Dislocation Density as a Differential ential Stress Indicator in Coesite and Quartz: TEM Constraints from Shear Experiments

Abstract Coesite, a key indicator of ultra-high pressure (UHP) metamorphism, provides critical insights into deep subduction and exhumation processes. However, the deformation mechanisms of intergranular coesite and its retrograde quartz are not well understood. This study investigated the defect structures of shear-deformed coesite and quartz using Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM) to elucidate their deformation behavior under varying high differential stresses, specifically in the range of 903 to 1607 MPa. The results indicate that EBSD is ineffective for analyzing defects in materials experiencing substantial plastic deformation. In contrast, TEM analysis visually demonstrates that dislocation density and the prevalence of dislocation tangles increase with rising differential stress in both coesite and quartz. For coesite, hot-pressed samples predominantly exhibited free dislocations and subgrain boundaries, whereas shear-deformed samples displayed dislocation tangles, indicative of enhanced dislocation production. Quartz samples similarly transitioned from free dislocations to dislocation tangles under higher differential stress. Empirical power function relationships between dislocation density (ρ) and differential stress (σ) were established: σ = 3.65 × 10−23 ρ2.39 for coesite and σ = 2.51 × 10−4 ρ0.64 for quartz. These findings elucidate the deformation mechanisms of coesite and quartz under high differential stress, suggesting that the experimental samples may closely reflect the natural deformation processes of intergranular coesite and its retrograde quartz. This study advances the understanding of the role of coesite and quartz in recording tectonic stresses and establishes a TEM-based methodology for estimating differential stress in UHP rocks.