Estimating concentrations of heat producing elements in the crust near the Sudbury Neutrino Observatory, Ontario, Canada

Because the concentrations of uranium and thorium in the crust must be determined precisely for the future geoneutrino observations planned at the Sudbury Neutrino Observatory, we investigate whether airborne radiometric surveys can be used to constrain crustal radioactivity. The regional airborne surveys cover a wide area with high spatial resolution (< 250 m), but are only sensitive to a very thin (25 cm) surficial layer. We calculate crustal heat production in the Sudbury region from airborne radiometric surveys and compare with measurements on outcrop and core samples, and with heat flow data. The concentrations of uranium, thorium, and potassium from radiometric surveys are correlated with geology, but heat production estimates are lower than values from rock samples. The radiometric surveys give a mean heat production of 0.8 ± 0.6 (σ) μW m− 3 for more than 176,000 values. The outcrop samples collected along a transect in the Superior Province yield an average heat production of 2.9 ± 2.4 (σ) μW m− 3 and core samples from drill holes yield an average of 2.5 ± 0.8 (σ) μW m− 3. The high heat production in the rock samples is consistent with surface heat flux measurements near Sudbury with a mean value that is 12 mW m− 2 higher than the average Canadian Shield. The study shows that airborne aeromagnetic surveys give useful information on lateral variations in surface heat production but are unlikely to provide the reliable values of heat production needed to calculate the crustal geoneutrino flux. Crustal heat production will be best calculated from heat flux data complemented by heat production measurements on rock samples. The high mean heat production in Sudbury Igneous Complex samples (≈ 1.5 μW m− 3) suggests that the main source of the melt sheet was the very radioactive upper crust of the Superior Province or that the melt sheet was extremely enriched relative to a lower crustal source.