Publication Date:
2022-05-26
Description:
Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 223 (2018): 141-158, doi:10.1016/j.gca.2017.11.030.
Description:
Hot-spring fluids emanating from deep-sea vents hosted in unsedimented ultramafic and mafic rock commonly
contain high concentrations of methane. Multiple hypotheses have been proposed for the origin(s) of this methane, ranging from synthesis via reduction of aqueous inorganic carbon (ΣCO2) during active fluid circulation to
leaching of methane-rich fluid inclusions from plutonic rocks of the oceanic crust. To further resolve the process(es) responsible for methane generation in these systems, we determined the relative abundances of several
methane isotopologues (including 13CH3D, a “clumped” isotopologue containing two rare isotope substitutions) in
hot-spring source fluids sampled from four geochemically-distinct hydrothermal vent fields (Rainbow, Von
Damm, Lost City, and Lucky Strike).
Apparent equilibrium temperatures retrieved from methane clumped isotopologue analyses average 310−42
+53 °C,
with no apparent relation to the wide range of fluid temperatures (96 to 370 °C) and chemical compositions (pH,
[H2], [ΣCO2], [CH4]) represented. Combined with very similar bulk stable isotope ratios (13C/12C and D/H) of
methane across the suite of hydrothermal fluids, all available geochemical and isotopic data suggest a common
mechanism of methane generation at depth that is disconnected from active fluid circulation. Attainment of equilibrium amongst methane isotopologues at temperatures of ca. 270 to 360 °C is compatible with the thermodynamically-favorable reduction of CO2 to CH4 at temperatures at or below ca. 400 °C under redox conditions characterizing intrusive rocks derived from sub-ridge melts. Collectively, the observations support a model where
methane-rich aqueous fluids, known to be trapped in rocks of the oceanic lithosphere, are liberated from host
rocks during hydrothermal circulation and perhaps represent the major source of methane venting with thermal
waters at unsedimented hydrothermal fields. The results also provide further evidence that water-rock reactions
occurring at temperatures lower than 200 °C do not contribute significantly to the quantities of methane venting at
mid-ocean ridge hot springs.
Description:
Financial support from the U.S. National Science Foundation
(NSF awards EAR-1250394 to S.O., and OCE-1061863 and OCE-0549829 to J.S.S.), the National Aeronautics
and Space Administration (NASA) (NNX-327 09AB75G to J.S.S., and the NASA Astrobiology Institute “Rock-
Powered Life” project under cooperative agreement NNA15BB02A to S.O.), the Alfred P. Sloan Foundation via
the Deep Carbon Observatory (to S.O. and J.S.S.), the U.S. Department of Defense (DoD) through a National Defense Science & Engineering Graduate (NDSEG) Fellowship (to D.T.W.), a Shell-MIT Energy Initiative Fellowship, and the Kerr-McGee Professorship at MIT (to S.O.) is gratefully acknowledged.
Keywords:
Methane
;
Hydrothermal vent fields
;
Fluid inclusions
;
Clumped isotopologues
;
Hydrogen isotope exchange
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
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