Multi-isotopic (O, H, C, S, Sr, B, Li) characterization of waters in a low-enthalpy geothermal system in Havza (Samsun), Turkey

Abstract

In this study, multi-isotopic (O, H, C, S, Sr, B, Li) compositions were used to perform geochemical characterization, determine the source and reservoir rocks, and explain the water-rock interaction mechanisms for geothermal fluids in the low-enthalpy Havza (Samsun) geothermal field (HGF) in Turkey. The geothermal water of Na-HCO3 water type has a wellhead temperature of 53°C, pH of nearly 7.7 and an EC value of 1140 µS/cm. The geothermal springs of Ca-HCO3 water type have an EC value of nearly 667 µS/cm and a temperature of nearly 26°C. The reservoir temperatures of the HGF geothermal system were calculated as 60–90°C and 108–160°C by silica and SO4-H2O oxygen isotope geothermometers, respectively. Stable isotope compositions (δ2H and δ18O) show that the geothermal well water is mixed with deeply circulated waters fed from higher elevations (∼ 830–1260 m), while the geothermal spring water is mixed with shallow cold water. The positive δ13C value (+3.01‰) indicates that the dissolved inorganic carbon (DIC) in the geothermal well water has no contribution from the atmospheric CO2 but originates in the metamorphic CO2 and marine limestones. However, the DIC in the geothermal springs is derived from C3 plants and silicate weathering. The values of 34SCDT show that the sulfate in the geothermal waters is due to the dissolution of sulfate minerals. The values of δ11B (-1.12 to +9.37 ‰) in the geothermal well and spring waters reflect both leaching of surrounding rocks and mantle-derived B. The reservoir rock may be Late Cretaceous-Permian limestones and Jurassic sandstones considering 87Sr/86Sr ratios (0.707108–0.707688) and δ7Li values (-7.44 to +6.18 ‰), whereas it might be Jurassic sandstones based on δ11B values. The strontium isotope composition of the geothermal spring water indicates the mixing of deep geothermal waters and cold groundwaters.