Radioactivity of rocks

Abstract

Although numerous previous measurements of the radioactivity of terrestrial materials have been made, most of these researches are of qualitative value only in their applications to geology, geophysics, and cosmology because of inadequate recognition of the analytical care necessary in order to obtain reliable results. In the present study of the radioactivity of terrestrial materials, a systematic program of standardization, calibration, and interchecking has been followed throughout. As part of an international intercalibration among the various workers in this field and a general program of helium age research, several hundred radioactivity measurements have been made. These results represent the most reliable collection of radioactivity determinations which have yet been made within the range of concentrations involved. By combining these newer measurements with the limited number of well-authenticated earlier analyses available, average values have been obtained as follows: 1.37 ± 0.17 × 10 −12 g Ra/g for 43 acidic igneous rocks, 0.51 ± 0.05 × 10 −12 g Ra/g for 7 intermediate igneous rocks, 0.38 ± 0.03 × 10 −12 g Ra/g for 54 basic igneous rocks, and 0.57 ± 0.08 × 10 −12 g Ra/g for 28 sedimentary rocks; 3.0 ± 0.3 × 10 −6 g U/g, 13 ± 2.0 × 10 −6 g Th/g, and a Th/U ratio of 5.0 for 26 acidic igneous rocks; 1.4 ± 0.2 × 10 −6 g U/g, 4.4 ± 1.2 × 10 −6 g Th/g, and a Th/U ratio of 2.6 for 6 intermediate igneous rocks; 0.96 ± 0.11 × 10 −6 g U/g, 3.9 ± 0.6 × 10 −6 g Th/g, and a Th /U ratio of 4.0 for 34 basic igneous rocks. These values are substantially lower than those obtained by Jeffreys in a compilation of most of the measurements reported prior to 1936. The present averages show a more marked decrease of radioactivity with increasing basicity, the Th/U ratios are considerably greater than those compiled by Jeffreys and are in better agreement with those to be expected from geochemical considerations. Two ultrabasic rocks were found to have radio-activities comparable to the low values for iron meteorites. Specific inaccuracies in earlier investigations have been, discovered. Estimates are made of the rate of production of heat by radioactive decay based on the above average values for the different rock types.