Evidence for a general mechanism for the origin of abiotic CH4 in serpentinizing hydrothermal systems
Abstract
Fluids rich in H2, such as those emanating from deep-sea hydrothermal systems, are of particular interest due to a favorable thermodynamic drive for the abiotic reduction of inorganic carbon (ΣCO2) to organic compounds. An abiotic origin of carbon compounds in natural waters is compelling due to their potential role in the origin and sustenance of life on early Earth. Placing unequivocal constraints on the contributions of abiotic organic synthesis to vent fluids, in which biotic sources also impact carbon budgets, remains a challenge. A previous study showed that abundant CH4 and other hydrocarbons in fluids venting at ultramafic-hosted deep-sea hydrothermal fields do not form in response to H2 generation during active serpentinization at the ultramafic-influenced Von Damm vent field [1], as hypothesized by previous studies. Rather, several lines of evidence demonstrate that hydrocarbons found in vent fluids are formed in H2-rich fluid inclusions in plutonic rocks. A calculated radiocarbon age for fluid ΣCO2 averaging 28,692 years (n=3) at Von Damm further emphasizes the slow kinetics of abiotic CH4 formation. Implications for this age will be discussed for fluid residence times within the Von Damm system, and extended beyond to other vent fields.
These results are corroborated by a clumped isotopologue analysis of dissolved CH4 from four geochemically-distinct hydrothermal vent fields (including Von Damm and Lost City) that yields apparent equilibrium temperatures that average 310 °C, with no apparent relation to the wide range of measured fluid temperatures (96-370 °C) and compositions [2]. All geochemical and isotopic data suggest a common mechanism of methane generation at depth that is disconnected from active fluid circulation. Leaching of pre-existing CH4 hosted in fluid inclusions can account for the uniformity of formation temperatures indicated for CH4 across multiple vent fields. Collectively, emerging evidence that supports the formation of CH4 over geologic timescales, rather than over much-shorter timescales of vent fluid circulation through the crust, challenges current paradigms while revealing possibilities to investigate formation of other organic molecules. [1] McDermott et al. (2015) Proc. Nat. Acad. Sci., pp. 7668-7672. [2] Wang et al. (2018) Geochim. Cosmochim. Acta, pp. 141-158.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T12C..06M
- Keywords:
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- 3035 Midocean ridge processes;
- MARINE GEOLOGY AND GEOPHYSICS;
- 8034 Rheology and friction of fault zones;
- STRUCTURAL GEOLOGY;
- 8135 Hydrothermal systems;
- TECTONOPHYSICS;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS