Présentations Colloques

Oral Presentation
Session 8.08: Mineral and Thermal water: an indicator of deep processes and source of economically valuable minerals
Sebnem Arslan
Noble gas isotopes and gas compositions of on-land and subaqueous thermal springs in the Koycegiz Lake and Dalaman plain area, Turkey
The purpose of this study is to demonstrate noble gas isotope and water and gas composition data from subaqueous thermal springs in Koycegiz Lake and from on-land hot springs located in the Koycegiz and Dalaman plains to contribute to hydrogeochemical conceptual modeling of the geothermal system in the area. To this end, seven locations were sampled for free gas and four were sampled for gas dissolved in water. Water samples are of the Na-Cl type. Koycegiz Lake on-land samples have relatively high discharge temperatures (between 37 and 40 °C) when compared to Dalaman plain samples (26-30 °C). Gases from the majority of the locations are composed mainly of nitrogen with variable amounts of CO2, Ar, O2, CH4, H2 and He. In one gas sample from Koycegiz Lake there is a major contribution of CH4. Air corrected helium isotope ratios of all samples are above the crustal value (0.02-0.05 Ra, Ra is the atmospheric 3He 4He ratio of 1.39x10-6), exhibiting a mantle component. The mantle helium percentages are calculated by using a two-component mixing model between mantle-He and crustal-He. Mantle-He percentages are between 2 and 6% for samples from subaqueous springs, 2% for samples from on-land springs located around Koycegiz, and 17-18% for on-land springs located in the Dalaman plain. High contributions of mantle helium in the Dalaman plain suggest that the faults of extensional tectonics in the area are enhancing the escape of gases originating from the upper mantle through the brittle parts of the crust and this affects the geothermal system. The differences in mantle helium contributions of subaqueous and on-land K Koycegiz Lake samples on the one hand and Dalaman plain samples on the other hand indicate that these samples are products of two different geothermal systems. **Acknowledgements- This study was supported by TUBITAK under Grant No. CAYDAG-112Y137**