High‐Angular Resolution Electron Backscatter Diffraction as a New Tool for Mapping Lattice Distortion in Geological Minerals

Abstract

Analysis of distortions of the crystal lattice within individual mineral grains is central to the investigation of microscale processes that control and record tectonic events. These distortions are generally combinations of lattice rotations and elastic strains, but a lack of suitable observational techniques has prevented these components being mapped simultaneously and routinely in earth science laboratories. However, the technique of high-angular resolution electron backscatter diffraction (HR-EBSD) provides the opportunity to simultaneously map lattice rotation and elastic strain gradients with exceptional precision, on the order of 0.01°for rotations and 10 −4 in strain, using a scanning electron microscope. Importantly, these rotations and lattice strains relate to densities of geometrically necessary dislocations and residual stresses. Recent works have begun to apply and adapt HR-EBSD to geological minerals, highlighting the potential of the technique to provide new insights into the microphysics of rock deformation. Therefore, the purpose of this review is to provide a summary of the technique, to identify caveats and targets for further development, and to suggest areas where it offers potential for major advances. In particular, HR-EBSD is well suited to characterizing the roles of different dislocation types during crystal plastic deformation and to mapping heterogeneous internal stress fields associated with specific deformation mechanisms/microstructures or changes in temperature, confining pressure, or macroscopic deviatoric stress. These capabilities make HR-EBSD a particularly powerful new technique for analyzing the microstructures of deformed geological materials.

Dislocation Creep of Olivine: Backstress Evolution Controls Transient Creep at High Temperatures

“…A comprehensive overview of all data acquisition and processing procedures for HR‐EBSD is provided by Wallis et al. (2019).…”

Dislocation interactions in olivine control postseismic creep of the upper mantle

“…Here we provide a summary of key points relevant to the interpretation of the present results. HR-EBSD measures lattice rotation and elastic strain heterogeneity by using cross-correlation to track small shifts in features within EBSD patterns [40][41][42][43] . Following acquisition of the EBSD data, one diffraction pattern was chosen from each grain, typically from a location with high pattern quality, to serve as a reference pattern for that grain.…”

“…We analyse these samples with high-angular resolution electron backscatter diffraction (HR-EBSD) (Methods), which maps lattice distortion by using cross-correlation to track shifts in subregions within diffraction patterns. Unlike conventional EBSD, which struggles to resolve the subtle microstructural changes associated with transient creep at small strains 39 , HR-EBSD provides exceptionally precise estimates of the density of geometrically necessary dislocations (GNDs, the fraction of the total dislocation density that generates net lattice curvature and long-range stress heterogeneity) and, importantly, maps heterogeneity in elastic strain and residual stress stored in the samples after the experiments [40][41][42][43] . We analyse the stress distributions in terms of the theory, established in the materials sciences [44][45][46][47] , for stress fields of a population of dislocations to test the causality between stress heterogeneity and the dislocation content (Methods).…”

The Effect of Grain Boundaries on Plastic Deformation of Olivine

“…where is the confining pressure and is the shear stress. For simplicity, we consider the local stress state at each point in the microstructure to be equal to this macroscopic stress state, although we recognize that local variations in stress can be significant (e.g., Wallis et al, 2019). For our analysis of slip transfer in olivine, we consider the set of slip systems listed in Table 1.…”