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The Cryosphere An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/tc-2020-135
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2020-135
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 08 Jun 2020

Submitted as: research article | 08 Jun 2020

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This preprint is currently under review for the journal TC.

Electron backscatter diffraction (EBSD) based determination of crystallographic preferred orientation (CPO) in warm, coarse-grained ice: a case study, Storglaciären, Sweden

Morgan E. Monz1, Peter J. Hudleston1, David J. Prior2, Zachary Michels1, Sheng Fan2, Marianne Negrini2, Pat J. Langhorne2, and Chao Qi3 Morgan E. Monz et al.
  • 1Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, Minnesota, USA
  • 2Department of Geology, University of Otago, Dunedin, New Zealand
  • 3Key Laboratory of Earth and Planetary Physics, Chinese Academy of Sciences, Beijing, China

Abstract. Microstructures provide key insights into understanding the mechanical behavior of ice. Crystallographic preferred orientation (CPO) develops during plastic deformation as ice dynamically recrystallizes, with the dominance of intracrystalline glide on the basal plane. CPO patterns in fine-grained ice have been relatively well characterized and understood in experiments and nature, whereas CPO patterns in "warm" (T > −10 ºC), coarse-grained, natural ice remain enigmatic. Previous microstructural studies of coarse-grained ice have been limited to c-axis orientations using light optical measurements. We have developed a new sample preparation technique, by constructing composite sections, to allow us to use electron backscatter diffraction (EBSD) to obtain a representative, bulk CPO on coarse-grained ice. We suggest that a grain sampling bias of large, branching crystals that appear multiple times as island grains in thin section may result in the typical multiple maxima CPOs previously identified in warm, coarse-grained ice that has been subjected to prolonged shear. CPOs combined from multiple samples of highly sheared ice from Storglaciären provide a more comprehensive picture of the microstructure and yield a pronounced cluster of c-axes sub-normal to the shear plane and elongate or split in a plane normal to the shear direction, and a concomitant girdle of a-axes parallel to the shear plane with a maximum perpendicular to the shear direction. This pattern compares well with patterns produced by sub-sampling data sets from experimentally sheared ice at high homologous temperatures up to strains of ~ 1.5. Shear strains in the margin of Storglaciären are much higher than those in experimental work. At much lower natural strain rates, dynamic recrystallization, particularly grain boundary migration, may have been more effective so that the CPO has been continuously reset and represents a smaller, final fraction of the shear history, rather than the entire finite strain history.

Morgan E. Monz et al.

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Latest update: 04 Jul 2020
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Short summary
We present a new sample preparation method that allows us to better characterize how crystals in glacial ice preferentially align as ice flows. We argue that a common pattern of alignment, with several favored orientations, may result from bias due to overcounting large crystals with complex 3D shapes that appear multiple times in 2D sections used for analysis. Our method effectively increases the sample size and reduces this bias. It results in a simpler pattern consistent with the ice flow.
We present a new sample preparation method that allows us to better characterize how crystals in...
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