Applicability of ICP-MS/AES, γ-ray and portable X-ray spectrometry (pXRF) data for rock-generated radiogenic heat calculations

Abstract

Rock-generated radiogenic heat is one of the key parameters in assessing geothermal potential. To calculate it accurately, it is necessary to know the K, U and Th contents of the rocks in a given region. We have compared measurements of these element concentrations using results obtained by three analytical methods: γ-ray spectrometry, pXRF spectrometry and ICP-MS/AES. The measurements were made on the same samples. By employing statistical methods used in analytical and clinical chemistry, it was quantitatively demonstrated that pXRF tends to overestimate, and γ-ray spectrometry may underestimate (though to a lesser extent), the concentrations of U and Th by comparison with ICP-MS/AES. The scale of over- and underestimation is the especially large for  samples with low contents of these elements. These discrepancies, especially in the case of pXRF measurements, lead to significant differences in the calculated radiogenic heat results, which can be many times higher (by up to two orders of magnitude) than those calculated on the basis of instrumental chemistry analyses. It can be concluded that both γ-ray spectrometry and pXRF spectrometry may be used for analyses to determine radiogenic heat. However, while the former method may be a primary research technique for this task (replacing instrumental chemical techniques), the latter should be used more as a screening technique, and may require calibration based on local standard samples obtained from the study area.