Monitoring of helium and carbon isotopes in the western Eger Rift area (Czech Republic): Relationships with the 2014 seismic activity and indications for recent (2000-2016) magmatic unrest

CO2 is the major component at all degassing sites. The δ13C values display a small range (−1.7 to −5.1‰) and the 3He/4He ratios vary from 1.9 to 5.9 Ra. The highest 3He/4He ratios are found at locations along the Počatky-Plesná Fault Zone, followed by the degassing site in the clay pit on the Nová Ves Fault and the locations on the ML fault at the edge of the CB. Although gas flow and CO2 concentrations in all degassing centers are very high, the fractions of mantle-derived helium are different, with presently up to 94% (in relation to the SCLM 3He/4He of 6.32 Ra) in the CB, up to 73% in the ML and up to 35% in the Karlovy Vary degassing center. At the locations in the eastern part of the CB a clear, progressive increase of the 3He/4He ratio has been observed since the first sampling campaigns there in 1993 and 1994, whereas at the other degassing sites the helium isotope ratio remained essentially the same. The progressive increase of the 3He/4He ratio in the eastern part of the CB, together with further short-time increases up to 6.3 Ra at one location (Bublák) before both the 2000 and 2008 earthquake swarms, indicate an ongoing magmatic process beneath this area, which seems to be associated with the occurrence of seismicity. The CB is located close to the Nový Kostel focal zone where since the beginning of our investigations four strong periods of seismicity (with magnitudes >3) occurred. The latest gas data confirm our earlier findings: time-series studies showed that in relation with seismic events, decreased 3He/4He ratios were repeatedly observed due to admixed seismically released crustal helium. Presently, the eastern part of the CB is the most active non-volcanic region in the European Cenozoic Rift System, with gas signatures similar to those found in free mantle-derived gases from the East African Rift system.