Determining olivine dislocation densities and deformation mechanisms by high angular resolution electron backscatter diffraction

Abstract:

The deformation of olivine is fundamental to plate-scale geodynamic processes, and its various deformation mechanisms may induce behaviours such as strain localisation and viscous anisotropy in the upper mantle. Improved characterisation of the lattice- to grain-scale processes by which creep occurs under a range of conditions will therefore better establish the role of different deformation mechanisms in controlling macroscopic tectonics.

Recent developments in high angular resolution electron-backscatter diffraction (HR-EBSD) have provided a new tool for analysing crystal plasticity and associated geometrically necessary dislocation (GND) densities across length scales of 10^-7–10^-3 m. This approach employs diffraction-pattern cross-correlation to achieve exceptional sensitivity in measurements of lattice rotation (< 0.01°) and GND density (~10¹¹ m^-2), and is being used to study dislocation processes and deformation mechanisms in a wide variety of materials-science applications.

We apply HR-EBSD analysis to olivine to characterise GNDs associated with different deformation mechanisms. We establish the ideal EBSD settings (i.e. step size and pattern binning) to optimise spatial resolution, GND sensitivity and mapping speed. GND distributions are measured in olivine single crystals and polycrystals deformed by dislocation creep, dislocation-accommodated grain-boundary sliding, or diffusion creep, in the laboratory or nature. We characterise the spatial distribution of dislocation structures, including the wavelength of periodic structures and the correlation between dislocation density and grain boundaries or triple junctions. A statistical assessment of GND distributions will be linked to variations in deformation mechanism. These results will provide a new tool for assessing deformation mechanisms in naturally deformed mantle rocks, thereby evaluating the validity of experimental flow laws under natural deformation conditions.