Geochemistry of Bazman thermal springs, southeast Iran

Abstract 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 (δ13C of CO2 and CH4, 3He/4He and 40Ar/36Ar) composition of dissolved and bubbling gases from the hottest vents are presented. Four groups of springs with temperature range of 27–44 °C discharge Na-Cl waters with total dissolved solids (TDS) of ∼800 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 SiO2 concentrations, Na-K and Na-K-Ca-Mg systems shows low equilibrium temperatures of up to 130 °C, consist with the temperatures estimated by alumino-silicate minerals saturation indices. The water isotopic composition (δ18O and δD) indicates meteoric origin with a small oxygen isotopic shift measured in the saline waters. The δ34S values of SO4 indicate influence of gypsum and anhydrite dissolution from the host rock. Dissolved and free gases are N2-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.