Compositional controls on the thermal conductivity of metamorphic rocks

Abstract

Thermal conductivity is essential for determining heat flow within the Earth, which is necessary for geothermal investigations, accurately modelling tectonic and volcanic processes, and predicting petroleum maturation. Although, currently, conductivity can be measured on hand samples it can be impracticable to make regional and subsurface models due to time and expense required. In this study, an analysis of the compositional controls on thermal conductivity of metamorphic rocks is completed. Thermal conductivity was determined using an optical thermal scanner on 168 metamorphic samples for which there are prior major oxide element analyses. Density is determined through models, as well as measured using Archimedes’ principle. The results show that thermal conductivity varies between 1.698 to 5.226 W m−1 K−1. When observing the relationships there is no trend between thermal conductivity and the major oxides. However, anisotropy has a log normal distribution with a mean of -2.098 and a standard deviation of 1.346 and produces a weak negative correlation with conductivity of -0.566. A correlation exists between SiO2 and K2O where a maximum anisotropy potential peaks between 60% to 65%, and approximately 5%, respectively. The modelled density is successful in determining the measured density, allowing the density for future samples to be determined indirectly. From the results of the study, more consideration needs to be taken in the future when observing the compositional controls for metamorphic rocks. A narrower range of rock types or chemistry could be considered, along with the mineralogy of the samples. Singular provinces should also be considered to determine if conductivity for metamorphic rocks occurs regionally. Furthermore, a focused study on how the P-T conditions of a singular rock type change with thermal conductivity could be undertaken. Such analyses will improve estimates of subsurface conductivity and the ability to accurately estimate crustal temperatures.