Geochemistry of Bazman thermal springs, southeast Iran

摘要

Thermal springs of the Bazman geothermal field, 27 km south of the Holocene Bazman volcano caldera in Makran continental margin, SE Iran, were studied for the first time. New data on major components, selected trace elements (Li, Rb, Cs, Ba, Sr, Fe, Al), water isotopic composition (H, O and S), chemical and isotopic (delta C-13 of CO2 and CH4, He-3/He-4 and Ar-40/Ar-36) composition of dissolved and bubbling gases from the hottest vents are presented. Four groups of springs with temperature range of 27-44 degrees C discharge Ma-CI waters with total dissolved solids (TDS) of similar to 800 to similar to 7000 mg/L from different aquifers composed of diverse type of rocks including granites and limestone. Only the hottest and most saline springs of the Bazman field with low bicarbonate are close to equilibrium with surrounding rocks whereas the others discharge immature waters. Geothermometry based on Sio(2) concentrations, Na-K and Na-K-Ca-Mg systems shows low equilibrium temperatures of up to 130 degrees C, consist with the temperatures estimated by alumino-silicate minerals saturation indices. The water isotopic composition (delta O-18 and delta D) indicates meteoric origin with a small oxygen isotopic shift measured in the saline waters. The delta S-34 values of SO4 indicate influence of gypsum and anhydrite dissolution from the host rock. Dissolved and free gases are N-2-rich (>95 vol.%) with a high He content (0.5 vol.%). A biogenic origin may be suggested for CH4 and CO2 based on their carbon isotopic characteristics (-62%. and -13%. vs. V-PDB, respectively). The R/Ra value of 0.5 indicates a contribution of about 6 % of He from the mantle. It is suggested that the Bazman thermal waters are heated at considerable depth (4-5 km) in a deep fault system most probably by the regional heat flow according to the local geothermal gradient. (C) 2019 Published by Elsevier B.V.