Abstract
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.
Highlights
Dislocations in geological minerals are fundamental to the creep processes that control large-scale geodynamic phenomena
The high-angular resolution electron backscatter diffraction (HR-electron backscatter diffraction (EBSD)) results exhibit distinct and wellresolved geometrically necessary dislocation (GND) substructure, including prominent boundaries and less pronounced, more closely spaced bands, both of which are in orientations consistent with slip-systems predicted to be activated by the loading direction
We have applied the HR-EBSD technique to olivine, the dominant mineral in Earth's upper mantle, to determine GND densities and distributions associated with creep
Summary
Dislocations in geological minerals are fundamental to the creep processes that control large-scale geodynamic phenomena. The application of quantitative EBSD-based dislocation density analysis to geological materials has been limited to date [8,15,16], and recently developed HREBSD methods have not yet, to the authors knowledge, been applied to common rock-forming minerals In this contribution, we develop and test the ability of HR-EBSD to derive dislocation density estimates for olivine, the most abundant mineral in Earth's upper mantle. Confidence in such extrapolations can only be gained if the same fundamental deformation mechanisms and processes can be demonstrated to have occurred in both the experimental and natural materials These considerations motivate detailed dislocation analysis (dislocation types, densities, and distributions) in order to model and interpret their role in accommodating strain. HR-EBSD provides both the high angular resolution necessary to resolve dislocation densities, distributions, residual stresses and elastic strains, as well as the large areal coverage of EBSD maps, typically up to a few hundred mm2 [9,13,46,47]
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