Monthly monitoring of gas and isotope compositions in the free gas phase at degassing locations close to the Novy Kostel focal zone in the western Eger Rif? Czech Republic

摘要

We report new data on gas and isotope compositions of mantle-derived exhalations from five locations in the Vogtland (Germany)/NW Bohemia (Czech Republic) area, close to the Novy Kostel focal zone, recorded by monthly sampling of gases over a three year period. This region is the locus typicus for the term "earthquake swarm"; since 1985/86 the largest numbers of earthquake swarms in the Vogtland/NW Bohemia have occurred in the Novy Kostel focal zone. Mantle-derived degassing has been studied at four locations within the Cheb Basin degassing centre, and at the Wettinquelle spring to the north on the edge of the Cheb Basin. The Bublák and U Mostku locations are on the Po?atky Plesná Fault Zone (PPZ) while the Kopanina and Dolni ?astkov locations are on the Mariánské Lázně Fault (MLF). The mantle-derived helium content at locations along the PPZ covers the range of the sub-continental lithospheric mantle (SCLM). Along the MLF the ~3He/~4He ratios are with up to 5.4Ra a little bit lower as at the PPZ locations. The CO_2/~3He ratios point to a predominantly magmatic source. Depending on the magnitude of gas flux the monthly sampling results indicated various major influences on the fluid signatures at the monitored locations due to seasonal cycles based on the different strong influence of CO_2/water interaction. Micro-seismicity occurred repeatedly during the monitoring period and seismically triggered geochemical anomalies were repeatedly observed at the locations close to the focal zone. The ~3He/~4He ratios drop from 5.9Ra (mean) up to 5.6Ra at Bublák and from 5.6Ra up to 5.3Ra at U Mostku. A decrease in ~3He/~4He ratios was observed prior to seismic events due to stress accumulation before rupturing, and also after the events due to the release of crustal-derived components within the area of the focal zone. Due to the higher 'helium baseline content' of the Kopanina gas the ~3He/~4He ratios decreased there only from 4.6 to 4.5Ra. At the Wettinquelle, the most distant from the focal zone, the ~3He/~4He ratios ranged between 2.3 and 2.5Ra altogether. Superimposed to the seismically induced anomalies a clear three month-lasting increase of mantle-derived helium was observed at the locations on the PPZ. The ~3He/~4He ratios increased to 6.3Ra at Bublák and to 6.0Ra at U Mostku. An increase in the 3He/4He ratios was also noted at Kopanina (to 4.7Ra) and Dolni ?astkov (to 5.4Ra), interpreted as indicating an ascent of less degassed magma (dyke intrusions) from the deeper lithospheric mantle reservoir. Superimposed geochemical effects initiated by the temporal and spatial evolution of a hidden magmatic-driven geodynamic process have therefore been observed in a continental rift system for the first time.