Ti‐in‐zircon and Ti‐in‐quartz thermometry using electron probe microanalysis: A promising approach in retrieving thermal conditions of granulite facies metamorphism and felsic magmatism in the Sandmata Complex, Aravalli Craton (NW India)

Abstract

The titanium (Ti) concentrations in zircon and quartz and corresponding thermometers have been regarded as powerful monitors that constrain petrogenetically meaningful temperature conditions of crystallization and metamorphism. The precise measurement of trace elements by electron probe microanalyzer (EPMA) is quite challenging as their x‐ray intensities are only slightly higher than the Bremsstrahlung background, leading to significant errors while quantifying the elemental abundances (e.g., Ti‐in‐zircon). We present EPMA‐based protocols for analysing Ti concentrations in zircon and quartz by considering recent analytical and instrumental improvements. High counting times at peak and background positions, simultaneous acquisition of Ti in multiple (three) spectrometers and data processing using the non‐central chi2 test for risk determination in sub‐counting method were employed in Ti‐in‐zircon and Ti‐in‐quartz protocols. Applying these protocols to the Mongolian garnet, San Carlos olivine and NIST 610 glass standards yielded Ti concentrations of 0.60 ± 0.02 wt%, 18 ± 3 ppm and 437 ± 26 ppm (in ±2σ), respectively. The Ti concentrations obtained from these standards are consistent with those published based on high‐precision analytical methods. We use the existing Ti‐in‐zircon and Ti‐in‐quartz thermometers to understand the thermal evolution of various lithologies in the Sandmata Complex, Aravalli Craton (north‐western India). The investigated samples include garnet–biotite gneiss, migmatite gneiss, mafic granulite and Anjana granite. The Ti‐in‐zircon temperatures calculated from the patchy‐zoned zircon cores yield HT–UHT conditions (892–959°C) for garnetiferous granite gneiss, migmatite gneiss and mafic granulite, whereas the zircon overgrowths yield temperatures of 725–871°C. The homogeneous zircon cores from Anjana granite yield temperatures (914–984°C) comparable to zirconium saturation conditions (920–960°C), indicating zircon crystallization from “hot” granitic melt. The oscillatory overgrowths on the zircon core obtain variable temperature conditions for inner overgrowth (811–877°C) and outer overgrowth (714–782°C), suggesting the episodic growth of zircon grains from different magma pulses. Additionally, the application of pressure‐dependent Ti‐in‐quartz thermometry yields quartz recrystallization conditions for garnet–biotite gneiss (430–556°C), migmatite gneiss (423–593°C), mafic granulite (430–679°C) and Anjana Granite (444–533°C). The combination of Ti‐in‐zircon thermometry, Ti‐in‐quartz and conventional thermobarometry demonstrates cooling histories for various high‐grade metamorphic and magmatic rocks in the Sandmata Complex. Based on this study, we emphasize that trace element thermometers can decipher high‐temperature metamorphism, which is otherwise erased in conventional thermometers due to diffusional readjustments during long‐lasting near‐peak and post‐peak metamorphism.

Atomic spectrometry update – a review of advances in environmental analysis

“…Applying this method to USGS reference glasses revealed that USGS BHVO-2G (basalt) had the highest degree of homogeneity for both Cl and F in geological samples. Ghosh et al 252 reported an EPMA method to determine Ti concentrations in zircon and quartz to retrieve thermal conditions of metamorphism and magmatism by Ti-in-zircon and Ti-in-quartz thermometry. To meet the challenges posed by X-ray intensities being only slightly above Bremsstrahlung background when measuring trace elements, this method combined long counting times at peak and background positions, simultaneous acquisition of Ti in three spectrometers and processing of data using the sub-counting method in which the total analytical time was split into smaller cycles to reduce any effects from surface damage on the final counts.…”