Heat from radioactive elements in young volcanics by γ-ray spectrometry

Radioactive heat production data for volcanic rocks ranging in age from 430 ka to the historical time and cropping out on the three main islands of the Aeolian arc (Southern Italy) are presented. They were derived from uranium, thorium and potassium concentration measured from γ-radiations originating from the decay of 214Bi (238U series), 208Tl (232Th series) and the primary decay of 40K. Concentration results from a NaI(Tl) detector, compared with independent determinations by means of mass spectrometry, reveal significant secular radioactive disequilibrium between 214Bi and 238U, in the rhyolitic rocks, which is ascribable to their young age. The use of the low-energy portion of the spectrum, where there are a number of γ-rays produced by 234Th, minimises the disequilibrium problem in the uranium determination. The Th/U ratio remains almost constant at 3.3 in all the rock types. Due to their low isotopic content, basalts show a low radioactive heat production rate (0.6 μW m−3), whereas the highest values (6.6–7.1 μW m−3) are found in trachytic and rhyolitic lavas.