Discovery of highly radioactive granite in the Bergen Region

Abstract

We report the discovery of the most radioactive granite ever found in Norway, namely the Løvstakken granite, located ~5 km southwest of Bergen. A preliminary gamma-spectrometry survey carried out in the Bergen Region, in autumn 2009, showed that the Løvstakken granite contained unusually high amounts of uranium and thorium. This finding was later confirmed by a more complete regional survey during the summer of 2010. We visited 281 sites and made 502 radiometric measurements in the Bergen Region and adjacent areas. Based on 87 measurements on the Løvstakken granite, we found that it contains ~18 ppm U, ~58 ppm Th and ~6% K on average (median values). Natural radioactivity is not harmful by itself, but the high uranium levels of the Løvstakken granite (i.e., up to ~69 ppm) cause concern in terms of radon hazard. In addition, geothermal gradients in the continental crust are strongly dependent on the amount of radioactive (i.e., heat-producing) elements it hosts. Our study indicates that the Løvstakken granite produces ~8 μW/m3 of heat. Such high heat-generation values may result in anomalously high temperatures in the subsurface of the Bergen Region, that in turn may render the use of geothermal energy economically interesting.