Helium isotopes, tectonics and heat flow in the Northern Caucasus

109 new measurements of He-3/He-4 equivalent to R in subsurface fluids of t he Northern Caucasus coupled with the data obtained previously allow region al regularities in the distribution of helium isotopic composition to be ex amined. Cis-Caucasian foredeeps show the lowest radiogenic R-values. The av erage R-av-value is slightly higher in gases of the Scythian plate beyond t he Stavropol arch. Within the arch, elevated R = (1.6-4.5) X 10(-7) indicat es an input of mantle-derived helium. This input is even more evident to th e south of Starvropol arch, in the Caucasian Mineral Water area, where the approximate to 8 Ma old laccolithes occur and R-values approach (5-11) X 10 (-7). The highest R-values, up to (0.7-0.9) X 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 crusta l thickness but reflect degassing of magmatic reservoirs including those ye t unknown. According to the recent Sr-Nd-O data, the young volcanic rocks are of mantl e affinity but they are contaminated by a crustal component. The average R- av-values in fluids and Sr-87/Sr-86 ratios in host magmatic rocks show an i nverse correlation suggesting mixing of crustal and mantle materials. R-val ues vary inversely with apatite fission-track ages of crystalline basement rocks. The ages increase westward of the Elbrus volcano, most likely record ing 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 CO2-bearing fluids, low values are typical of CH4 gases, a few N-2-rich gases display highly variable R. R elationships 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. Copyrigh t (C) 2000 Elsevier Science Ltd.