Isotope-Geochemical Features and Genesis of Gases in the East Carpathian Region

The paper presents data on the composition of a gas phase of underground fluids in the East Carpathian region, including ³He/⁴He, ⁴⁰Ar/³⁶Ar, and ⁴He/²⁰Ne ratios. The argon isotope composition of these gases was used to estimate the fractions of atmospheric Aratm and radiogenic ⁴⁰Ar_rad formed in the rocks, N₂/Aratm ratio, and to reveal the admixture of nonatmogenic (“excess”) nitrogen in most samples. The CO₂ content in gases positively correlates with the fraction of mantle component in fluid helium. At the same time, the CO₂ content shows a negative correlation with the total helium (and light ³He enriching mantle derivatives), thus excluding the simultaneous influx of CO₂ and helium from a common mantle source in the fluids. A wide spectrum of ³He/⁴He = R in gases of the region spanning three orders of magnitude confirms the concept of mixing of the crustal and mantle components in the helium. However, even gases with similar R values show a wide scatter of He concentrations. This is mainly caused by the additional influx of other gases: CH₄ formed during OM transformation or CO₂ released during the thermal metamorphism of carbonate sequences. Correlation of the CH₄/³He ratio and the helium isotope composition in the Carpathian gases indicates the crustal origin of hydrocarbons, which formed economic gas pools in the Ciscarpathian Trough and the adjacent part of the Folded Carpathians. Lateral chemical and isotope variations revealed in the underground fluids are related to the tectonic zoning of the region. The helium isotope variations are also consistent with the geodynamic setting of the region (thinning of the crust and lithosphere towards the Pannonian Basin, growth of the background conductive heat flow and corresponding ascent of isotherms). In combination with geothermal data, they reflect specifics of the mantle heat-and-mass flow discharge.