On the average amount and vertical distribution of radioactivity in the continental crust

The average concentrations of uranium, thorium and potassium can be estimated independently using heat flow data and geochemical mass budgets. The range of values for the average heat production is 0.7-1.0 μW/m3, which translates into a range of 15-26 mW/m2 for the mantle heat flow beneath old cratons. The vertical distribution of radioactivity is studied using the linear heat flow relation of Birch. We compare the apparent thickness D deduced from it to the structure of the upper crust determined by independent methods. Apparent thickness is significantly affected by the averaging effects of horizontal heat conduction. The reduced heat flow deduced from the Birch relation represents an average heat flow which is free from the shallowest radioactivity variations. It integrates the heat production distribution over lateral distances as large as 300 km. To compute the mantle heat flow beneath continents, it is therefore necessary to use a large-scale average of heat production which is only available through geochemical budgets