Noble gas isotopes and the chemical composition of geothermal gases from the eastern part of the Buyuk Menderes Graben (Turkey)

Wiersberg T., SÜER S., GÜLEÇ N. T., Erzinger J., PARLAKTUNA M.

JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, cilt.208, sa.3-4, ss.112-121, 2011 (SCI-Expanded, Scopus)

Özet

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 Buyuk Menderes Graben system in western Anatolia (Turkey). Gas from both locations is composed mainly of CO2 with minor contributions of N-2, H2S, CH4, O-2, Ar, H-2, C2+ and He, with higher contents of H2S. He and C2+ in Tekke Hamam samples. Air corrected helium isotope ratios reveal a mantle origin for samples from both locations, but He-3/He-4 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 He-3/He-4 ratio of 1.39 x 10(-6)). Kizildere gas contains less He-4 (0.7-2.2 ppmv) and He-3 (similar to 2- x 10(-6) ppmv) than gas from Tekke Hamam (2.5-3.5 ppmv and similar to 1 x 10(-5) ppmv, respectively). CO2/He-3 ratios of 2-8 x 10(11) at Kizildere and 0.7-1 x 10(11) at Tekke Hamam are higher than average mantle values (similar to 2 x 10(9)). Within the Kizildere geothermal field, reservoir temperatures and CO2/He-3 and CH4/C2H6 ratios increase and He-3/He-4 ratios decrease from southwest to northeast. Interpolation of a linear trajectory between He-3/He-4 and temperature from Kizildere delivers temperature of 173 degrees C-184 degrees C for the source reservoir of Tekke Hamam, in good agreement with measured reservoir temperatures from well TH-2 at Tekke Hamam (171 degrees C). The low helium content, the variable helium isotope composition and the higher CO2/He-3 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 He-3/He-4 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 CO2/He-3 ratios at Kizildere, compared to Tekke Hamam, but such process alone cannot explain the overall high CO2/He-3 ratios (>7 x 10(10)) and calls for contribution of CO2 from interaction between geothermal fluids and carbonate reservoir rock. Noble gas abundances indicate mixing with cold meteoric water in the reservoir. (C) 2011 Elsevier B.V. All rights reserved.