An active subcontinental mantle volatile system in the western Eger rift, Central Europe:: Gas flux, isotopic (He, C, and N) and compositional fingerprints

The composition and flux of gas emanations, and the isotopic ratios of CO2, He and N-2 of 74 mineral springs and dry gas vents (mofettes) in the western Eger rift (Czech Republic) have been analyzed. Four geochemically similar, but tectonically separate, gas escape centers are distinguishable, out of which 3 show a free gas flux >85000 dm(3) h(-1). All gases from the centers are CO2-rich (>99 vol.%) and have delta(13)C values ranging from -1.8 to -4.0 parts per thousand. He-3/He-4 ratios are as high as R/R-a = 5, and are among the highest measured in Europe. The discharge of the gas mixture decreases with distance from the emanation centers with both decreasing fractions of CO2 and delta(13)C values, whereas the fractions of N-2 and trace gases increase. These changes in chemical and isotopic composition are associated by a decrease in R/R-a ratios from about 5 in the centers to <2 in the peripheries. The changes of the contents and isotopic composition of CO2 can be explained by physico-chemical fractionations of CO2 between gaseous and aqueous phases. Towards the periphery, the contents of free CO2 and its delta(13)C are reduced by dissolution of CO2 in groundwater, whereby the content of N-2 increases. He-3/He-4 ratios give evidence for mixing of He from both a deep-seated magmatic and a crustal source. The gas emanation centers, with their strongly magmatic delta(13)C value of about -2.7 parts per thousand, seem to outline the intersections of the Eger rift and the Marianske Lazne fault, which are considered to represent a deep-reaching fracture system that enables the ascent of gases from a magmatic body in the European subcontinental mantle (SCM). Therefore, the European SCM is suspected to be the main source of CO2. The most mantle-like He land probably N-2) occurs in the centers of gas release. The total regional gas Aux in the western Eger rift is determined to be 3.6 x 10(8) mol a(-1). When related to the investigated area of 1500 km(2), flux densities greater than 0.24 x 10(6), 52, and 0.65 mol km(-2) a(-1) for CO2, N-2 and He respectively are calculated. Copyright (C) 1999 Elsevier Science Ltd.