Thermal conductivity of granitoids of varying composition up to 300 °C and implications for crustal thermal models

SUMMARY A thermal conductivity profile through the upper crustal column is an essential ingredient in any thermal modelling. Granitoid is one of the major constituents of the upper crust in the Archaean cratons. Although granitoids have a wide range in composition, yet data on their thermal conductivity at elevated temperatures are very limited. At present, a single value is commonly used to characterize the decrease in thermal conductivity with temperature for the upper crust. We are reporting thermal conductivity measured at 25 °C, 50 °C and thereafter at 50 °C intervals up to 300 °C on 34 granitoid samples of four compositionally different types. The samples are alkali granite, biotite granite, granodiorite and metasomatised granodiorite from one of the Archaean cratons of the Indian shield, known as Bundelkhand Craton. Before studying the samples at elevated temperatures, these have been studied for their physical, petrological and geochemical characteristics. By 300 °C, the thermal conductivity decreases on the average by 28–31 per cent for metasomatised granodiorite, alkali granite and biotite granite, and in stark contrast by 16 per cent for granodiorite. Expressing the thermal conductivity variation with temperature as λT = λRT (1 + bT)–1, two distinct temperature coefficient (b) values have been found, 1.1 × 10–3 to 2.2 × 10–3 K–1 for alkali feldspar granite to monzogranite and 0.4 × 10–3 to 1.2 × 10–3 K–1 for granodiorite to tonalite to quartz diorite. One of the implications of this outcome is illustrated by applying these two distinct temperature coefficients for the upper crust for a 1-D generic model with a surface heat flow and appropriate radiogenic heat production of the crustal column in arriving at crustal temperature–depth profiles. The temperature differences at the base of a 40-km crust vary as much as 90 °C. Further, the temperature coefficient can be expressed as b = 0.71 × λRT– 0.63 for the alkali feldspar granite to monzogranite, whereas b = 0.83 × λRT– 1.26 for the granodiorite to tonalite to quartz diorite, which will be useful in determining the temperature coefficient of various types of granitoid from thermal conductivity at room temperature (λRT).