Carrier and dilution effects of CO₂ on thoron emissions from a zeolitized tuff exposed to subvolcanic temperatures

Radon (²²²Rn) and thoron (²²⁰Rn) are two isotopes belonging to the noble gas radon (sensu lato) that is frequently employed for the geochemical surveillance of active volcanoes. Temperature gradients operating at subvolcanic conditions may induce chemical and structural modifications in rock-forming minerals and their related ²²²Rn-²²⁰Rn emissions. Additionally, CO₂ fluxes may also contribute enormously to the transport of radionuclides through the microcracks and pores of subvolcanic rocks. In view of these articulated phenomena, we have experimentally quantified the changes of ²²⁰Rn signal caused by dehydration of a zeolitized tuff exposed to variable CO₂ fluxes. Results indicate that, at low CO₂ fluxes, water molecules and hydroxyl groups adsorbed on the glassy surface of macro- and micropores are physically removed by an intermolecular proton transfer mechanism, leading to an increase of the ²²⁰Rn signal. By contrast, at high CO₂ fluxes, ²²⁰Rn emissions dramatically decrease because of the strong dilution capacity of CO₂ that overprints the advective effect of carrier fluids. We conclude that the sign and magnitude of radon (sensu lato) changes observed in volcanic settings depend on the flux rate of carrier fluids and the rival effects between advective transport and radionuclide dilution.