Dislocations in geological minerals are fundamental to the creep processes that control large-scale geodynamic phenomena. However, techniques to quantify their densities, distributions, and types over critical subgrain to polycrystal length scales are limited. The recent advent of high-angular resolution electron backscatter diffraction (HR-EBSD), based on diffraction pattern cross-correlation, offers a powerful new approach that has been utilised to analyse dislocation densities in the materials sciences. In particular, HR-EBSD yields significantly better angular resolution (<0.01°) than conventional EBSD (~0.5°), allowing very low dislocation densities to be analysed. We develop the application of HR-EBSD to olivine, the dominant mineral in Earth's upper mantle by testing (1) different inversion methods for estimating geometrically necessary dislocation (GND) densities, (2) the sensitivity of the method under a range of data acquisition settings, and (3) the ability of the technique to resolve a variety of olivine dislocation structures. The relatively low crystal symmetry (orthorhombic) and few slip systems in olivine result in well constrained GND density estimates. The GND density noise floor is inversely proportional to map step size, such that datasets can be optimised for analysing either short wavelength, high density structures (e.g. subgrain boundaries) or long wavelength, low amplitude orientation gradients. Comparison to conventional images of decorated dislocations demonstrates that HR-EBSD can characterise the dislocation distribution and reveal additional structure not captured by the decoration technique. HR-EBSD therefore provides a highly effective method for analysing dislocations in olivine and determining their role in accommodating macroscopic deformation.
Bibliographical noteFunding Information:
David Kohlstedt generously contributed deformed single crystals for characterisation. The authors are grateful to John Wheeler for the use of his software to calculate components of the dislocation density tensor from conventional EBSD data. We thank Nick Timms for his constructive review of the manuscript. D. Wallis, L. Hansen and A.J. Wilkinson acknowledge support from the Natural Environment Research Council Grant NE/M000966/1 . T.B. Britton acknowledges support for his research fellowship from the Royal Academy of Engineering . Research data supporting this paper can be found on the Oxford Research Archive ( http://www.ora.ox.ac.uk/ ).
© 2016 The Authors
- Dislocation density
- Electron backscatter diffraction
- Geological materials
- Olivine slip systems