Ice deformation microstructures and related mechanisms
Any large-scale plastic flow of even huge bodies of any material is actually conducted by deformation on smaller scales; down to crystallite and subgrain as well as atomic scales. As these deformation mechanisms significantly control the rheology of the material, knowledge on these processes is essential to understand and predict ice dynamics seriously. The mechanical properties of the ice are furthermore modified by recrystallisation processes induced mainly by deformation energy and temperature changes.
The deformation and recrystallisation processes leave behind traces in the grain morphologies and orientations (microstructures) which can be used to identify and under certain assumptions to quantify the relevant mechanisms. These traces are e.g. grain sizes, grain shapes, crystal orientation distributions (fabrics), subgrain boundary occurrence, subgrain boundary misorientations and types, dislocation densities and types.
We study deformation microstructures in ice core samples from Greenland and Antarctica with the following experimental methods:
- Microstructure Mapping: The ice sample surface is polished and exposed to controlled sublimation, which acts as thermal grooving and reveals grain and subgrain features in very high resolution. These features are imaged by optical light microscopy (~3µm/pix) or Large Area Scanning Macroscope (LASM, ~5µm/pix).
- Automated fabric analyzer: The classical polarisation microscopy principles are used to determine c-axes orientations. Automatic control of polarizers, lambda-quarter plate and viewing direction by a computer software, as well as automatic samples stage scanning enables fast acquisition of data (http://www.earthsci.unimelb.edu.au/facilities/analyser/).
- Electron Backscatter Diffraction (EBSD, in cooperation with Dr. Martyn Drury, Utrecht University, The Netherlands): Diffraction of backscattered electrons in an Scanning Electron Microscope (SEM) enables the determination of full crystal orientations (c- and a-axes) of ice core samples in a high spatial (~3µm) and angular resolution (relative orientation ~0.5°). We use EBSD to characterize and quantify subgrain boundaries and the dislocation types they are composed of (Example see in Fig. 1).
Fig. 1: a) EBSD map of a part of a grain (EDML 655.9 m) with several types of subgrain boundaries (highlighted subsets). b) Overall grain orientation. c) Misorientation rotation axes of subgrain boundaries in a. d) Interpretation. Further details can be found here.
- X-ray Laue Diffraction (in co-operation with Dr. Atsushi Miyamoto, ILTS, Japan): X-ray diffraction is the most traditional method to measure orientations of single crystals. We can apply it to ice polycrystals, because natural ice crystals from ice sheets are rather large. Our main application of the method is the characterisation of subgrain boundaries and related dislocation arrangements (Example see in Fig. 2).
Fig. 2: a) Microstructure Mapping image with points of X-ray Laue measurements indicated. b) Orientations plotted as misorientations from point to point. b) Overall grain orientation. d) Interpretation. Further details can be found here.
Running Projects
- Deformation of ice on the grain scale by means of high-resolution crystal-orientation measurements (DFG funded: WE 4695/1-2)
- Micro-structure analysis of NEEM deep ice cores including textural parameters (grain size, grain shape, sub-grain boundary occurrence, slip line occurrences) and fabrics (c-axes distributions)
