Characterizing Dislocation Substructures in Creep-Deformed Olivine Using Electron Channeling Contrast Imaging

Abstract

Olivine is the dominant mineral in Earth's upper mantle and therefore controls mantle rheology and the mechanics of plate tectonics. The constitutive laws for dislocation-mediated deformation of olivine depend on the nature, density, and arrangements of dislocations within crystals. Hence, imaging and characterizing these defects is important, albeit challenging. Traditional imaging approaches involve (1) transmission electron microscopy (TEM), which samples small areas and requires extensive preparation and (2) oxidation decoration methods that have low spatial resolution and cannot distinguish dislocations of opposite Burgers vectors. Here, we apply electron channeling contrast imaging (ECCI) to unlock insight into the deformation structures within olivine, and combined with electron backscatter diffraction (EBSD) and weighted Burgers vector (WBV) mapping as an informative route to characterize dislocation substructures in bulk materials. Specifically, we have used an ECCI workflow based on selected-area electron channeling patterns (SA-ECPs) and we apply this workflow to a single crystal of San Carlos olivine that was deformed by creep at high temperature. ECCI micrographs reveal subgrain boundaries, surface threading dislocations, and dislocation loops across representative areas. The observations demonstrate that this workflow can reliably reveal the complexity of subgrain boundaries in olivine, which can host multiple dislocation types and exhibit non-planar geometries. Despite the limited number of slip systems in olivine, subgrain boundaries can form complex, mixed assemblies. Overall, such observations can provide a variety of constraints on dislocation types, morphologies, and distributions, which are required to parameterize and calibrate models of transient and steady-state dislocation creep in olivine and other materials.