QUANTITATIVE MAPPING OF TI-IN-QUARTZ FOR THERMOBAROMETRY: APPLICATION TO HIGH-PRESSURE GRANULITE

In high-grade rocks such as high-pressure granulites, the Ti content of quartz commonly shows a large variability in composition. This apparent complexity requires a careful evaluation of the chemical data, if possible combined with a detailed microstructural study. Only then, meaningful temperatures can be derived using Ti-in-quartz thermometry. In this study, we demonstrate that cathodoluminescence (CL) mapping is essential for this purpose, since the CL emission (captured using a blue filter) on quartz is proportional to the Ti content. Quantitative maps of TiO₂ were produced by combining qualitative CL maps with spot analyses obtained by Electron Probe Micro-analyzer (EPMA). This mapping approach produces temperature maps which raise the possibility of recovering detailed records of T conditions in a microstructural context. Measurements of Ti in quartz, were performed simultaneously with a JEOL JXA8230 EPMA using three WDS spectrometers, whereas CL maps were acquired using a coupled panchromatic CL system. The XMapTools software was used for calibration and to generate temperature maps (calibration of Thomas et al., 2010). Results obtained for the high-pressure granulite from the Passos Nappe (SE – Brazil) Southern Brasília Orogen show an heterogenous distribution of Ti in quartz. The Ti content decreases towards the rim of crystals, from 134 ppm to 26 ppm. These rocks have a pelitic protolith; they recorded metamorphic peak conditions at high-pressure granulite facies followed by a phase of near isothermal decompression. Highest temperatures obtained for 1.0 GPa (~ 800 ˚C) are in good agreement with early retrograde conditions. This result indicates that Ti-in-quartz thermometry shows, in the studied sample, the temperature of quartz formation, which occurred during melt crystallization. The described method is appropriate to retrieve quartz-forming reactions and the associated conditions in granulitic rocks.

Thomas et al. (2010). Contrib. Mineral. Petrol., 160(5),743–759