Abstract
We present noble gas isotope and gas composition data from eight production wells of the Kizildere geothermal field and from six gas discharging pools from the adjacent Tekke Hamam geothermal field, situated in the eastern segment of the Büyük Menderes Graben system in western Anatolia (Turkey). Gas from both locations is composed mainly of CO 2 with minor contributions of N 2, H 2S, CH 4, O 2, Ar, H 2, C 2+ and He, with higher contents of H 2S, He and C 2+ in Tekke Hamam samples. Air corrected helium isotope ratios reveal a mantle origin for samples from both locations, but 3He/ 4He ratios are lower and more variable at Kizildere (0.96–2.06 R a) than at Tekke Hamam (2.36–2.91 R a, R a is the atmospheric 3He/ 4He ratio of 1.39 × 10 − 6 ). Kizildere gas contains less 4He (0.7–2.2 ppmv) and 3He (~ 2–5 × 10 − 6 ppmv) than gas from Tekke Hamam (2.5–3.5 ppmv and ~ 1 × 10 − 5 ppmv, respectively). CO 2/ 3He ratios of 2–8 × 10 11 at Kizildere and 0.7–1 × 10 11 at Tekke Hamam are higher than average mantle values (~ 2 × 10 9). Within the Kizildere geothermal field, reservoir temperatures and CO 2/ 3He and CH 4/C 2H 6 ratios increase and 3He/ 4He ratios decrease from southwest to northeast. Interpolation of a linear trajectory between 3He/ 4He and temperature from Kizildere delivers temperature of 173 °C–184 °C for the source reservoir of Tekke Hamam, in good agreement with measured reservoir temperatures from well TH-2 at Tekke Hamam (171 °C). The low helium content, the variable helium isotope composition and the higher CO 2/ 3He ratios at Kizildere can be explained by temperature-dependent partial removal of mantle-derived helium, e.g. through vapor separation, followed by addition of radiogenic helium. Kizildere samples derive from the remaining water phase after gas separation, while Tekke Hamam samples may represent the gas phase liberated by the former process. The pristine 3He/ 4He ratio of helium at Kizildere can be calculated as 2.2 R a, close to helium isotope ratios observed at Tekke Hamam. Partial removal of helium may also account for the higher and T-dependent CO 2/ 3He ratios at Kizildere, compared to Tekke Hamam, but such process alone cannot explain the overall high CO 2/ 3He ratios (> 7 × 10 10) and calls for contribution of CO 2 from interaction between geothermal fluids and carbonate reservoir rock. Noble gas abundances indicate mixing with cold meteoric water in the reservoir.
Published Version
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