Tracing deep thermal water circulation systems in the E–W trending Büyük Menderes Graben, western Turkey

Abstract

We present information on the spatial variation of helium isotope ratios and several conservative geochemical constituents that serve as natural tracers to identify thermal water flow systems along the E–W trending Büyük Menderes Graben. The air-normalized R/RA ratios (where R=sample 3He/4He and RA=air 3He/4He) of geothermal fluids in the graben range from 3.69 to 0.2. R/RA ratios of the high temperature fluids display regular variation along the graben, from high mantle values in the east to low crustal values in the west. The highest R/RA ratios showing significant mantle He signatures occur at the eastern segment of the graben. In the central section, R/RA ratios are lower and vary between 1.82 and 1.14. The lowest ratio of 0.20 RA, indicating a value of high crustal helium up to 98%, was observed in the Germencik Field, where one of the highest temperatures recorded was 232°C, at the western segment of the graben. A regional variation is also apparent in the CO2–He systematics. Mantle-derived carbon proportions were found to be generally less than 2% but similar to the R/RA ratios, the higher mantle carbon proportions occur at the eastern segment of the graben.The spatial distribution of all the data points over the entire extent of the graben shows a regularly decreasing trend of mantle-derived He and CO2 contributions toward the west. These results were interpreted as a progressive dilution process of mantle volatiles resulting from the systematic addition of 3He-depleted crustal CO2 components.Finally, because the R/RA ratios of geothermal fluids in the graben decrease continuously in E-to-W direction, we propose a continuous dilution process involving the addition of crustal radiogenic 4He associated with deep circulating fluid flow. This conclusion is compatible with the B and Cl− concentrations of the thermal waters that show a westward-increasing trend. This trend appears to be controlled by water–rock interaction and may be linked to a possible thermal water circulation system trending from E to W toward the Aegean Sea.