Abstract
Quartz and ice both exhibit distinctive microstructures when deformed at low stress and high homologous temperature, known as grain-boundary migration (GBM) microstructures. These 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 to quartz. Differential stress was 0.7â6.0 MPa, confining pressure 4â9 MPa, and T â3° to â25 °C. There is a clear transition at âź â12 °C from granular microstructures produced by rotation recrystallization at high stress and low T, to GBM microstructures at low stress and high T, accompanied by an increase in strength of the crystallographic preferred orientation (CPO). Samples with GBM microstructure show lobate grain-boundaries and âisland grainsâ where several distinct and separate areas have the same crystallographic orientation, representing lobes of a single grain isolated on the cut surfaces. The size of lobes and island grains define an array with respect to stress with a slope of 0.9, similar to but offset from the slope of published stress/grain-size data. This suggests the possibility of determining paleostress from GBM microstructures in both ice and quartz.
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â˘Experimentally deformed ice shows a transition to GBM microstructures at âź â12 °C.â˘GBM microstructures in ice resemble those at T > 500 °C in naturally deformed quartz.â˘A new grain-boundary bulge piezometer fits ice data with GBM microstructures.