EBSD map analysis for antigorite deformation micro-textures related to dislocations: Implications for the effect of antigorite crystal structure on plastic deformation

Abstract

The identification of dislocations and associated Burgers vectors in minerals is important to discuss the mechanisms of crystal plastic deformation of rocks. Observation of dislocations is normally carried out using transmission electron microscopy (TEM). However, the necessary sample preparation and observation can be challenging in particular for hydrous minerals that are sensitive to beam damage. In addition, the spatial scale at which TEM can be used to observe dislocations is limited, making it difficult to evaluate the process of bulk-rock deformation. Recently, the improved accuracy and indexing rate in Electron Backscatter Diffraction (EBSD) mapping and the ease of analyzing mapping data using a toolkit for analysis provided in programming software have made it possible to observe the microstructural features related to dislocations using EBSD measurements, such as small misorientations less than a few degrees of angular differences within a single grain. In this study, we examine how EBSD observations of natural antigorite-rich serpentinite samples can be used to derive information about the nature of dislocations and associated antigorite deformation mechanisms. In particular, we focused on the effect of different lengths in the crystal undulations, expressed as M- and m-values, of antigorite on the deformation processes and the resulting crystal preferred orientation (CPO) patterns of antigorite in antigorite schist samples. The resulting antigorite CPOs are all like the B-type CPO patterns that have been widely reported from natural antigorite schists regardless of the wavelength of the curved antigorite crystals. However, misorientation analyses using the EBSD maps suggest dislocations characteristic of the [100](001) slip system, responsible for A-type CPO formation, are more common in antigorite-rich serpentinites with shorter wavelengths, whereas dislocations characteristic of the [hk0](001) slip systems, responsible for G-type CPO formation, are more common in those with longer wavelengths. Therefore, the dislocation microstructures observed in this study do not provide evidence that the B-type CPO was formed by dislocation creep. However, these dislocation microstructures may preserve evidence of different deformation stages before and after the formation of the B-type CPO. This implies that, as M- and m-values tend to decrease under high-pressure conditions, A- and G-type CPOs are likely to form in deeper and relatively shallower domains, respectively. These results suggest the strength of schistosity, grain shape, and CPO strength in ductilely deformed antigorite-rich serpentinites may be affected by differences in the dominant deformation mechanisms and dislocation microstructures influenced by the wavelength of antigorite.