Nitrogen and Carbon Isotope Composition of Gases in High-$${{p}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}$$ Waters in the North Caucasus

Abstract

The paper presents the first data on the nitrogen isotope composition of the associated gases of mineral waters in the North Caucasus and newly acquired data on the carbon isotope composition of the CO2 and CH4. Most of the samples were taken from the high- $${{p}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}$$ waters of the Elbrus and Kazbek volcanic areas in the Greater Caucasus. It is shown that δ15N changes in the range of –3.9 to +5.6‰ and synchronously increases in gases of the Caucasus Spa Area (CSA) with an increase in the N2 and CH4 concentrations. This correlation points to a genetic association of non-atmogenic nitrogen with sedimentary methane-generating processes. The values of δ13С in carbon dioxide in the Elbrus area vary from –11.8 to –3.0‰. The mean value of δ13С (СО2) tends to increase north of Elbrus volcano, and this trend may be a consequence of the increasing content of СО2 produced by the thermal decomposition of sedimentary carbonates in the composition of gases, as well as a result of low-temperature interaction of the high- $${{p}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}$$ waters with the Jurassic and Cretaceous carbonate rocks. It is shown that high- $${{p}_{{{\text{C}}{{{\text{O}}}_{{\text{2}}}}}}}$$ waters associated with Elbrus volcano often contain up to 12.4% CH4. This methane is characterized by high δ13С of –32.0 to –17.2‰. Analysis of the distribution of concentrations and δ13С values of СН4 around Elbrus and the presence of N2 with positive δ15N values in the gases suggest a crustal genesis of the methane. The role of magmatic activity in this situation is reduced to producing a temperature anomaly that activates carbon isotope exchange in the CO2–CH4 system.