Active Degassing of Deeply Sourced Fluids in Central Europe: New Evidences From a Geochemical Study in Serbia.

We report on the results of an extensive geochemical survey of fluids released in the Vardar zone (central-western Serbia), a mega-suture zone at the boundary between Eurasia and Africa plates. Thirty-one bubbling gas samples are investigated for their chemical and isotopic compositions (He, C, Ar) and cluster into three distinct groups (CO₂-dominated, N₂-dominated, and CH₄-dominated) based on the dominant gas species. The measured He isotope ratios range from 0.08 to 1.19 Ra (where Ra is the atmospheric ratio), and reveal for the first time the presence of a minor (<20%) but detectable regional mantle-derived component in Serbia. δ¹³C values range from -20.2‰ to -0.1‰ (versus PDB), with the more negative compositions observed in N₂-dominated samples. The carbon-helium relationship indicates that these negative δ¹³C compositions could be due to isotopic fractionation processes during CO₂ dissolution into groundwater. In contrast, CO₂-rich samples reflect mixing between crustal and mantle-derived CO₂. Our estimated mantle-derived He flux (9.0 × 109 atoms m^-2 s^-1) is up to 2 orders of magnitude higher than the typical fluxes in stable continental areas, suggesting a structural/tectonic setting favoring the migration of deep-mantle fluids through the crust. [ABSTRACT FROM AUTHOR]