δ11B in Mud Volcanic Waters of the Kerch–Taman Mud-Volcanic Area (Crimea–Caucasus Region): Genesis and Formation Conditions of the Boron Mineralization

The isotopic-geochemical characteristics of boron in waters of 21 mud volcanoes in the Kerch–Taman area have been determined. Boron concentration varies from 14 to 550 mg/L, and δ¹¹B values range from +8.3 to +54.7‰ (+21‰ on average). Low δ¹¹B values (~+14‰) were also obtained in two freshwater samples collected from aquifers of Neogene–Quaternary sediments. These data reflect the high heterogeneity of boron isotopic characteristics in waters of various genesis, which are discharged on the Earth’s surface within the Kerch and Taman peninsulas. It is shown that the increase of boron concentration in the mud volcanic waters is provided by the contribution of isotopically light (δ¹¹B ~ +10‰) boron. The inverse relationship between δ¹¹B and δ¹⁸O values in water was found. It indicates common mechanisms of ¹⁰B and ¹⁸O enrichment in the mud volcanic waters. These processes are temperature-dependent. They demonstrate a significant correlation of boron concentrations ([B]) and its isotopic composition (δ¹¹B) with T(Mg-Li)-temperatures in the range from ~40 to 130^oC. Thus, the chemical ([B]) and isotopic (δ¹¹B) variations observed in the studied mud volcanic waters reflect different depths of the formation of water salt composition and, respectively, the different temperature stages of catagenetic transformation of sedimentary strata. The revealed patterns are probably related to the smectite transformation into illite, which takes place in the clayey strata of the Maikop Group and is accompanied by the release of large volumes of dehydration waters with high δ¹⁸O values (up to +14‰). The most probable source of boron with low δ¹¹B values is destructive smectites.