MR31A-02
Localisation of shear in ice by recrystallisation on kinks and grain boundary networks: new EBSD data in old experiments

Wednesday, 16 December 2015: 08:20
301 (Moscone South)
David John Prior1, Narayana Golding2, William B Durham3, Meike Seidemann1, Sabrina Diebold4 and Johannes H P De Bresser5, (1)University of Otago, Geology, Dunedin, New Zealand, (2)Massachusetts Institute of Technology, Cambridge, MA, United States, (3)MIT 54-720, Cambridge, MA, United States, (4)Utrecht University, Utrecht, Netherlands, (5)Utrecht University, Utrecht, 3584, Netherlands
Abstract:
Relatively high stress (>1MPa), low temperature (<258K) ice creep experiments, to significant strain, have been possible some decades, using a gas confining pressure to prevent sample failure. Imaging the microstructures that result from these experiments has been problematic, primarily because of the fine size of some or all of the grains. We have now developed protocols to enable electron backscatter diffraction (EBSD) in of fine-grained water ice with a very high (>95%) success rate and have quantified the microstructures of >50 confined medium experiments.

The indium jackets of many confined media experiments have an irregular rumpled surface after deformation. The “rumples” have wavelengths and amplitudes larger than the starting grain size. EBSD maps show anastomosing traces along which fine recrystallised grains are developed. The traces follow some of the original grain boundaries – the scale of connectivity of the network of recrystallized grains is on the scale of several grains (or larger) and broadly corresponds to the wavelength of the rumples on the jacket. In some samples the original grains are kinked and recrystallized grains occur along kink traces. In samples where such recrystallisation is well-developed, these bands contribute of the anastomosing network. Recrystallised grain size describes a broad piezometer relationship that is consistent with much coarser recrystallized grain sizes from higher temperature, lower stress unconfined experiments.

We suggest that the dynamic recrystallisation process is a mechanism that enables localization and weakening in ice. The high stress confined media experiments give some insight into processes that may occur in terrestrial ice sheets and cannot be accessed at natural conditions. Some preliminary modeling shows that a dynamic recrystallisation process is consistent with the magnitude of weakening on ice-stream margins in Antarctica.

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