Transmission Kikuchi diffraction study of submicrotexture within ultramylonitic peridotite

Deformed polycrystalline rocks show various crystallographic textures reflecting their imposed deformation histories. However, the textures of ultrafine grains, which are in the submicrometer to nanometer order, may be overlooked depending on the analytical technique used. Thus, we report the first application of transmission Kikuchi diffraction (TKD), which is capable of high-spatial-resolution crystallographic texture analysis, to a fine-grained ultramylonitic peridotite sample in a scanning electron microscope. We successfully obtained TKD maps with an effective spatial resolution of ~ 80 nm and with highly reliable indexing using a conventional W-filament scanning electron microscope with a standard electron backscattered diffraction (EBSD) system. Olivine grains, which were clearly visualized by TKD, were slightly elongated in a direction subparallel to the macroscopic lineation texture. Their shapes were nonuniform with serrated grain boundaries, strongly indicating that the sample has been deformed dominantly by dislocation activity, even though the grain size is in the order of several micrometers or smaller. The combined TKD–transmission electron microscope (TEM) analysis indicated that a slip-system transition from the [100] slip to the [001] slip might have occurred, although the crystals’ preferred orientation patterns were not completely overwritten. The transition might have been sufficiently affected by water infiltration, high differential stress, or both along the transform fault. Thus, TKD efficiently analyzed the crystallographic textures and characterized the subgrain boundaries of polycrystalline rocks consisting of submicrometer-order grains. Moreover, combining the EBSD, TKD, and TEM methods allowed us to perform multiscale analyses of the crystallographic textures of ultrafine-grained deformed rocks, seamlessly linking the millimeter- to nanometer-order scales.