Composition-based estimates of the thermal properties of New Zealand basement rocks

Abstract

ABSTRACT We present composition-based estimates and the first national maps of thermal conductivity and heat production of New Zealand's basement terranes. Our thermal conductivity estimates are based on a geometric mean of four mineral groupings, quartz, olivine, pyroxene and other (expected to be predominantly feldspars). Thermal conductivities of each of these mineral groups are derived from regression of thermal conductivity in log space as a function of proportion of each of these groups. We compare our thermal conductivity mixing model with a previously developed model using major oxide composition, and find both models work well for New Zealand basement rocks, providing an opportunity to calculate thermal conductivity from either major oxide composition or mineralogy. We note spatial patterns in estimated thermal properties, with high thermal conductivities and heat production in the Buller Terrane and intruding batholiths, the opposite in the central Brook Street, Murihiku and Maitai terranes and eastern Caples and Waipapa terranes, and intermediate thermal conductivity and heat production in the eastern Torlesse terranes. Rocks with high thermal conductivity tend to also have high heat production (and vice versa) because thermally conductive quartz-rich rocks also tend to contain high concentrations of radiogenic heat producing elements.