Comparative Assessment of Chemical and Isotopic Composition of Geothermal Fluids in the Eastern Part of the Buyuk Menderes Graben (Western Turkiye)

Abstract

In this study, we comparatively discuss chemical and isotopic characteristics of thermal waters from several geothermal fields within the eastern part of the Buyuk Menderes graben, Western Turkiye. The studied thermal waters with a wide range of temperature (33 to 242 degrees C) and pH value (6.10 to 9.38) show water types varying from Ca-Mg-HCO3-SO4 to Na-HCO3-SO4. The chemical composition of waters is controlled by several processes, which include temperature, circulation depth, extent of water-rock interaction, dissolution/precipitation, mixing, cation exchange and microbial activity. All thermal waters are of meteoric origin and generally have deep circulation. delta C-13 data indicate that marine limestone and mantle-derived CO2 are the major sources of carbon in thermal waters and delta S-34 values imply that the sulfate is originated from the Neogene gypsums. At discharge temperatures, all thermal waters are saturated with various carbonate, silica and clay minerals, which is supported by the XRD analysis of scaling materials. The REY composition of the scaling samples showed that the limestone is the source rock for the rare earth elements. Thermal waters with a positive 18O shift of 2.7 to 4.6 parts per thousand have reservoir temperatures of 170-245 degrees C, whilst other waters with a shift of <1 parts per thousand have reservoir temperatures in the range of 79 to 166 degrees C. Regarding the distribution of temperature, electricity production seems to be more suitable in the western part of the region, although relatively low-temperature areas in the east also look promising.