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 CH4 isotopologues ((CH4)-C-12, (CH4)-C-13, (CH3D)-C-12, and (CH3D)-C-13) 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 Delta(CH3D)-C-13 values ((CH3D)-C-13 abundance relative to stochastic) of 0.82-1.77 parts per thousand, which correspond to high apparent temperatures of isotopologue equilibrium (T Delta(CH3D)-C-13 = 278-490 degrees C). Hydrothermal well fluids from the Krafla and Nimafjall geothermal fields in Iceland yielded the lowest Delta(CH3D)-C-13 values, and thus the highest Delta(CH3D)-C-13-based temperatures averaging 438(-45)(+55) degrees C. Those samples also show the most pronounced departures in delta D-CH4 and delta C-13(CH4) values expected for isotopic equilibrium with respect to delta DH2O and delta(CCO2)-C-13. In contrast, CH4 samples from natural steam vents in other Iceland locations and in Nisyros have slightly higher Delta(CH3D)-C-13 values (with T-Delta 13CH3D = 351(-35)(+42) degrees C) and have delta D-CH4 and delta C-13(CH4) values that are consistent with those expected for isotopic equilibrium with both H2O and CO2. The short fluid residence times (1-50 years) in systems that are exploited for geothermal energy, such as Krafla and Nimafjall, combined with the proximity of a hot magma chamber, favor the preservation of kinetic signals. The initial disequilibrium delta D-CH4 and delta C-13(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 Delta(CH3D)-C-13-based temperatures at Krafla and Nimafjall could therefore represent nonequilibrium signals associated with either pyrolysis or abiotic generation of CH4 in a superheated vapor or supercritical water phase (>374 degrees 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 degrees C) kinetic signals would be erased in days to months, implying rapid migration and quenching of CH4 into the overlying sub critical (<300 degrees C) hydrothermal reservoir fluids. In systems with longer fluid residence times such as Nisyros, equilibrium isotopologue distributions at temperatures of similar to 350 degrees C are consistent with long fluid residence times on the order of > 100 years. Our calculations further reveal that CO2-CH4 isotopic equilibration requires unreasonably long fluid residence times, suggesting that any apparent C-13 equilibrium may be coincidental. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Research suggests that methane from geothermal areas can originate from thermal cracking of organic matter or have an abiotic origin, with geological features and fluid residence times influencing the isotopic equilibrium distribution of methane. Geothermal fields in Iceland exhibit higher temperatures and shorter fluid residence times, while Nisyros shows longer fluid residence times and equilibrium isotopic distributions.