Geochemistry of CO2-rich gas emissions in the Carpathians: Multiscale geological sources and implications for orogenic degassing

Degassing of deep-seated fluids is a key process occurring in orogenic systems, yet its sources and controlling mechanisms remain poorly constrained. The Carpathians represent a major degassing province in Europe, where CO2 emissions are concentrated in the Neogene–Quaternary volcanic arc and carbonate-rich flysch nappes along tectonized suture zones (Magura, Pieniny and Ceahl˘au-Severin suture zone), while CH4 of mostly thermogenic origin dominates in the Outer Flysch belt. We present the first regional geochemical dataset and map of CO2 and CH4 emissions in the Western and Eastern Carpathians, integrating chemical and isotopic analyses with lithological and structural constraints. Helium isotopes reveal variable mantle–crustal mixing: elevated R/Ra values (>3) near long-dormant volcanic centres, especially Ciomadul, reflect persistent deep magmatic reservoirs with 60-70% mantle/magmatic 3He input, whereas radiogenic 4He signatures dominate non-volcanic flysch and metamorphic regions, producing low R/Ra values (~0.02). CO2 acts as the primary carrier of mantle He, but metamorphic devolatilization of marls and carbonates at 5–20 km depth provides the principal crustal CO2 source, consistent with “orogenic CO2 degassing” described in other collisional belts. Degassing sites cluster along nappe boundaries and fault zones, where enhanced permeability enables rapid volatile ascent. Carbon isotopes and CO2/3He ratios confirm heterogeneous carbon sources of the CO2 gases emitted at the surface, with mantle and crustal inputs at different proportions. In general, the biogenic CO2 contributions are negligible, with the majority of samples plotting along a mantle-limestone mixing line, indicating significant crustal-derived CO2 up to 80-95% for non-volcanic areas, and 40-70% for volcanic areas. The carbon isotopes and CO2/3He ratios are variably modified by groundwater interaction (dissolution and precipitation processes). Mantle-derived He flux averages are 1.59 × 10-13 g m-2 s-1 for Ciomadul volcano, 8.64 × 10-14 g m-2 s-1 for the Eastern Carpathians volcanic area and 3.46 × 10-14 g m-2 s-1 for the Eastern Carpathians non-volcanic area. CO2 fluxes show average values of 1.4 × 106 g km-2 y-1 for the Ciomadul volcanic area, 1.18 × 108 g km-2 y-1 for the volcanic area of the Eastern Carpathians and 5.1 × 107 g km-2 y-1 for the non-volcanic area of the Eastern Carpathians. Mantle-derived He fluxes coupled with CO2/3He indicates a 4.66 Mt. year-1 mantle CO2 flux for the Carpathians. These values match with other active orogens, highlighting the Carpathians as a key setting to investigate volatile transport, crust–mantle interactions, and their contribution to the global carbon cycle.