Helium, radon, and hydrothermal circulation associated with the Carnmenellis radiothermal granite of southwest England

Abstract

The Carnmenellis granite of southwest England has higher than average radioelement concentrations and a heat flow greater than 100 mW m −2 , more than twice the United Kingdom average. Measurements were made of 4 He concentrations and 222 Rn activities in surface waters over the granite and its immediate host rocks. High values (greater than 48 ppb 4 He, 160 c/min 222 Rn) for either or both gases are interpreted as indicating discharge of groundwater from the upwelling limbs of hydrothermal convection cells. 4 He has a spatial distribution which agrees well with the observed variations of heat flow within the pluton, but 222 Rn is also influenced by lithology and depth of groundwater flow. Although the distribution of convection cells is partly determined by the presence of NW‐SE faults, there is close similarity with theoretical models of groundwater circulation. It is concluded that 4 He and 222 Rn surveys provide a simple but effective method for mapping areas of modern hydrothermal activity.