Geochemistry of dissolved gases from the Eastern Carpathians - Transylvanian Basin boundary

•We show the results on the volatiles dissolved in mineral waters discharged in Romania.•δ13CTDIC and He indicate the presence of fluids from different crustal sources besides contributions of mantle-derived fluids.•The compositions are the consequence of mixings and occurrence of secondary processes (degassing and carbonate precipitation).•Our findings prove that presence of mantle-components may be present also at ~ 50 km distance away from the volcanic centres.

We show the results of a study on the volatiles dissolved in mineral waters discharged over a 200 km-long transect along the Rodna-Bârgău area and Călimani-Gurghiu-Harghita volcanic chain (Eastern Carpathians, Romania). All of the collected mineral water samples carry dissolved gas with carbon dioxide content up to 1.99 cm3STP/gH2O, and helium content up to 2.3 × 10− 5 cm3STP/gH2O. Carbon (δ13CTDIC total dissolved inorganic carbon, ranging from − 15.6 to 5.32‰ vs. VPDB), He systematics (He isotopes in the range of 0.38-0.99 Ra, Ra = air-normalized 3He/4He ratio) and CO2/3He ratio spanning over four orders of magnitude from MORB-like values of 2.92 × 109 to crustal-type values of 3.02 × 1013, coherently indicate the presence of fluids from different crustal sources (e.g. sediments, hydrocarbon reservoirs) besides minor, but detectable contributions of mantle/magmatic-derived fluids (up to 16.45%). Our investigations show that the wide range of chemical and isotopic composition can be explained in terms of mixings among different gas sources feeding the groundwater and the contemporary occurrence of gas-water interactions like degassing and deposition of carbonates, affecting the circulating waters after their infiltration.

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