Tracking the thermal properties of the lower continental crust: Measured versus calculated thermal conductivity of high-grade metamorphic rocks (Southern Granulite Province, India)

Abstract

In this study, the bulk thermal conductivity (TC) of 26 rock samples representing different types of granulite-facies rocks, i.e., felsic, intermediate and mafic granulites, from the Southern Granulite Province, India, is measured at dry and saturated conditions with the optical-scanning method. Thermal conductivity is also calculated from modal mineralogy (determined by XRD and EPMA), applying several mixing models commonly used in thermal studies. Most rocks are fine- to medium-grained equigranular in texture. All samples are isotropic to weakly anisotropic and possess low porosities (<2%). Measured TC values range between 2.5 and 3.0Wm−1K−1 for felsic granulites, between 2.5 and 3.5Wm−1K−1 for intermediate granulites and between 2.4 and 2.7Wm−1K−1 for mafic granulites. Considering this data and literature compilations, rocks representative for the lower continental crust typically display values between 2 and 3Wm−1K−1 at ambient temperature and pressure conditions. Depending on the mixing model and the mineral TC value used in the calculations, measured and calculated bulk TC could be properly fitted. For mean values of mineral TCs, the harmonic mean provides an almost perfect fit, with a mean deviation of −1±6% (1σ). However, the implication of that correspondence would be that minerals and pores are predominantly aligned parallel, which is in apparent contrast to the texture of the rocks studied here. The geometric mean, which does not consider any layering of minerals or pores in the rock and, thus, should be in better harmony with the textural characteristics of the studied high-grade rocks, matches the measured TC data very well, if minimal minerals TCs reported in the literature are applied (mean deviation 5±8%). Thus, if samples appropriate for laboratory measurements (in terms of sample size or physical-chemical-mechanical condition) are not available, bulk TC of high-grade metamorphic rocks with low anisotropy and porosity could be satisfactorily good assessed from modal mineralogy, using the data sets for mineral TC applied in this study. Further work is required on the applicability of mixing models to compute TC of other rock types, e.g., of igneous and sedimentary rocks.