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

The isotopic-geochemical characteristics of boron in the waters of 21 mud volcanoes of the Kerch-Taman region have been determined. Boron concentration varies from 14 to 550 mg/L, and δB values range from +8.3 to +54.7‰ (+21‰ on average). Low δB 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 and discharged on the Earth's surface within the Kerch and Taman peninsulas. It is shown that the increase of boron concentration in mud volcanic waters is provided due to the input of boron with low isotopic values (δB ∼+10‰). The inverse relationship between δB and δO values in water was found. It indicates unified mechanisms of enrichment of B and O in the mud volcanic waters. These processes are temperature-dependent. They demonstrate a significant correlation between boron concentrations ([B]) and isotopic values (δB) with T(Mg−Li)-temperatures in the range from ∼40 to 130°C. Thus, the chemical ([B]) and isotopical (δB) variations observed in the studied mud volcanic waters reflect different depths of water chemical composition formation and, respectively, different temperature stages of catagenetic transformation of sedimentary strata. The revealed patterns are probably related to the reaction of smectite transformation into illite, which takes place in clayey strata of the Maikop series and is accompanied by the release of large volumes of dehydration waters with high δO values (up to +14‰). The most probable source of boron with low δB values is destructive smectites.