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 CO 2 emissions are concentrated in the Neogene–Quaternary volcanic arc and carbonate-rich flysch nappes along tectonized suture zones (Magura, Pieniny and Ceahlău-Severin suture zone), while CH 4 of mostly thermogenic origin dominates in the Outer Flysch belt. We present the first regional geochemical dataset and map of CO 2 and CH 4 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/R a values (>3) near long-dormant volcanic centres, especially Ciomadul, reflect persistent deep magmatic reservoirs with 60-70% mantle/magmatic 3 He input, whereas radiogenic 4 He signatures dominate non-volcanic flysch and metamorphic regions, producing low R/R a values (~0.02). CO 2 acts as the primary carrier of mantle He, but metamorphic devolatilization of marls and carbonates at 5–20 km depth provides the principal crustal CO 2 source, consistent with “orogenic CO 2 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 CO 2 / 3 He ratios confirm heterogeneous carbon sources of the CO 2 gases emitted at the surface, with mantle and crustal inputs at different proportions. In general, the biogenic CO 2 contributions are negligible, with the majority of samples plotting along a mantle-limestone mixing line, indicating significant crustal-derived CO 2 up to 80-95% for non-volcanic areas, and 40-70% for volcanic areas. The carbon isotopes and CO 2 / 3 He 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. CO 2 fluxes show average values of 1.4 × 10 6 g km -2 y -1 for the Ciomadul volcanic area, 1.18 × 10 8 g km -2 y -1 for the volcanic area of the Eastern Carpathians and 5.1 × 10 7 g km -2 y -1 for the non-volcanic area of the Eastern Carpathians. Mantle-derived He fluxes coupled with CO 2 / 3 He indicates a 4.66 Mt. year -1 mantle CO 2 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. • CO 2 emissions in the Carpathians are located along volcanic ranges and suture zones. • He isotope ratios reveal elevated R/R a (>3) values near dormant volcanic regions. • In non-volcanic areas metamorphic decarbonation is a key source of orogenic CO 2 . • CO 2 degassing is structurally controlled, occurring along nappes and major faults. • Mantle-He fluxes coupled with CO 2 / 3 He indicates a 4.66 Mt. year -1 mantle CO 2 flux for the Carpathians.
Kis et al. (Fri,) studied this question.