High temperature generation and equilibration of methane in terrestrial geothermal systems: Evidence from clumped isotopologues.

Fluids emanating from geothermal areas contain trace quantities of methane and other simple hydrocarbons. These hydrocarbons are thought to derive from thermal cracking of organic matter dissolved in circulating meteoric or seawater or found in pre-existing organic-rich sedimentary rocks, but an abiotic origin has also been proposed. We measured the relative abundances of four CH 4 isotopologues (12CH 4 , 13CH 4 , 12CH 3 D, and 13CH 3 D) in hydrothermal gases discharged by steam vents and geothermal wells from Iceland and Nisyros island (Greece) in order to investigate the origin of methane. Measured methane samples yielded consistently low Δ13CH 3 D values (13CH 3 D abundance relative to stochastic) of 0.82–1.77‰, which correspond to high apparent temperatures of isotopologue equilibrium ( T Δ 13 CH 3 D = 278–490 °C). Hydrothermal well fluids from the Krafla and Námafjall geothermal fields in Iceland yielded the lowest Δ13CH 3 D values, and thus the highest Δ13CH 3 D-based temperatures averaging 438 - 45 + 55 °C. Those samples also show the most pronounced departures in δD CH4 and δ13C CH4 values expected for isotopic equilibrium with respect to δD H2O and δ13C CO2. In contrast, CH 4 samples from natural steam vents in other Iceland locations and in Nisyros have slightly higher Δ13CH 3 D values (with T Δ 13 CH 3 D = 351 - 35 + 42 °C) and have δD CH4 and δ13C CH4 values that are consistent with those expected for isotopic equilibrium with both H 2 O and CO 2. The short fluid residence times (1–50 years) in systems that are exploited for geothermal energy, such as Krafla and Námafjall, combined with the proximity of a hot magma chamber, favor the preservation of kinetic signals. The initial disequilibrium δD CH4 and δ13C CH4 values are consistent with a thermogenic origin from immature organics dissolved in hydrothermally heated groundwater, but an abiotic origin cannot be excluded. The high apparent Δ13CH 3 D-based temperatures at Krafla and Námafjall could therefore represent nonequilibrium signals associated with either pyrolysis or abiotic generation of CH 4 in a superheated vapor or supercritical water phase (>374 °C), considered to exist in the roots of the system above the magmatic heat source. Isotopologue equilibration calculations demonstrate that under such conditions (e.g. > 400 °C) kinetic signals would be erased in days to months, implying rapid migration and quenching of CH 4 into the overlying subcritical (<300 °C) hydrothermal reservoir fluids. In systems with longer fluid residence times such as Nisyros, equilibrium isotopologue distributions at temperatures of ~ 350 °C are consistent with long fluid residence times on the order of > 100 years. Our calculations further reveal that CO 2 –CH 4 isotopic equilibration requires unreasonably long fluid residence times, suggesting that any apparent 13C equilibrium may be coincidental. [ABSTRACT FROM AUTHOR]