Mantle helium traces in the Elbrus–Kazbek sector of the Greater Caucasus and adjacent areas

The isotope composition of He contained in gas phase of subsurface fluids in the Greater Caucasus orogene and adjacent areas is revisited combining data obtained before 1998 with 54 new measurements. We report 3He/4He = R ratios (hereafter, R) in CO2-rich springs from the vicinities of the Elbrus and Kazbek active volcanoes and CH4-rich gases from mud volcanoes in eastern Georgia. The central segment of the orogene between Elbrus and Kazbek volcanoes differs from its western and eastern segments in the highest R-values exceeding 5 × 10− 6 (∼ 3.6 Ra). The halo of the elevated R-values extends northward, far from the Mt. Elbrus into the Scythian Plate, indicating the recent magmatic activity in this region as well. The origin of this halo is discussed along with temperatures of the mineral springs. The high R-values are also observed near Mt. Kazbek and further to the south, in mud volcanoes of eastern Georgia, indicating hidden magmatic activity.