Radioactive Elements in the Continental Crust

Abstract

THE radioactive elements are particularly useful as indicators of the chemical composition of the deeper parts of the crust. Thorium, uranium, and potassium show a strong concentration towards the surface of the Earth and are enriched in the continental crust. The abundance of thorium, uranium and potassium can be estimated by two independent methods; a physical method based on heat-flow data, and a geochemical method based on geological premises. These estimates differ by a factor of two (Table 1). The estimates for the heat-flow data are from Clark and Ringwood1, and are based on a 37-km-thick two-layer crust. The upper layer is 16 km thick, resting on a layer 21 km thick. Calculations of the concentrations of thorium, uranium and potassium within each layer for different heat flows are given in Table 2. It is noteworthy that the concentrations of the radioactive elements in the 16-km layer in regions with high heat flow are comparable with the geochemical estimates made on the basis of the new abundance data by Clark et al.2. The concentrations calculated for the lower layer are similar to those in basalts. This could indicate that this layer has a basaltic composition. How ever, metamorphic processes, including generation of granite magma, and the cycling of elements within the crust could also account for the low concentrations of radioactive elements in the deep crust3–5. These processes would result in a more mafic average composition of the lower crust as compared with the higher crust3–8. There is also some indication5 that high-grade metamorphic rocks, especially those of granulite facies, have lower concentrations of thorium and uranium than magmatic rocks of comparable chemical composition, so that the upward concentration of thorium and uranium in the crust could be more marked than that of potassium.