.70.2 with an activation energy of 18325kJ/mol. A fugacity exponent determined at 800 degrees C was 1.00.2. 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 epsilon 10-2.97 +/- 0.23 sigma 1.7 +/- 0.2d-0.51 +/- 0.13fH2O1.0 +/- 0.fenced idth="0.25emwith stress, sigma in MPa, grain size, d in m, fH2O in MPa, and T-k 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 >10m.