HELIUM AND CARBON ISOTOPE SYSTEMATICS IN CRUSTAL FLUIDS FROM THE EIFEL, THE RHINE GRABEN AND BLACK-FOREST, FRG

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 He-3, which varies from < 1% up to approximately 70% of the total He present. The presence of mantle-derived He-3 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 He-3 and surface volcanics, He-3 occurs over a larger geographic area than do surface volcanics. A correlation between the distribution pattern of mantle-derived He-3 and tectonic activity, local seismicity or heat flow is not apparent from the present data set. DeltaC-13CO2-values for CO2-rich mineral waters vary from -25 to -5 parts per thousand (PDB) and indicate a contribution of organic-derived 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)c > 6 and -8 parts per thousand < deltaC-13CO2 < -5 parts per thousand is diluted by a "crustal" end-member with (R/R(a))c < 0.1 and deltaC-13CO2 < -15 parts per thousand. The observed relationship between He and C strongly points to the presence of mantle-derived CO2 in some groundwaters, particularly in waters from the East and West Eifel districts, and the Vogelsberg and Kaiserstuhl volcanic sites.