Compound-Specific Carbon Isotope Decoupling in the Çıralı (Yanartaş) Ophiolitic H2–CH4 System: Constraints on Multistage Gas Evolution in the Tekirova Peridotite, SW Türkiye

The Çıralı (Yanartaş) gas seepage system in the Tekirova Peridotite, SW Türkiye, is an ophiolitic H2–CH4 system associated with serpentinized ultramafic rocks and fault-controlled pathways. This study evaluates molecular compositions, C1–C3 compound-specific carbon isotope data, and methane hydrogen isotope compositions from four newly sampled gas outlets together with previously published data for which the full parameter set is available. The new samples contain 85.5–91.6% CH4 and 2.6–5.3% H2, with δ13C-CH4 values of −7.3 to −4.4‰ and δ2H-CH4 values of −121 to −104‰. The C1–C2 isotope reversal index (RI12) ranges from +13.6 to +14.5‰, whereas the C2–C3 isotope proximity index (PI23) ranges from 0.2 to 1.3‰. These values indicate the clear isotopic decoupling of methane from ethane and propane, whereas the C2–C3 pair shows comparatively small isotopic differences. The same relative pattern occurs at all four outlets, indicating consistency between samples. The results are consistent with a multistage model in which C2+ components are compatible with a thermogenic contribution, whereas methane may have been affected by serpentinization-related H2 input, fracture-controlled flow, and shallow ophiolitic modification. Catalytic CO2 hydrogenation is evaluated not as a demonstrated field-scale process but as a hypothesis requiring testing in chromitite–serpentinite-related microenvironments.