Geochemical and isotopic characteristics of geothermal discharges in the Emet Basin, Western Anatolia, Turkey

Abstract

Abstract The geochemical and isotopic compositions of low-temperature thermal waters in the N–S-oriented Emet Basin, located in Western Anatolia, Turkey, were comprehensively investigated. Three distinct physicochemical processes affecting the chemical composition of the thermal waters were distinguished at this site: dissolution of CaCO3, gradual transformation of Ca–HCO3-type waters to Ca–SO4-type waters by the addition of SO4, and mixing of deep Na–HCO3-type waters with Ca–SO4-type waters. The first two processes are associated with the dissolution of carbonates and evaporites, respectively, and predominantly control the chemistry of thermal waters in the basin. The third process is observed locally and is manifested as CO2-rich thermal waters with relatively higher total ionic concentration. The majority of the thermal waters lie close to the local meteoric water line with a deuterium excess of +17‰. The single CO2-rich thermal water in the basin, located in the Yenicekoy area, shows a slight enrichment in 18O and a shift from the local meteoric water line. The δ34SSO4 and δ18OSO4 values of the dissolved sulfate range from 14.49‰ to 22.46‰ (vs. V-CDT) and 11.9‰–17.8‰ (vs. V-SMOW), respectively, which indicates that the source of sulfate in the thermal waters is evaporites such as gypsum and anhydrite in the basin. The δ13CDIC values of the thermal waters vary between −0.63‰ and −10.36‰ (vs. V-PDB). By using a binary mixing model, the δ13C value of the deep carbon source associated with the formation of CO2-rich thermal waters was calculated as 0‰, which is a typical value of marine limestones. Thermal decomposition of marine limestones and dissolution of carbonates dominate the carbon budgets of thermal waters, whereas the organic carbon contribution is generally less than 20% and the mantle-derived carbon contribution was found to be insignificant. Air-corrected 3He/4He ratios of the thermal waters ranging from 0.34 RA to 1.57 RA indicate that radiogenic (crustal) helium is dominant in the region, although up to 19.4% of the total helium is of mantle origin. The 3He/heat ratios vary from 0.4\u202f×\u202f10−15\u202fcm3 STP/J to 13.2\u202f×\u202f10−15\u202fcm3 STP/J, which are very close to the ratios in the continental environment. This indicates that the dominant heat source in the region is radiogenic.