A Subgrain‐Size Piezometer Calibrated for EBSD

Abstract

We calibrate a subgrain-size piezometer using electron backscatter diffraction (EBSD) data collected from experimentally deformed samples of olivine and quartz. Systematic analyses of angular and spatial resolution test the suitability of each data set for inclusion in calibration of the subgrain-size piezometer. To identify subgrain boundaries, we consider a range of critical misorientation angles and conclude that a 1°threshold provides the optimal piezometric calibration. The mean line-intercept length, equivalent to the subgrain-size, is found to be inversely proportional to the von Mises equivalent stress for data sets both with and without the correction of Holyoke and Kronenberg (2010,

Intracrystalline deformation microstructures in natural olivine with implications for stress estimation

“…7 c). This case has also been reported when measuring grain size and subgrain size in previous literature 16 , 19 , 47 . Because case-1 and -3 LAMBs occur at a small step size, which suggests the existence of a high lattice curvature, these two variants of LAMBs are primarily composed of real SGBs 30 – 32 .…”

“…Second, the choices of denoising filters and boundary smoothing also affect the appearance of LAMBs or SGBs 55 , 56 . Third, the direction of profiles (e.g., parallel to either X- or Y-axes or random in the specimen reference frame 19 , 57 , 58 ) can also yield contrasting intercept lengths of neighboring LAMBs or SGBs, especially when LAMBs or SGBs tend to have a preferred direction like our sample (Fig. 1 e and Supplementary Fig.…”

“…3 a9, b9 and c9), implying that the intercept length approximates subgrain size, that LAMB density approaches SGB density, and that the calculated stress is most likely sound in this situation, although an even smaller step size (< 0.5 μm) is suggested to be required to capture nearly all SGBs and authentic subgrain size in the experimentally deformed olivines 19 . It is noteworthy that the value of this critical step size apparently depends on the spacing of adjacent SGBs in a grain, which needs to be at least twice the step size 19 .…”

“…The third is the subgrain-size piezometer, which is built on the empirical relation between the spacing (i.e., subgrain size) of adjacent low-angle boundaries (i.e., subgrain boundary (SGB) or dislocation wall) in the minerals and stress. This piezometer has also been calibrated for olivine 6 , 10 , 11 , 19 – 21 , quartz 19 , 22 and calcite 12 , 23 . Similarly, there is also an inverse correlation between subgrain size and stress.…”

“… 7 , 9 , 25 , 26 . Besides, they can also be calculated indirectly via high-angular resolution electron backscatter diffraction (EBSD) 27 and/or traditional EBSD mapping 13 , 14 , 16 , 19 .…”