Mantle helium signal in the West Carpathians, Poland

Abstract

Mantle derived helium with R/RA 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/RA), indicating complex processes of CO2 derivation. The R/RA 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.