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

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 Buyuk Menderes Graben. The air-normalized R/R-A ratios (where R = sample He-3/He-4 and R-A = air He-3/He-4) of geothermal fluids in the graben range from 3.69 to 0.2. R/R-A 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/R-A ratios showing significant mantle He signatures occur at the eastern segment of the graben. In the central section, R/R-A ratios are lower and vary between 1.82 and 1.14. The lowest ratio of 0.20 R-A, 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 degrees 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/R-A ratios, the higher mantle carbon proportions occur at the eastern segment of the graben.