The Effect of Grain Boundaries on Plastic Deformation of Olivine

The plastic deformation of olivine has been studied for decades. However, the precise role of grain boundaries during deformation in, for example, the dislocation-accommodated grain-boundary sliding regime, remains poorly understood. Specifically, we lack knowledge regarding the manner in which grain boundaries interact with other defects, such as dislocations, during deformation. To investigate the interaction of dislocations and grain boundaries, we analyzed the structure and distribution of grain boundaries in a polycrystalline aggregate of Fo₅₀ deformed in torsion (Hansen, Zimmerman, Dillman, & Kohlstedt, 2012, https://doi.org/10.1016/j.epsl.2012.04.016). We characterized the microstructure of the aggregate using electron-backscatter diffraction and transmission electron microscopy in three perpendicular directions. An increase in plastic strain is associated with the development of a strong crystallographic preferred orientation and a grain-boundary plane distribution that evolves from a uniform distribution to one dominated by (010)-type planes. We use the m' factor, to evaluate the potential for transmission of dislocations across grain boundaries based on the relative orientations of slip systems between neighboring grains. With progressive deformation, our analysis indicates an increase in abundance of apparently slip-transparent boundaries until moderate strains ((Formula presented.) = 4) are reached. Based on these observations, we propose that specific types of grain boundaries are created by dislocation activity and that the input of dislocations into grain boundaries facilitates grain-boundary sliding. Our results provide insight into the microphysics of olivine deformation and highlight the importance of the coupled study of inter- and intragranular mechanisms during rock deformation.