The aim of this study was to understand geochemistry of thermal fluids circulating in the basin at the southwest of Tekman (Erzurum, Turkey) Geothermal Province in Eastern Anatolia as well as to estimate reservoir temperature and its heat source by assessment of helium and carbon isotopic compositions of liquid and gas samples.Deep thermal and cold shallow groundwaters are NaCl type (Cerme and Ilipinar springs) and Ca-Mg-HCO 3 type, respectively. The discharge temperatures of thermal waters vary between 29 and 57°C. The reservoir temperatures were estimated by solute silica and cation geothermometers vary from 80 to 110°C. CO 2 is the dominant gas in geothermal fluids with variable amounts of nitrogen, helium and CH 4 . The isotopic ratios of helium that range from 1.03 to 1.54Ra c show a range crustal to magmatic-type values. The isotopic composition of carbon (CO 2 ) obtained from the bubbling and dissolved gases shows the variation from -0.2 to 3.4‰ vs. PDB. The mantle derived fluids interact at shallower levels with circulating meteoric waters and originate geothermal systems from which equilibration temperatures were estimated up to 192°C by gas geothermometers.
The thermal fluids vented over Eskisehir province have been investigated for their origin and to estimate the geothermal potential of the area. Thermal waters as well as bubbling and dissolved gases were collected and analysed for their chemical and isotopic features. Their isotopic composition varies in the range from -11.5 to -7.7 ‰ for δO, -84 and -57 ‰ for δH, and 0-7.2 TU for tritium. The gases (bubbling and dissolved) are mostly N-dominated with a significant amount of CO. The helium isotopic ratios are in the range of 0.2-0.66 R/Rac, indicate remarkable mantle-He contribution ranging between 2 and 10 % in the whole study area. Considering the estimated geothermal gradient about three times higher than the normal gradient, and the reservoir temperatures estimated to be between 50 and 100 °C using quartz and chalcedony geothermometers, a circulation model was built where possible mixing with shallow waters cool down the uprising geothermal fluids.
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