In situ gamma-ray measurements of 40K, 232Th and 238U in high-grade metamorphic rocks from the Sowie Mountains (Sudetes, SW Poland)

Abstract

This paper reports on the abundance of primordial radionuclides (40K, 232Th and 238U) in characteristic lithologies from the Sowie Mountains (SW Poland). In situ gamma-ray measurements were conducted at 12 localities hosting exposed augen gneiss, flaser gneiss, granulite, homophanized gneiss, hornblendite, layered gneiss, layered migmatite, migmatic gneiss, mylonitic gneiss and mylonitic granulite. The activity concentration of 40K varied from 180 Bq kg–1 (mylonitic granulite) to 845 Bq kg–1 (layered gneiss). The activity concentrations associated with 228Ac (232Th) varied from 10 Bq kg–1 (mylonitic granulite) to 53 Bq kg–1 (homophanized gneiss), while activity associated with 226Ra (238U) varied from 9 Bq kg–1 (mylonitic granulite) to 43 Bq kg–1 (layered gneiss). An augen gneiss adjacent to the Intra-Sudetic Fault showed the highest combined Th + U activity (89 Bq kg–1). The average Th/U ratio (3.6) calculated for gneiss outcrops of the Sowie Mountains falls within the range reported for biotite gneiss (3.5–4). Subsurface outcrops from the Walim-Rzeczka underground complex did not give noticeably higher 238U activities. The results obtained for the Sowie lithologies are compared with those obtained by similar methods for similar rocks as reported in literature sources.