Transitional grain-size-sensitive flow of milky quartz aggregates

Abstract:

Fine-grained (~15 µm) milky quartz aggregates exhibit reversible flow strengths in triaxial compression experiments conducted at T = 800-900^oC, P_c = 1.5 GPa when strain rates are sequentially decreased (typically from 10^-3.5 to 10^-4.5 and 10^-5.5 s^-1), and then returned to the original rate (10^-3.5 s^-1), while samples that experience grain growth at 1000^oC (to 35 µm) over the same sequence of strain rates exhibit an irreversible increase in strength. Polycrystalline quartz aggregates have been synthesized from natural milky quartz powders (ground to 5 µm) by HIP methods at T = 1000^oC, P_c = 1.5 GPa and t = 24 hours, resulting in dense, fine-grained aggregates of uniform water content of ~4000 ppm (H/10⁶Si), as indicated by a broad OH absorption band at 3400 cm^-1. In experiments performed at 800^o and 900^oC, grain sizes of the samples are essentially constant over the duration of each experiment, though grain shapes change significantly, and undulatory extinction and deformation lamellae indicate that much of the sample shortening (to 50%) is accomplished, over the four strain-rate steps, by dislocation creep. Differential stresses measured at T = 800^oC decrease from 160 to 30 MPa as strain rate is reduced from 10^-4.6 to 10^-5.5 s^-1, and a stress of 140 MPa is measured when strain rate is returned to 10^-4.5 s^-1. Samples deformed at 1000^o and 1100^oC experience normal grain growth, with grain boundary energy-driven grain-coarsening textures superposed by undulatory extinction and deformation lamellae. Differential stresses measured at 1000^oC and strain rates of 10^-3.6, 10^-4.6, and 10^-5.5 s^-1 are 185, 80, and 80 MPa, respectively, while an increased flow stress of 260 MPa is measured (following ~28 hours of prior high temperature deformation and grain growth) when strain rate is returned to 10^-3.6 s^-1. While all samples exhibit lattice preferred orientations, the stress exponent n inferred for the fine-grained 800^oC sample is 1.5 and the stress exponent of the coarse-grained 1000^oC sample is between ~3 and 4. Our value for n of fine-grained quartz samples (and previously reported values of n < 3 for quartz aggregates with added water) may attest to a component of diffusion creep and grain boundary sliding that accompanies dislocation creep.