Methane thermometry in deep-sea hydrothermal systems: Evidence for re-ordering of doubly-substituted isotopologues during fluid cooling
Résumé
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, 13 CH3D and 12 CH2D2, 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 D 12 CH2D2-D 13 CH3D data establish that methane is in internal equilibrium at 343 #$% &'(°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 D 13 CH3D values are indistinguishable from those at Rainbow, 12 CH2D2 abundances are marginally higher. At Lost City, D 13 CH3D data show a range of values, which at face value correspond to apparent temperatures of between 265 #,' &,-°C and
Domaines
Océan, Atmosphère
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