Natural laboratory NW Bohemia: Comprehensive fluid studies between 1992 and 2005 used to trace geodynamic processes

Comprehensive studies of CO 2 ‐rich fluids close to the swarm earthquake region Nový Kostel at the Czech‐German border have been started 15 years ago and have in particular included two extended chemical and isotope monitoring studies lasting for several years each. The regional surface distribution patterns of the fluid signatures including the identification of the origin of fluid components are the focus of the detailed studies. Three degassing centers (Cheb basin, Mariánské Lázně, and Karlovy Vary) with high CO 2 flux and the same level of δ 13 C values, but different levels of 3 He/ 4 He ratios, have been identified. The studies have located the CO 2 source and have investigated seismically induced changes in fluid characteristics on the basis of unique weekly sampling campaigns at selected locations. A seismically triggered release of crustal helium was confirmed by both monitoring campaigns. Finally, indications for a presently active magmatic process beneath the Cheb basin have been found. In contrast to volcanically active regions, magma accumulation in the study area takes place at the crust‐mantle boundary and is not yet accompanied by heat transfer to the surface. Likewise, reactive magma‐derived components are absent in the degassing fluids. The area of investigation has the potential to be a natural laboratory for fundamental studies of active geodynamic processes. The results of our fluid monitoring, including the stunning observation of mantle‐derived free fluids marked by 3 He/ 4 He ratios within the subcontinental mantle range, are supported by geophysical findings from seismic studies and geologic indications.