Helium isotopes, tectonics and heat flow in the Northern Caucasus

Abstract

109 new measurements of 3He/ 4He ≡ R in subsurface fluids of the Northern Caucasus coupled with the data obtained previously allow regional regularities in the distribution of helium isotopic composition to be examined. Cis-Caucasian foredeeps show the lowest radiogenic R-values. The average R av-value is slightly higher in gases of the Scythian plate beyond the Stavropol arch. Within the arch, elevated R = (1.6–4.5) × 10 −7 indicates an input of mantle-derived helium. This input is even more evident to the south of Starvropol arch, in the Caucasian Mineral Water area, where the ≈8 Ma old laccolithes occur and R-values approach (5–11) × 10 −7. The highest R-values, up to (0.7–0.9) × 10 −5, are observed further to the south, in the central segment of the Greater Caucasus, where recent volcanism is manifested. Enhanced R-values do not correlate with the crustal thickness but reflect degassing of magmatic reservoirs including those yet unknown. According to the recent Sr-Nd-O data, the young volcanic rocks are of mantle affinity but they are contaminated by a crustal component. The average R av-values in fluids and 87Sr/ 86Sr ratios in host magmatic rocks show an inverse correlation suggesting mixing of crustal and mantle materials. R-values vary inversely with apatite fission-track ages of crystalline basement rocks. The ages increase westward of the Elbrus volcano, most likely recording the thermal degradation of the Greater Caucasus since the pre-Cainozoic magmatic activity. A direct correlation between R av-values and background conductive heat flow densities implies that discharge of the mantle melts into the crust is the common cause of the geochemical, geochronological and geothermal regularities observed. Elevated R-values are generally observed in CO 2-bearing fluids, low values are typical of CH 4 gases, a few N 2-rich gases display highly variable R. Relationships between the major gas constituents and noble gas isotopes are discussed. Fractionation, loss, and gain of these species are considered as the processes controlling the compositions of underground fluids.