Thermal properties of carbonatite and anorthosite from the Superior Province, Ontario, and implications for non-magmatic local thermal effects of these intrusions

Abstract

Igneous intrusions are important to the thermomechanical evolution of continents because they inject heat into their relatively cold host rocks, and potentially change the distribution of radiogenic heat production and thermal properties within the crust. To explore one aspect of the complex evolution of the continental crust, this paper investigates the local thermal effects of two intrusive rock types (carbonatites and anorthosites) on the Archean Superior Province of the Canadian shield. We provide new data on their contrasting properties: rock density near 298 K, thermal diffusivity, and heat capacity up to 800 K (which altogether yield thermal conductivity), plus radiogenic element contents. The volumetrically small carbonatites have widely varying radiogenic heat production (2–56 µW m−3) and moderate thermal conductivity at 298 K (~ 1 to 4 W m−1 K−1) which decreases with temperature. The massive Shawmere anorthosite has nearly negligible radiogenic heat production (< 0.002 µW m−3) and low, roughly temperature-independent thermal conductivity (~ 1.6 W m−1 K−1). Steady-state thermal structures within and around these intrusions, which have quite different shapes and physical properties, were modeled using a pipe geometry for carbonatite and a tabular geometry for anorthosite. We found that the thermal aureoles of these intrusion types persist for hundreds of millions of years after the magmatic heat advected by the intrusions has dissipated. Longevitity of aureoles is due to the high radiogenic element concentrations of the small carbonatite intrusions, and to the low thermal conductivity of the Shawmere anorthosite. Our findings apply to other anorthosite bodies, which vary little in composition and mineralogy, whereas results for carbonatites depend on variations in their radiogenic content.