A new empirical calibration of the quartz c-axis fabric opening-angle deformation thermometer

The opening-angle of quartz c-axis fabrics (OA) is strongly temperature dependent and has proven to be a powerful deformation thermometer for natural metamorphic rocks. Previous considerations of empirical data have identified a linear correlation between OA and temperature between 250 and 650 degrees C, and no correlation above 650 degrees C. However, possible effects of pressure have not been investigated. We expanded the data set of OA versus temperature, including data from rocks deformed over 300-1050 degrees C and 2.5-15 lobar. Disregarding possible effects of pressure, the OA-temperature relationship can be described by two linear correlations for the intervals similar to 250-650 degrees C and similar to 650-1050 degrees C: T (degrees C) = 6.9 OA (degrees) + 48 (250 degrees C <= T <= 650 degrees C and OA <= 87 degrees) T () = 4.6 OA (degrees) + 258 (650 degrees C <= T <= 1050 degrees C and OA >= 87 degrees) The change on the curve slope of the OA-temperature relationship correlates approximately to the low- to high-quartz transition and to changes in the dynamic recrystallization mechanism from subgrain rotation to grain boundary migration. The available data suggest that pressure has a secondary effect accompanying the major temperature dependence of OA, which is particularly important for temperatures above 650 degrees C, where the correlation between OA and temperature is less pronounced. For fixed pressures, the OA has logarithmic relationships with temperature over the range 250-1050 degrees C. The following thermometer equation is formulated from a multiple regression: T (degrees C) = 410.44 lnOA (degrees) + 14.22 P (kbar) -1272 An uncertainty of +/- 50 degrees C is inherited from the petrological temperature estimates of the natural samples. The data suggest the gradual increasing importance of prism {[}c] slip relative to <a> slip in quartz with rising temperature. Under conditions of `average' geological strain rate and water weakening, prism {[}c] slip dominates for deformation above similar to 700 degrees C. (C) 2016 Elsevier B.V. All rights reserved. (AU)