Thermal conductivity at elevated temperature, density and geochemical signatures for the massive rhyolites of the Bundelkhand Craton, central India

Abstract

SUMMARY In spite of the fact that rhyolite constitute a vital part of the key tectonic environments, such as continental rift-arc systems and oceanic islands, the data on the thermal and physical properties are scarce, which hinders the exact thermal modelling of these regions. Here, we have investigated the thermal conductivity from room temperature (25 °C) to elevated temperatures (up to 300 °C) for 11 massive rhyolite samples, collected from the greenstone belt of the Bundelkhand Craton, central India. The petrographical, geochemical (major oxide and trace elements) and physical (density and porosity) properties have been studied to characterize the samples before measurement of thermal conductivity at elevated temperatures. Geochemical results indicate that these rhyolites are high-K (K2O: 3.6–5.4 wt. per cent), calc-alkaline in nature with enriched REE signatures {(La/Yb)N: 9.4–22.3, (Gd/Yb)N: 1.2–1.9} and are similar to FI-type Archaean rhyolites. The density of these rhyolites depicts a narrow range between 2590 and 2690 kg m−3, with an average of 2637 kg m−3 and negligible porosity. Their thermal conductivity at room temperature varies between 2.5 and 3.3 W m−1 K−1, with an average of 2.8 W m−1 K−1; the decrease in thermal conductivity from room temperature to 300 °C ranges between 16 and 32 per cent, with an average of 23 per cent; and the temperature coefficient of thermal conductivity b, in the expression λT = λRT (1 + bT)−1, varies between 0.7 × 10−3 and 1.7 × 10−3 K−1 with an average of 1.1 × 10−3 K−1. Our study reveals that the massive rhyolites have an almost similar density as their intrusive equivalent like Bundelkhand granitoids, but their thermal properties, such as thermal conductivity at room temperature (λRT), decrease in thermal conductivity with temperatures (Δλ) and the temperature coefficient of thermal conductivity (b), lies between the two extreme variety of the granitoids, that is (i) alkali feldspar granite to monzogranite and (ii) granodiorite to tonalite to quartz diorite. We suggest that the temperature coefficient of the massive rhyolite can be expressed as b = 0.81 × λRT–1.21, which will be useful in determining the thermal conductivity of such rhyolites at elevated temperatures from their thermal conductivity at room temperature (λRT). Thermal and physical parameters reported for rhyolites will provide important constraints in various geophysical and thermo-mechanical modelling for the rhyolitic terrains.