Helium isotopes in gases of Mineral Waters in the western Caucasus

The He isotope composition as an indicator of mantle-derived component was studied in gases from mineral springs, stratal waters, and mud volcanoes developed west of the Teberda River valley (10 objects) and two springs in the central segment of the Greater Caucasus orogen between the active El’brus and Kazbek volcanoes. In the western segment of the orogen, the values of ³He/⁴He = R_corr vary in the range of (46–114) × 10⁻⁸ = (0.33–0.81)R_atm, where R_atm =1.4 × 10⁻⁶ is the atmospheric ratio. They are substantially lower as compared with values in the vicinity of El’brus and Kazbek and close to those in samples from the central segment equal to (70–134) × 10⁻⁸ = (0.50–0.96)R_atm, as well as to the values previously recorded in the Caucasian Mineral Waters (CMW) area. Moreover, the content of ³He in them is notably higher as compared with its crustal radiogenic level characteristic of mud volcanoes in the Taman Peninsula, where R = (1.4–2.8) × 10⁻⁸ = (0.01–0.02)R_atm. Nitrogen-methane gas from northern piedmonts of the western Caucasus also contain nonatmogenic components, including the radiogenic ⁴⁰Ar (⁴⁰Ar/³⁶Ar = 900), “excess” nitrogen (∼87% of the total N₂ concentration in sample) and the mantle He. These data specify the distribution of mantle derivates along the orogen strike and age of intrusive magmatic activity in its different segments.