Hydrogeochemistry of geothermal waters in eastern Turkey: Geochemical and isotopic constraints on water-rock interaction

Abstract

The hydrogeochemistry of geothermal waters from 31 geothermal fields in eastern Turkey is investigated with regard to major ion compositions, stable (δ18O-δ2H-δ34S) and tritium (3H) isotope systematics. Discharge temperature of studied waters varies from 24 to 65 °C. Four different geochemical processes were found to control the major ion concentrations of waters which include dissolution of carbonates, fluid-mineral interaction, oxidation of sulfur-bearing minerals, and chloride enrichment. The northern, central and southern provinces are represented by different local meteoric water lines (LMWL) with deuterium excess of 15.0, 13.9 and 16.5‰ V-SMOW, respectively. The stable isotope values of the thermal waters are close to LMWLs and indicate a meteoric origin. The enrichment in oxygen isotope composition (0.6 to 7.7‰ V-SMOW) in some thermal waters resulted from water–rock interaction process and 18O-exchange process between CO2 and H2O. The δ34S and δ18O contents in dissolved sulfate cover a wide range from 6.2 to 32‰ V-CDT and from −2.5 to 14.8‰ V-SMOW, respectively, indicating that the sulfate isotope systematics of the majority of waters is governed by dissolution of terrestrial sulfate and marine evaporites. The reservoir temperatures estimated by chemical and isotopic geothermometers of KMg (27–127 °C), silica (29–179 °C) and 18OSO4-H2O (51–196 °C), and by the silica-enthalpy mixing model (130 to 235 °C) yielded inconsistent results. The geological factors (e.g., relatively thick crust, low surface heat flux, absence of ideal cover units) in eastern Turkey have resulted in the development of low- or moderate-temperature geothermal systems.