Significance of high-temperature deformational microstructures in quartz and ice

Quartz and ice both exhibit distinctive microstructures when deformed at low stress and high homologous temperature, with large but variable grain sizes, amoeboid grain shapes with extensive grain-boundary lobes, low internal lattice distortion, and few subgrains. Strong crystallographic preferred orientations (CPO) suggest dislocation creep. These microstructures (commonly referred to as GBM) reflect rapid grain-boundary migration and grain growth during deformation. They are difficult to reproduce experimentally in silicate minerals, and no correlation has been established between quantifiable aspects of the microstructure and deformational conditions. We carried out direct shear experiments to investigate the effect of stress and temperature (T) on GBM microstructures in ice, which is crystallographically analogous in several respects to quartz. The experiments were carried out on standard ice with an initial grain size of 260 µm at constant differential stress in the range 1.2 - 6.4 MPa, confining pressure of 5 - 9 MPa, and temperature in the range -25 to -3°C. The experiments show a clear transition in ice at around -12°C, from granular microstructures produced by sub-grain rotation at high stress and low T, to GBM microstructures at low stress and high T, accompanied by a marked increase in the strength of the CPO. The samples with GBM microstructure show lobate grain-boundaries, and "island grains" with closely similar orientations that are likely to represent lobes isolated on the 2-D cut surfaces. The mean size of bulges and island grains define an array in stress/grain-size space. This array aligns with the dynamically recrystallized grain-sizes in the high-stress/low-T samples, and also with recrystallized grain-size data obtained at low stress in unconfined experiments by previous workers. The results suggest that all these microstructures may be controlled by stress via the dislocation density in the same way as the dynamically recrystallized grain size in metals and silicates. If so, this opens up the possibility of determining paleostress from high-T (>500°C) microstructures in quartz.