Isotopic and kinetic constraints on methane origins in Icelandic hydrothermal fluids

Stefánsson, A. et al. (2024) Isotopic and kinetic constraints on methane origins in Icelandic hydrothermal fluids. Geochimica et Cosmochimica Acta, 373, pp. 84-97. (doi: 10.1016/j.gca.2024.03.015)

Abstract

The origin of methane in hydrothermal fluids has long been a subject of debate – whether it is abiotic or biotic. In this study, we aim to unravel and quantify the sources of CH4 in active hydrothermal systems by adopting a holistic approach analyzing well characterized high-temperature hydrothermal fluids (∼230–310 °C) in Iceland. We employ a broad variety of geochemical and isotope indicators, encompassing chemical and isotope compositions of the targeted fluids. These signatures are then compared with results from chemical and isotope kinetic models and data from sedimentary-hosted hydrothermal systems. Carbon species in these fluids include CO2 (2.60–184 mmol/kg), CH4 (2.39·10−4–0.325 mmol/kg), dissolved organic carbon (4.78·10−3–0.112 mmol/kg), and CO (1.89·10−6–4.16·10−4 mmol/kg). Carbon and helium isotopes suggest a relatively uniform mantle-derived source of CO2 (δ13C-CO2: −4.80 to −1.50 ‰, CO2/3He: 1.49·109–4.14·1010 14C-CO2: 0.11–2.42 pMC). Methane, in contrast, has multiple sources. Overall, chemical equilibria among carbon species (CO2, CH4, CO) is not attained, suggesting kinetic controls. Tritium content (<0.8–1.42 TU) and hydrologic constraints indicate relatively short hydrothermal fluid residence times (∼5–200 years), with occasional inputs from older water components. Within this short timeframe, CH4 concentrations vary from lower, to significantly higher than those calculated using CO2 reduction kinetics. The isotope composition (δD-CH4: −172 to −138 ‰, δ13C-CH4: −32.0 to −24.6 ‰; 14C-CH4: 0.36–11.54 pMC) and geochemical and isotope modeling suggest that the majority (>80–90 %) of CH4 originates from a radiocarbon inactive source, i.e. mantle CH4, reduction of mantle CO2 and/or old organic matter, with relatively small contributions from both marine (<20 %) and terrestrial (<10 %) dissolved organic carbon. Measured isotopic compositions of CH4 do not match those expected for mantle-derived CH4 as well as values generated from reduction of mantle-derived CO2. Instead, differences in δD-CH4 and δ13C-CH4 values exist between systems fed by meteoric water and those fed by seawater, challenging the assumption of a uniform CO2 source and invariable reaction mechanisms. Differences between systems are best explained by variable extent of thermal decomposition and primary variations in the isotope composition of marine and terrestrial organic matter. Also, δD-CH4 and δ13C-CH4 values in meteoric water-fed systems closely resemble those in the Öxarfjördur sedimentary-hosted systems. In summary, our data supports a predominant thermogenic origin of CH4 in both seawater and terrestrial hydrothermal fluids in Iceland. The source of organic matter appears to be a combination of modern dissolved organic carbon and older sedimentary deposits. In addition, some of the hydrothermal systems studied (Krafla, Reykjanes, Theistareykir) which are characterized by low CH4 concentrations, may contain a significant portion of CH4 that may originate from CO2 reduction.

• Item Type: Articles
• Additional Information: This work is part of the GECO project and has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement # 818169. The work was further supported by Rannís grant # 152680, Orkusjódur Landsvirkjunar and 2019 EAOG Research Award.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Garnett, Dr Mark
• Authors: Stefánsson, A., Ricci, A., Garnett, M., Gunnarsson-Robin, J., Kleine-Marshall, B. I., Scott, S. W., Lelli, M., Cardoso, C. D., Pik, R., Santinelli, C., Ono, S., Barry, P. H., Broadley, M. W., Byrne, D., Halldórsson, S. A., and Fiebig, J.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Geochimica et Cosmochimica Acta
• Publisher: Elsevier for the Geochemical Society
• ISSN: 0016-7037
• ISSN (Online): 1872-9533
• Published Online: 27 March 2024
• Data DOI: 10.5281/zenodo.10812895