Electron backscatter diffraction analysis to determine the mechanisms that operated during dynamic recrystallisation of quartz-rich rocks

Determination of the controlling nucleation and recrystallisation mechanisms from a samples microstructure are essential for understanding how the microstructure formed and evolved through time. The aim of our research was to apply a quantified analytical approach to the identification of the controlling nucleation, recrystallisation and microstructural modification mechanisms. We used electron backscatter diffraction to quantify the microstructures of naturally deformed quartz-rich rocks which were deformed at various temperature and pressure conditions. Our results show that ratios of the recrystallised grain size to the subgrain size with values less than 1 (0.5–0.7 in the data presented here) suggest bulge nucleation, whereas ratios of ∼1 suggest subgrain rotation nucleation. Other supporting evidence for subgrain rotation nucleation is an increase in misorientation from the centre of an original protolith ‘parent’ grain to the edge. All samples show evidence for modification of the microstructure due to grain boundary sliding including increased misorientation angles between grains and movement of recrystallised grains between parent grains. By systematically analysing sample microstructures it is possible to separate out evidence to determine the controlling nucleation and recrystallisation mechanisms, as well as being able to identify microstructure modification mechanisms. Using microstructural quantification via EBSD allows a systematic methodology to analyse samples from any location from an objective viewpoint.

► We quantified quartz-rich microstructures using EBSD.
► Ratio of subgrain to recrystallised grain size constrains the nucleation mechanism.
► Bulge nucleation dominates in regimes 1 and 3.
► SGR nucleation dominates regime 2.
► Microstructures are modified by twinning and grain boundary sliding.