СO₂-Rich Thermal Waters of the Neutrino Research Tunnel (Baksan Neutrino Observatory, North Caucasus)

Isotope–geochemical (δ²Н and δ¹⁸O in Н₂О, δ¹⁵N in N₂, δ¹³С in СО₂ and СН₄, ³Не/⁴Не) studies of СО₂-rich thermal waters (up to 41.3^oC) flowing out at the end of the 4-km long Neutrino research tunnel are carried out. The obtained values of δ¹³С(СО₂) (–8.0…–6.4 ‰) indicate the volcanogenic genesis of CO₂. The presence of the volcanogenic component in gases is also emphasized by high values of ³Не/⁴Не (346 × 10^–8). At the same time, nitrogen present in the gas phase is characterized by values of δ¹⁵N = +1.3‰, which points to its crustal genesis. In the tunnel, one of the gas outlets is recorded to have an elevated concentration of CH₄ (up to 0.5%). This methane is characterized by high values of δ¹³C (–33.5 and –26.0‰), which, in general, are typical of other СО₂-rich springs of the Eastern Elbrus region. In contrast to the other surface cold CO₂-rich springs of the North Caucasus, which are almost indistinguishable from surface waters in terms of the δ²H and δ¹⁸O values, the CO₂-rich thermal springs of the tunnel are noticeably enriched in the heavy oxygen isotope (¹⁸O). This is a consequence of oxygen exchange between the waters and the host rocks at elevated temperatures. In the δ¹⁸O–δ²H diagram, the figurative points of thermal waters form a well-pronounced trend that reflects the mixing of these isotopically heavy and isotopically light infiltration waters.