The different degassing behaviour of upper mantle-derived fluids in the western Eger rift area - a contribution to find an optimal ICDP location
Zusammenfassung der Projektergebnisse
Our detailed fluid studies have displayed the western Eger rift area as natural laboratory to study fundamental fluid-coupled geodynamic processes recommending this area as very suitable for a future ICDP Proposal. The actual geodynamic situation offers the unique chance to find out significant links between different tectonic environment, magma moving and occurring earthquake swarm seismicity within a continental rift system. Therefore such drilling would make progress in the understanding of fluid-coupled geodynamic processes in the lithosphere as a whole. 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 Nový Kostel focal zone, recorded by monthly sampling of gases over a three year period. Mantle-derived degassing has been studied at four locations within the Cheb basin gas escape 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 Počatky Plesná Fault Zone (PPZ) while the Kopanina and Dolni Častkov locations are on the Mariánské Láznĕ Fault (MLF). The portion of mantlederived helium was higher along the PPZ than along the MLF. The monthly sampling results indicated various major influences on the fluid signatures at the monitored locations due to seasonal cycles. Microseismicity occurred repeatedly during the monitoring period and seismically triggered geochemical 3 4 anomalies were repeatedly observed at the locations close to the focal zone. A decrease in He/ He 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. Surprisingly, superimposed to the seismically induced anomalies a clear three month-lasting increase of mantle-derived helium was observed. Along the PPZ the 3He/4He ratios increased to 6.3 Ra at Bublák and to 6.0 Ra at U Mostku. Due to limited frame of the project only the degassing behaviour along the Počatky Plesná could be studied in detail. The quantifying of the CO2 degassing by comprehensive studies of soil CO2 concentrations and soil CO2 fluxes as well gas flux measurements along the PPZ fit well with the data from isotope time series. The Bublák flux characteristic indicates together with the subcontinental helium isotope signature that this structure acts as highly permeable fluid transport pathway (conduit) down to the lithospheric mantle. The progressive temporal and spatial increase of the 3He/4He ratios first along the PPZ and after this at the MLF probably indicates a connection between both of these faults at deeper crustal levels. The three month-lasting increase of the 3He/4He ratios in spring 2006 was interpreted as indicating the ascent of less degassed magma (dike intrusion) from the deep lithospheric mantle. Such an effect, initiated by the temporal and spatial evolution of a hidden magmatic-driven geodynamic process, has been observed, for the first time, in a non-volcanic continental rift system. At present the western Eger rift is the most active part of the European Cenozoic Rift system. The results of the project make the western Eger rift area to a first rate natural analogue to study processes which are of interest at the CO2 sequestration (e.g. evaluation of CO2 migration, tracing CO2 degassing). The established tool (comprehensive isotope time series) is also of interest for the geothermal exploration and useful at studies due to volcanic risk evaluation. The message of the ongoing hidden magmatic process at the Czech-German border region has resulted in manifold media release (internet, press, radio, and television). Media release was also given to the Award of the Geochemical Society of Japan received during the 16th Annual V.M. Goldschmidt Conference August 2006.
Projektbezogene Publikationen (Auswahl)
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2006. Combined geochemical and geophysical studies on the western Eger (Ohře) rift. Central Europe, Geochim. Cosmochim. Acta
Kämpf H., Heuer B., Bräuer K.
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2006. Free gases as tracers for mantle-related geodynamic processes in the European Cenozoic rift system. Geochim. Cosmochim. Acta
Bräuer K., Kämpf H., Niedermann S., Strauch G.
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2007. Combined gas-geochemical, geophysical and mantle-xenolithe studies on the Vogtland/NW-Bohemia intraplate mantle degassing field (western Eger rift, Central Europe). In: “Mantle plumes –A multidisciplinary approach” (eds. J.R. Ritter and U.R. Christiansen) Springer-Verlag Berlin-Heidelberg-New York, pp.127-158
Kämpf H., Geissler W.H., Bräuer K.
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2008. Isotopic characterisation of magmatic volatiles as new tool for tracing of hidden active magmatic processes. Geochim. Cosmochim. Acta 72 (12), A109
Bräuer K., Kämpf H., Strauch G.
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2008. The gas flow at mineral springs and mofettes in the Vogtland/NW Bohemia: an enduring long-term increase. Geofluids 8, 274-285
Koch U., Bräuer K., Heinicke J., Kämpf H.
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2008. The Hartoušov Mofette Field in the Cheb Basin Western Eger Rift (Czech Republic): a Comperative Geoelectric, Sedimentologic and Soil Gas Study of a Magmatic Diffuse CO2-Degassing Structure. Z. geol. Wiss. 36, 177-193
Flechsig C., Bussert R., Rechner J., Schütze C., Kämpf H.
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2008. The natural laboratory NW Bohemia – Comprehensive fluid studies between 1992 and 2005 used to trace geodynamic processes. Geochem., Geophys., Geosys. 9, Q04018
Bräuer K., Kämpf H., Niedermann S., Strauch G., Tesar J.
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2009. Earthquake swarms in non-volcanic regions: what fluids have to say. Geophys. Res. Lett. 36, L17309
Bräuer K., Kämpf H., Strauch G.
