Heat production and thermal conductivity of rocks from the Pikwitonei–Sachigo continental cross section, central Manitoba: implications for the thermal structure of Archean crust

Abstract

The Pikwitonei and Sachigo subprovinces of central Manitoba provide a cross-sectional view of the Superior Province crust. In cross section, the upper to mid-level crust is composed of synformal greenstone belts surrounded by tonalitic gneisses, both of which are intruded by granitoid plutons. This crustal structure persists downward into the granulite facies, where keels of the greenstone belts can be found. To constrain thermal models of the crust, we measured heat production and thermal conductivity in 60 rocks from this terrain using standard gamma-ray spectrometry and divided bar techniques. Large vertical and lateral heterogeneities in heat production in the upper crust are evident; heat production is high in granites and metasedimentary rocks, intermediate in tonalite gneisses, and low in the portions of greenstone belts dominated by mafic meta-igneous rocks. In the deeper granulite facies rocks, heat production decreases by a factor of two in the tonalitic gneisses and remains low in the high-grade mafic rocks. When applied to the Pikwitonei–Sachigo crust cross section, the laboratory data here do not support step function or exponential models of the variation of heat production with depth. However, estimates of surface heat flow and surface heat production for various sites in the crustal model yield the well-known linear relationship between surface heat production and surface heat flow observed for heat-flow provinces for both one- and two-dimensional models. This demonstrates that determinations of heat production with depth based on inversion of the linear heat-production–heat-flow relationship are nonunique.