NON-MAGMATIC THERMAL EFFECTS OF INTRUSIONS

Igneous intrusions not only inject magmatic heat into their host rocks, they also change the distribution of radiogenic heat production and thermal properties within the crustal section. Here we investigate these non-magmatic effects of intrusions on thermal conduction in the crust, using laboratory measurements and thermal models. We selected several carbonatites, a pegmatite and anorthosite from the Canadian shield, and determined their thermal conductivity from measurements of heat capacity, thermal diffusivity and density. The carbonatites and pegmatite have high widely varying radiogenic heat production (~1 to ~56 µWm^-3) and variable thermal conductivity (from ~4 to 1 m^-1 K^-1, that decreases at higher temperatures. The anorthosite has very low radiogenic heat production of 0.0016 µWm^-3 and low, temperature-independent thermal conductivity near 1.6 W m^-1 K^-1.

We modeled steady-state thermal structures within and around pipe-like carbonatite intrusions. We show that a combination of high radiogenic element concentrations and low thermal conductivity can enable the thermal aureole to persist for hundreds of millions of years after the magmatic heat has entirely dissipated. Models of tabular anorthosite bodies show that they also remain warmer than their host rock, despite having virtually no radiogenic heat production. This effect is due entirely to the anorthosite’s low thermal conductivity relative to the host tonalite.