Cryogenic Electron Backscatter Diffraction (cryo-EBSD)
Understanding the physical properties of natural ice flow requires accurate microstructural characterisation. Cryo-EBSD (Prior et al., 2015), which is usually coupled with a scanning electron microscope (SEM), is a state-of-the-art technique; it revolutionised the quantitative microanalysis of ice in the last few years, enabling spatially resolved quantification of ice microstructure.
Figure 1. Schematic drawing that illustrates the principles of EBSD data collection from the surface of ice samples. Technical details of cryo-EBSD can be found in Prior et al. (2015).
Figure 2. EBSD map of an ice sample deformed under direct shear to 100% of lateral extension. The colour indicates crystal orientation (Fan et al., in prep).
Prior, D. J., Lilly, K., Seidemann, M., Vaughan, M., Becroft, L., Easingwood, R., Diebold, S., Obbard, R., Daghlian, C., Baker, I., Caswell, T., Golding, N., Goldsby, D., Durham, W. B., Piazolo, S., & Wilson, C. J. L. (2015). Making EBSD on water ice routine. Journal of Microscopy, 259(3), 237–256. https://doi.org/10.1111/jmi.12258
High-Angular Resolution Electron Backscatter Diffraction (HR-EBSD)
Elastic interactions of dislocations (defects of crystals) govern geophysical processes such as post-seismic deformation, plate-boundary formation, and post-glacial rebound. HR-ESBD (Wilkinson et al., 2006) is a state-of-the-art technique; it revolutionised the quantitative microanalysis of rock-forming minerals, such as quartz and olivine, providing measurements of intragranular distortion and elastic strain gradients with exceptional precision.
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Figure 3. Schematic drawing that illustrates the principles of HR-EBSD data collection and processing (adopted from Britton et al. (2013)). Technical details of HR-EBSD can be found in Wilkinson et al. (2006).
Figure 4. Map of stress heterogeneity via processing HR-EBSD data collected from a naturally deformed olivine grain (Fan et al., in prep).
Wilkinson, A. J., Meaden, G., & Dingley, D. J. (2006). High-resolution elastic strain measurement from electron backscatter diffraction patterns: New levels of sensitivity. Ultramicroscopy, 106(4–5), 307–313. https://doi.org/10.1016/j.ultramic.2005.10.001
Britton, B., Holton, I., Meaden, G., & Dingley, D. (2013). High angular resolution electron backscatter diffraction: measurement of strain in functional and structural materials. Microscopy and Analysis, 27(4), 8.
Micro-computed Tomography (micro-CT)
Natural ice and rock are characterised by heterogeneous microstructures in 3-D. The geometry of air bubbles, including bubble size and shape, is crucial for understanding the deformation history of ice flow. The latest development of cryogenic micro-CT provides accurate measurement of air-bubble geometry in 3-D at micrometre scale (Movie 2).
Figure 6. Schematic drawing of the principles of micro-CT (adopted from Boerckel et al. (2014)).
Movie 1. Processed micro-CT data showing air bubbles within an ice core collected from a fast-deforming margin in Whillans ice stream, Antarctica (Fan et al., in prep).
Boerckel, J. D., Mason, D. E., McDermott, A. M., & Alsberg, E. (2014). Microcomputed tomography: Approaches and applications in bioengineering. In Stem Cell Research and Therapy (Vol. 5, Issue 6). BioMed Central Ltd. https://doi.org/10.1186/scrt534
Fabric Analyser (Automated Polarising Optical Microscope)
The SEM-related techniques have a more restricted limit on sample size. Fabric Analysers can provide microstructural quantification (Peternell et al., 2011) for samples with relatively large sizes (e.g., natural ice cores). Fabric Analyzer coupled with an in-situ stage can measure the evolution of microstructure during deformation (Movie 1).
Movie 2. Left: Microstructural evolution of a deforming ice-thin section; colours indicate c-axes orientations. Compression from top to bottom. Right: Orientations of c-axes protected on a pole figure (Fan et al., in prep).
Peternell, M., Russell-Head, D. S., & Wilson, C. J. L. (2011). A technique for recording polycrystalline structure and orientation during in situ deformation cycles of rock analogues using an automated fabric analyser. Journal of Microscopy, 242(2), 181–188. https://doi.org/10.1111/j.1365-2818.2010.03456.x