5:15 PM - 7:15 PM
[SMP28-P10] Constructing CPO patterns from EBSD measurements to be used in quartz c-axis fabric opening-angle thermometry: issues and proposed protocol
Keywords:EBSD, Quartz, Dynamic recrystallization, Quartz c-axis fabric opening-angle geothermometer
It is well known that the opening-angle of c-axis pole figure or fabric of quartz aggregates deformed by dislocation creep (Figure 1) is roughly proportional to the deformation temperature (°C), and this relationship is the basis of the quartz c-axis fabric opening-angle geothermometer (OA thermometer). The relationship can be explained semi-quantitatively by the deformation temperature dependence of the slip systems active in quartz, and the thermometer has been calibrated using a range of natural samples. There is active research on the effects of deformation parameters such as strain rate and fluid pressure on the opening-angle, with the aims of strengthening our understanding of the theoretical background to the OA thermometer and quantifying the associated errors. However, the opening-angle is also potentially influenced by the methods used to measure the crystal orientations and construct the quartz c-axis fabric (CPO). Few studies have examined these effects.
In recent years, the development of electron backscatter diffraction (EBSD) methods has made it possible to obtain large data sets of crystal orientations in a short time. In addition, the MTEX (open-source MTEX toolbox for MATLAB) has become a widely used standard tool for constructing CPO from such crystal orientation data. In this study, we investigated the factors that could affect the opening-angle when constructing a CPO based on EBSD measurements and using MTEX and considered the implications for the OA thermometer. We studied a quartz schist sample collected from the central part of the Shikoku region of the Sanbagawa metamorphic belt. Only quartz-dominant regions were selected for analysis.
Following previous studies, we used values of the MTEX function GOS (grain orientation spread) to examine the amount of internal strain within individual grains, and distinguish between relict grains that had accumulated plastic strain and relatively strain-free recrystallized grains. A comparison of the CPO patterns associated with these two grain populations shows that the opening-angle defined by recrystallized grain orientations was several degrees larger than that defined by the orientations of the relict grains. One degree difference in the opening-angle results in difference in the estimated deformation temperature of several degrees. In MTEX, the MUD values (multiples of uniform density) are calculated from crystal orientations using kernel density estimates to define contours. For these estimates, the half-width variable (angular radius of the smoothing cap) has a significant effect on the geometry of the contours (the larger the value, the smoother and more averaged the contour, and the smaller the value, the more it reflects local density differences). The results with half widths of 3–30°show that application of larger half width angles results in smaller opening-angles. The above results indicate that when constructing quartz CPO and using OA thermometers using EBSD and MTEX, it is necessary to define protocols for identifying recrystallized grains and the appropriate value of the half width for contouring.
In published results used to construct OA thermometers some workers focus only on the orientations of recrystallized grains or subgrains, whereas others include orientations of relict grains such as ribbon-shaped grains. As we show, the choice of whether or not to include relict grains affects the resulting opening-angle. In view of this lack of consistency in previous studies, we propose to measure the opening-angle for both types of CPO, one constructed from all grains, and the other constructed from selected recrystallized grains, and use the estimated temperatures as an appropriate range for the deformation temperature. In many publications, the c-axis orientations are plotted on an equal-area projection, and contoured using programs that display contours based either on MUD or the percentage of points that fall within a specific counting area. From our experience we propose that the half width for the smoothing cap be set to 5°when constructing the contoured CPO. This results in diagrams that are similar in their smoothness as those used in papers that propose the OA thermometers.
In recent years, the development of electron backscatter diffraction (EBSD) methods has made it possible to obtain large data sets of crystal orientations in a short time. In addition, the MTEX (open-source MTEX toolbox for MATLAB) has become a widely used standard tool for constructing CPO from such crystal orientation data. In this study, we investigated the factors that could affect the opening-angle when constructing a CPO based on EBSD measurements and using MTEX and considered the implications for the OA thermometer. We studied a quartz schist sample collected from the central part of the Shikoku region of the Sanbagawa metamorphic belt. Only quartz-dominant regions were selected for analysis.
Following previous studies, we used values of the MTEX function GOS (grain orientation spread) to examine the amount of internal strain within individual grains, and distinguish between relict grains that had accumulated plastic strain and relatively strain-free recrystallized grains. A comparison of the CPO patterns associated with these two grain populations shows that the opening-angle defined by recrystallized grain orientations was several degrees larger than that defined by the orientations of the relict grains. One degree difference in the opening-angle results in difference in the estimated deformation temperature of several degrees. In MTEX, the MUD values (multiples of uniform density) are calculated from crystal orientations using kernel density estimates to define contours. For these estimates, the half-width variable (angular radius of the smoothing cap) has a significant effect on the geometry of the contours (the larger the value, the smoother and more averaged the contour, and the smaller the value, the more it reflects local density differences). The results with half widths of 3–30°show that application of larger half width angles results in smaller opening-angles. The above results indicate that when constructing quartz CPO and using OA thermometers using EBSD and MTEX, it is necessary to define protocols for identifying recrystallized grains and the appropriate value of the half width for contouring.
In published results used to construct OA thermometers some workers focus only on the orientations of recrystallized grains or subgrains, whereas others include orientations of relict grains such as ribbon-shaped grains. As we show, the choice of whether or not to include relict grains affects the resulting opening-angle. In view of this lack of consistency in previous studies, we propose to measure the opening-angle for both types of CPO, one constructed from all grains, and the other constructed from selected recrystallized grains, and use the estimated temperatures as an appropriate range for the deformation temperature. In many publications, the c-axis orientations are plotted on an equal-area projection, and contoured using programs that display contours based either on MUD or the percentage of points that fall within a specific counting area. From our experience we propose that the half width for the smoothing cap be set to 5°when constructing the contoured CPO. This results in diagrams that are similar in their smoothness as those used in papers that propose the OA thermometers.