Abstract
Abstract Helium and 13 C(CO 2 ) isotope compositions of gas discharges are used in conjunction with 3 He/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 3 He/ 4 He ratios, δ 13 C(CO 2 ) values, and CO 2 / 3 He 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 3 He/ 4 He ratio), −1.47‰ VPDB to −4.01‰ VPDB, and 4.66 × 10 9 to 11.4 × 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 CO 2 / 3 He ratios and δ 13 C (CO 2 ) values reveal that while the principal CO 2 contribution was from decomposition of marine carbonates, mantle-derived CO 2 contributions to the total CO 2 contents of the geothermal fluids can reach up to ∼32%. The 3 He/enthalpy ratios were calculated in range from 0.016 × 10 −12 cm 3 STP/J to 0.275 × 10 −12 cm 3 STP/J, and are generally comparable to the theoretical 3 He/heat production ratio of the upper mantle. On the basis of 3 He/enthalpy ratios, the maximum magmatic heat contribution in the field was estimated as ∼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.
Published Version
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