We investigate the helium, carbon and oxygen-hydrogen isotopic systematics and CO(2)/(3)He ratios of 8 water and 6 gas samples collected from 12 geothermal fields in western Anatolia (Turkey). (3)He/(4)He ratios of the samples (R) normalized to the atmospheric (3)He/(4)He ratio (R(A) = 1.39 x 10(-6)) range from 0.27 to 1.67 and are significantly higher than the crustal production value of 0.05. Fluids with relatively high R/R(A) values are generally found in areas of significant heat potential (Kizildere and Tuzla fields). CO(2)/(3)He ratios of the samples, ranging from 1.6 x 10(9) to 2.3 x 10(14), display significant variation and are mostly higher than values typical of an upper mantle source (2 X 109). The delta(13)C (CO(2)) and delta(13)C (CH(4)) values of all fluids vary from -8.04 to +0.35 parts per thousand and -25.80 to -23.92 parts per thousand (vs. PDB), respectively. Stable isotope values (delta(18)O-delta D) of the geothermal waters are conformable with the Mediterranean Meteoric Water Line and indicate a meteoric origin. The temperatures calculated by gas geothermometry are significantly higher than estimates from chemical geothermometers, implying that either equilibrium has not been attained for the isotope exchange reaction or that isotopic equilibration was disturbed due to gas additions en route to the surface. Evaluation of He-CO(2) abundances indicates that hydrothermal degassing and calcite precipitation (controlled probably by adiabatic cooling due to degassing) significantly fractionate the elemental ratio (CO(2)/(3)He) in geothermal waters. Such processes do not affect gas phase samples to anywhere near the same extent. For the gas samples, mixing between mantle and various crustal sources appears to be the main control on the observed He-C systematics: however, crustal inputs dominate the CO(2) inventory. Considering that limestone is the main source of carbon (similar to 70 to 97% of the total carbon inventory), the carbon flux from the crust is found to be at least 20 times that from the mantle. As to the He-inventory, the mantle-derived component is found to vary up to 21% of the total He content and is probably transferred to the crust by fluids degassed from deep mantle melts generated in association with the elevated geotherm and adiabatic melting accompanying current extension. The range of (3)He/enthalpy ratios (0.000032 to 0.19 x 10(-12) cm(3) STP/J) of fluids in western Anatolia is consistent with the release of both helium and heat from contemporary additions of mantle-derived magmas to the crust. The deep faults appear to have facilitated the deep circulation of the fluids and the transport of mantle volatiles and heat to the surface. (C) 2007 Elsevier B.V. All rights reserved.