Griesshaber, Erika; O’Nions, R. K.; Oxburgh, E. R.
Helium and carbon isotope systematics in crustal fluids from the Eifel, the Rhine Graben and Black Forest, F.R.G.
In: Chemical Geology, Vol. 99, Nr. 4: S. 213-235
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Helium and carbon isotope analyses are reported for groundwater samples from the Rhenish Massif, the Rhine Graben and Black Forest. Most of the samples contain a resolvable component of mantle-derived 3He, which varies from < 1\% up to ∼ 70\% of the total He present. The presence of mantle-derived 3He in deep groundwaters is discussed in relation to the distribution of surface volcanics, tectonic activity, regional seismicity and heat flow. Although there is a crude relationship between the presence of mantle-derived 3He and surface volcanics, 3He occurs over a larger geographic area than do surface volcanics. A correlation between the distribution pattern of mantle-derived 3He and tectonic activity, local seismicity or heat flow is not apparent from the present data set. δ13CCO2-values for CO2-rich mineral waters vary from - 25 to - 5‰ (PDB) and indicate a contribution of organicderived carbon to the CO2-bearing groundwater. In geographically restricted areas simple mixing relationships exist between the isotope composition of carbon and that of helium. This can be approximated by a two-component model, where a \"magmatic\" end-member with ( R R1)c6 and -8‰<δ13CCO2<- 5‰ is diluted by a \"crustal\" end-member with ( R Ra)c<0.1 and δ13CCO2<-15‰. The observed relationship between He and C strongly points to the presence of mantlederived CO2 in some groundwaters, particularly in waters from the East and West Eifel districts, and the Vogelsberg and Kaiserstuhl volcanic sites.