ROLE OF RECRYSTALLIZATION ON THE CHARACTERISTIC ICE FABRIC FORMATION

It is generally recognized that the fabric pattern of ice in polar ice sheets gradually changes from the random pattern to characteristic ones as the single maximum or the small circle girdle according to increase of the depth. We have considered three main mechanisms for the development of the ice fabric, 1) rotation of basal plane acompanied by deformation 2) grain groth 3) recrystallization. With respect to the mechanism 1), we obtained a quantitative relationship between the rotation angle of c-axis of individual grains in compressed polycrystalline ice and the amount of uniaxial strain by experiments, and the obtained result of concentration tendency of c-axes with increasing strain was successfully applied to the observed fabric change to the single maximum in Dye-3 cores /1/. The recrystallization mechanism 3) should prevail at a region where ice temperature is comparatively high. For example, at the depth deeper than 1800m at Byrd station Antarctica, the fabric transforms from the single maximum to the small circle girdle. Several laboratory experiments hitherto carried out succeeded to form the small girdle fabric pattern by the uniaxial compression tests /2-4/, no formation mechanism has ever been proposed. In the present work, we have carried out experiments of observing recrystallization nuclei produced at the grain boundary of artifially grown bicrystals when they were subjected to the compressive deformation. Five bicrystal specimens of the planer shape were prepared so that c-axes of both crystals were parallel to the plane but rotated each other by 45° (for tests 1,3 and 5) and 30° (tests 2 and 4) around the rotation axis perpendicular to the plane. They were subjected to the uniaxial compression up to 10 % total strain at a temperature -2°C. After released from the testing machine, specimens were examined under microscope. Many recrystallization nuclei were found along the boundary. A photomicrograph of a recrystallized grain (hereafter call subgrain) etched by ethylenedichloride solution of formvar is shown in Fig. 1