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 weste rn Eger rift (Czech Republic) have been analyzed. Four geochemically simila r, but tectonically separate, gas escape centers are distinguishable, out o f which 3 show a free gas flux >85000 dm(3) h(-1). All gases from the cente rs 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 ar e among the highest measured in Europe. The discharge of the gas mixture de creases with distance from the emanation centers with both decreasing fract ions of CO2 and delta(13)C values, whereas the fractions of N-2 and trace g ases increase. These changes in chemical and isotopic composition are assoc iated by a decrease in R/R-a ratios from about 5 in the centers to <2 in th e 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 c ontent of N-2 increases. He-3/He-4 ratios give evidence for mixing of He fr om both a deep-seated magmatic and a crustal source. The gas emanation cent ers, 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 Marian ske Lazne fault, which are considered to represent a deep-reaching fracture system that enables the ascent of gases from a magmatic body in the Europe an 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) occu rs 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 i nvestigated area of 1500 km(2), flux densities greater than 0.24 x 10(6), 5 2, and 0.65 mol km(-2) a(-1) for CO2, N-2 and He respectively are calculate d. Copyright (C) 1999 Elsevier Science Ltd.