Methane thermometry in deep-sea hydrothermal systems: Evidence for re-ordering of doubly-substituted isotopologues during fluid cooling

Deep-sea hydrothermal fluids are often enriched in carbon dioxide, methane, and hydrogen. Methane effuses from metal-rich black smokers such as the Rainbow hydrothermal field, at temperatures higher than 200 °C. At the Lost City field, CH4 emanates from alkaline fluids at < 100 °C. The abundance of the rare, mass-18 CH4 isotopologues, 13CH3D and 12CH2D2, can mitigate degeneracies in the conventional isotopic signatures of methane. We studied the isotopologue compositions of methane from the Rainbow, Lucky Strike, Von Damm, and Lost City hydrothermal fields. At Rainbow, where the vented fluids are at ∼360°C, our coupled Δ12CH2D2 - Δ13CH3D data establish that methane is in internal equilibrium at °C. This may track the formation temperature of abiotic methane, or it may be the result of equilibration of methane isotopologues within the carrier fluid. Lucky Strike and Von Damm have fluid temperatures < 300°C and although Δ13CH3D values are indistinguishable from those at Rainbow, 12CH2D2 abundances are marginally higher. At Lost City, Δ13CH3D data show a range of values, which at face value correspond to apparent temperatures of between °C and °C, far hotter than fluid temperatures. A unique aspect of the Lost City data is the range of large 12CH2D2 excesses. The Δ12CH2D2 data correspond to temperatures of between °C and °C, showing a near-perfect match with fluid temperatures. We find that mixing scenarios involving microbial methane may not account for all of the isotope data. We suggest that Δ12CH2D2 values, unlike Δ13CH3D values, are prone to near-complete re-equilibration at host fluid temperatures. We suggest that 13CH3D isotopologue data are consistent with abiotic methane being synthesized at ∼350 °C. On the other hand, 12CH2D2 isotopologue ordering records post formation residence temperatures. We explore a possible mechanism decoupling the re-equilibration systematics of the doubly-substituted isotopologues.