ABSTRACT

Mantle derived helium with R/R_AM ratios up to 1, which accompanies CO₂ exhalations and cold CO₂-rich carbonated waters, has been found to escape through the 40 km thick continental lithosphere in the Outer Carpathians. Revised ⁴He/²⁰Ne ratios of mantle and crust (6818 and 2.53 × 10⁷, respectively) allow the estimation of the respective contribution of mantle, crust and air-derived helium to the bulk helium. Although isotopic ratios of noble gases attest to a mixing process which takes place between crust and mantle gases, carbon isotope ratios of the CO₂ (δ¹³C of -8.9 to -1.8 per mil) do not correlate with helium isotope ratios (R/R_A), indicating complex processes of CO₂ derivation. The R/R_A ratios in methane-dominated gases are similar to those in CO₂-rich gases, designating similar mode of helium supply. The CO₂/³He ratios are up to 2 orders of magnitude higher than the secular CO₂/³He ratio of mantle gas emanations at ocean ridges. It suggests that the recycled crustal carbon rather than mantle carbon is the principal source of CO₂. The fluids of interest with their noble gases and other isotopic signatures and high concentrations of dissolved carbonates are similar to hydrothermal solutions observed in active marginal basins.