RESTRAINTS ON THIN SECTION ANALYSIS OF GRAIN GROWTH IN UNSTRAINED POLYCRYSTALLINE ICE

Abstract

Tests were performed at -1°C to evaluate the effects of a free surface and the thickness dimensions of thin sections on the growth of grains in fine-grained, pore-rich, strain-free polycrystalline ice. Results show that negligible growth of grains occurs when the mean size of grains is more than 1.5 to 2 times the section thickness. Grain growth in thicker sections was significant for the fact that grain boundary migration, leading to 3-4 fold increases in average grain size, was virtually unaffected by the presence of large numbers of bubbles in the ice. Nor was there any evidence to indicate any concentrating of bubbles along migrating boundaries. Grain boundary grooving was a characteristic feature of most sections undergoing grain growth. This implies actual migration of grooves during grain growth. The fact that the total length of grooves decreased with increasing grain size also implies some process of groove consumption during grain growth. Three-dimensional grain growth measurements in bulk samples compared favorably with those obtained from sections two to three times thicker than the mean grain diameter.