Helium and C-13(CO2) isotope compositions of gas discharges are used in conjunction with He-3/enthalpy ratios to identify major heat sources of the Simav Geothermal Field, located on one of the major active graben system in western Anatolia (Turkey). The air-corrected He-3/He-4 ratios, delta C-13(CO2) values, and CO2/He-3 ratios of gas samples collected at the wellheads range from 1.36 R-A to 1.57 R-A (where R-A is the atmospheric He-3/He-4 ratio), 1.47 parts per thousand VPDB to -4.01 parts per thousand. VPDB, and 4.66 x 10(9) to 11.4 x 10(9), respectively. The air-corrected helium ratios indicate a dominant crustal source for helium with significant mantle-derived helium contributions (up to 19.4%) to the total helium contents. Additionally, the combined CO2/He-3 ratios and delta C-13 (CO2) values reveal that while the principal CO2 contribution was from decomposition of marine carbonates, mantle-derived CO2 contributions to the total CO2 contents of the geothermal fluids can reach up to similar to 32%. The He-3/enthalpy ratios were calculated in range from 0.016 x 10(-12) cm(3) STP/J to 0.275 x 10(-12) cm(3) STP/J, and are generally comparable to the theoretical He-3/heat production ratio of the upper mantle. On the basis of He-3/enthalpy ratios, the maximum magmatic heat contribution in the field was estimated as similar to 55% which is consistent with the average surface heat flow estimates for western Anatolia implying that at least half of the heat input to the overall heat budget of the Simav Geothermal System originating from a magmatic source. Finally, because there is no evidence of any active or recent volcanism in the region, mantle volatiles and magmatic heat contribution were attributed to the extension-related mantle melt generation providing mantle heat and volatile input beneath the crust. (C) 2015 Elsevier Ltd. All rights reserved.