285-8 Practical Protocol for Quartz c-Axis Opening-Angle Thermometry Using EBSD Measurements

Session: Rock Deformation and the Dynamics of Mountain Building: A Session Honoring the Scientific Contributions of John P. Platt, Part II

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The microstructure of deformed metamorphic rocks is a valuable source of information about the magnitude and distribution of stresses and strains within the lithosphere. However, metamorphic rocks generally have complex burial and exhumation histories and are likely to have undergone deformation under various conditions. Despite this complexity some studies make the assumption that microstructures record conditions associated with peak metamorphism. Quartz c-axis fabric opening-angle thermometry (OA thermometry) is an important tool that can help determine the temperature at the time the observed quartz-rich rock structures developed. The opening-angle (OA: the angle separating peripheral legs of a girdle distribution) is defined by the contoured quartz c-axis pole figures plotted on a Schmidt net. Several studies have demonstrated an empirical relationship between the OA and the deformation temperature of quartz. OA could also depend on water fugacity and strain rate, and the uncertainties of OA thermometer is generally thought to be about ±50°C. However, the influence of the approach followed to draw the contoured pole figures and the resultant contour smoothness has not been systematically studied and is a potential additional source of uncertainty. Some older studies relied on hand-drawn contours, which introduces user bias.

With the use of electron backscatter diffraction (EBSD) methods it is possible to obtain detailed maps of crystal orientations in rock samples systematically. In MTEX (toolbox to analyse crystal orientation data), misorientation angle thresholds and kernel density estimations can be used to define grain boundaries and grain orientations of samples and to derive contoured pole figures. The results are reproducible and independent of the user. In this study we used MTEX analyses of EBSD data of quartz schist samples from the Sanbagawa belt of SW Japan to examine possible effects on OA by changes in: i) grain selection, ii) angular radius of the smoothing function used to contour the pole figure, iii) misorientation angle threshold, and iv) spatial resolution. All factors influence the resulting OA but the most important is the choice of smoothing function. We propose a protocol for constructing contoured pole figures using MTEX and EBSD data. The derived pole figures are compatible with existing calibrations of the OA thermometer. Our results also have the potential to contribute to evaluating the effects on OA of water fugacity and strain rate.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7061

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