Radioactivity and radiogenic heat in Sierra Nevada plutons

Abstract

A radiometric traverse was made across granitic rocks in the Huntington Lake area of the Sierra Nevada, California. Field γ-ray counting rates were recorded, and samples were taken and subsequently evaluated in the laboratory for U, Th, and K content by γ-ray spectrographic analyses. These concentrations were converted to heat-generation values, using the factors given by Birch: 1 ppm U = 0.73 μcal/g yr, 1 ppm Th = 0.20 μcal/g yr, 1 per cent K = 0.27 μcal/g yr. Three principal plutons were sampled: a stock of pyroxene quartz diorite on the western border of the Sierra, hornblende-biotite granodiorite of the ‘Dinkey Creek type’ on the western slope of the range, and the Mount Givens granodiorite occupying the higher elevations east of Huntington Lake. Laboratory results indicate that the Mount Givens granodiorite averages nearly twice the U and Th content and, therefore, nearly twice the radiogenic heat production of Dinkey Creek granodiorite; the pyroxene quartz diorite is quite low in radioisotope content, and consequently in heat production, compared with the Mount Givens and Dinkey Creek plutons. Average values are The ratio of Th to U does not vary appreciably between the Mount Givens and Dinkey Creek plutons, averaging 3.24 in the Dinkey Creek and 3.06 in the Mount Givens. If the radioactivity of the Mount Givens granodiorite can be considered to be typical of the younger granitic rocks of the Sierra Nevada, and that of the Dinkey Creek granodiorite typical of the older plutonic rocks, a large central part of the batholith may have considerably higher radioactivity and radiogenic heat production than the older rocks on the western flank of the Sierra.