GRAIN-SIZE-INDUCED WEAKENING OF H2O ICES-I AND ICES-II AND ASSOCIATEDANISOTROPIC RECRYSTALLIZATION

Grain-size-dependent flow mechanisms tend to be favored over dislocati on creep at low differential stresses and can potentially influence th e rheology of low-stress, low-strain rate environments such as those o f planetary interiors. We experimentally investigated the effect of re duced grain size on the solid-state flow of water ice I, a principal c omponent of the asthenospheres of many icy moons of the outer solar sy stem, using techniques new to studies of this deformation regime. We f abricated fully dense ice samples of approximate grain size 2 +/- 1 mu m by transforming ''standard'' ice I samples of 250 +/- 50 mu m grain size to the higher-pressure phase ice II, deforming them in the ice I I field, and then rapidly releasing the pressure deep into the ice I s tability field. At T less than or equal to 200 K, slow growth and rapi d nucleation of ice I combine to produce a fine grain size. Constant-s train rate deformation tests conducted on these samples show that defo rmation rates are less stress sensitive than for standard ice and that the fine-grained material is markedly weaker than standard ice, parti cularly during the transient approach to steady state deformation. Sca nning electron microscope examination of the deformed fine-grained ice samples revealed an unusual microstructure dominated by platelike gra ins that grew normal to the compression direction, with c axes prefere ntially oriented parallel to compression. In samples tested at T great er than or equal to 220 K the elongation of the grains is so pronounce d that the samples appear finely banded, with aspect ratios of grains approaching 50:1. The anisotropic growth of these crystallographically oriented neoblasts likely contributes to progressive work hardening o bserved during the transient stage of deformation. We have also docume nted remarkably similar microstructural development and weak mechanica l behavior in fine-grained ice samples partially transformed and defor med in the ice II field.