APPLICATION OF EBSD AND EDS TO QUANTIFYING DEFORMATION MECHANISMS AND RHEOLOGY IN ROCKS

Distinguishing the microphysical mechanisms by which rocks deform in different parts of Earth’s lithosphere is critical to understanding how plate boundaries deform at the larger scale. I demonstrate how EBSD and EDS can be used to quantify two critical aspects of rock rheology: deformation mechanisms and flow stress.

Deformation mechanisms suggested for crust and mantle rocks include diffusion creep, dislocation creep, and grain-boundary sliding, each of which exhibit unique microstructural characteristics. Diffusion creep is characterized by small-scale chemical differentiation of specific minerals (e.g. olivine or quartz) from less mobile phases such as sheet silicates, graphite or oxides. Material may be precipitated in veins, or as fibrous or optically continuous overgrowths on the margins of pre-existing grains. This process commonly results in a moderate to strong shape-preferred orientation but a random lattice preferred orientation (LPO). Dislocation creep is characterized by the formation of strong LPO, subgrains and/or abundant grain boundary migration, and, in some cases, ribbon grains formed by dislocation glide. Grain-boundary sliding may result in a homogeneous distribution of different phases (neighbor-switching), may produce diamond-shaped grains and grain boundaries that are aligned across several grain diameters, and may significantly weaken or randomize the LPO. I use EBSD to distinguish these features through submicron-scale microstructure maps and LPO measurements, including within materials that are too fine-grained to characterize optically.

EBSD has also made possible the rapid quantification of grain size. Of particular use in understanding rheology is the size of dynamically recrystallized grains formed during dislocation creep, as this scales inversely with the flow stress-- a proxy for ‘rock strength’. Measuring dynamically recrystallized grain sizes using optical techniques poses several issues of distinguishing recrystallized grains from subgrains or deformed host grains, particularly for small grain sizes. High quality EBSD maps, however, can be used to separate dynamically recrystallized grains, subgrains, and their host grains simply and methodically, adding much greater precision and accuracy to recrystallized grain size measurements.