Spatial variations in gas and stable isotope compositions of thermal fluids around Lake Van: Implications for crust-mantle dynamics in eastern Turkey

摘要

We investigate the helium (~3He/ ~4He) and carbon (δ ~(13)C) isotope compositions and relative abundance ratios (CO _2/ ~3He) of gas samples together with the stable isotope compositions of dissolved carbon and sulfur and the oxygen and hydrogen isotopic compositions of the associated water phase from a number of geothermal fields located around Lake Van in eastern Anatolia, Turkey. The mantle-derived helium component, which is likely transferred to the crust beneath eastern Turkey by recent magmatism, is found to constitute up to 96% (e.g. Nemrut Caldera) of the total He content in fluids. As regards the spatial distribution of He, samples collected from areas of Pliocene-Quaternary volcanics are characterized by a wide and generally higher range of R/R _A ratios (0.93 to 7.76 R _A) compared to those of non-volcanic regions (0.85 to 1.0 R _A). CO _2/ ~3He ratios vary over a wide range (2.4×10 ~5-3.8×10 ~(13)) but are mostly higher than that of the nominal upper mantle (~2×10 ~9). Oxygen-hydrogen isotope values of the waters are conformable with the Global Meteoric Water Line and indicate a local meteoric origin. Sulfate in waters is most probably derived from dissolution of marine carbonates and terrestrial evaporite units. Temperatures calculated by SO _4-H _2O isotope geothermometry lie between 40 and 199°C, and are in poor agreement with reservoir temperatures estimated from silica geothermometers. Discordant temperatures may be due to either the relatively slow rate of isotopic equilibrium between water and sulfate or mixing of geothermal water with sulfate-bearing shallow waters which may modify the δ ~(18)O value. The δ ~(13)C (CO _2) values of gas samples are consistently lower than those of their water counterparts, consistent with loss of CO _2 from waters by degassing. Mixing between mantle and various crustal C-sources appears to be the main control on the C-isotope composition. The principal origin of CO _2 in all samples is crustal lithologies, mainly limestone (~85 to 98% of the total carbon inventory): thus, the crustal carbon flux is at least 10 times that from the mantle. There is a broad correlation between high ~3He/ ~4He values and thinner crust in the western part of the Lake Van region, where several historically-active volcanoes are located. This observation indicates that localized volcanic and magmatic activity exerts the primary control on the balance between mantle and crustally-derived volatiles in the region.