Regional distribution pattern of carbon and helium isotopes from different volcanic fields in the French Massif Central: Evidence for active mantle degassing and water transport

We report new data of the gas and isotope (δ13C and 3He/4He) composition of 26 free gas emanations escaping from bubbling springs and mofettes in six different Cenozoic volcanic areas of the French Massif Central, which belongs to the European Cenozoic Rift System (ECRIS). In all six units the CO2 concentrations in the free gas phase are >96vol% throughout, at most locations even >99vol%. The δ13C values range between −3.6 and −6.3‰ (relative to PDB) and the 3He/4He ratios between 1.05 and 6.36Ra (Ra=3He/4He in air). These isotope signatures indicate an origin of helium and CO2 predominantly in the subcontinental mantle. In addition, the gas composition and the isotope ratios of C, N, He, Ne, and Ar were monitored approximately once a year at two locations in the Mont-Dore Massif during a time span of five and twelve years, respectively. As a result robust data have been obtained, suggesting mixing of variable fractions of atmosphere-derived and sedimentary nitrogen. 3He/4He ratios overlapping the characteristic range of the subcontinental lithospheric mantle (SCLM) indicate fluid transport in deep reaching faults from the magmatic reservoir to the surface and suggest that minor fractions of mantle-derived nitrogen are present also. In comparison with MORB (Mid-Ocean Ridge Basalt), the CO2 is slightly enriched in 13C, confirming the findings in the Eifel area (Germany) and the western Eger Rift (Czech Republic), two other regions of the ECRIS where SCLM–derived gases escape at the surface.