A note on the fluxes of abiogenic methane and hydrogen from mid‐ocean ridges

Abstract

[1] The concentrations of methane and hydrogen in hydrothermal vent fluids issuing from mid-ocean ridges tend to fall into two groups, one with high concentrations of these gases in ultramafic-hosted vent fields and a second group with relatively lower concentrations in basalt-hosted vent fluids. Ultramafic-hosted systems, however, appear to be restricted to slow-spreading ridges and constitute only a fraction of the hydrothermal systems found there. In this note, the hydrothermal fluxes of methane and hydrogen have been calculated by estimating the percentages of the total subsurface hydrothermal circulation that circulate through each type of host rock. Even though the percentage of the total subsurface flow that is affected by serpentinization appears to be rather small (8%), it still appears that this process produces about 70% of the total mid-ocean flux of these gases. The total production of methane and hydrogen is calculated to be about 20 × 10 9 mol yr −1 and 190 × 10 9 mol yr −1 , respectively. The hydrogen flux is comparable to that most recently calculated on the basis of the rate of hydration of mantle rock in newly formed crust and the stoichiometry of the serpentinization reaction. This suggests that, except for the production of methane, a major portion of the hydrogen produced in the subsurface is not consumed before venting. Citation: Keir, R. S.(2010), A note on the fluxes of abiogenic methane and hydrogen from mid-ocean ridges, Geophys.

The geochemistry of methane isotopologues

“…For those interested in the race towards measuring intact methane isotopologues, readers are referred to Jones (2012). Figure 1.1 | Some of the many reservoirs and fluxes of methane on Earth, and their relative magnitudes (Kvenvolden, 1988;Keir, 2010; IPCC, 2013;Kirschke et al, 2013 .5 | Zero-point energy lowering and the origin of non-stochastic clumped isotopologue composition at equilibrium. The zero-point energy (the energy of the ground state, the quantum state with the lowest possible energy) of a molecule of methane is lowered upon substitution of heavy isotopes.…”

“…The abundance and isotopic composition of methane venting from submarine hydrothermal fields that are relatively free of sediment cover has been described at oceanic spreading centers characterized by a range of spreading rates (e.g., Welhan, 1988b;Charlou et al, 2002;McCollom and Seewald, 2007;Proskurowski et al, 2008;Cannat et al, 2010;Charlou et al, 2010;Proskurowski, 2010;McDermott et al, 2015;McDermott, 2015). In general, fluids that have interacted with ultramafic rocks are substantially enriched in CH 4 relative to fluids that have reacted with mafic rocks (Keir, 2010), although there are exceptions in which high-CH 4 fluids are associated with apparent mafic substrates (e.g., basalt) (Charlou et al, 2000).Several distinct geochemical processes have been proposed to account for the presence of abiotic CH 4 in submarine hydrothermal fluids. Some have proposed that CH 4 is formed by reduction of aqueous inorganic carbon (i.e., ∑CO2) in subsurface reaction zones during convective circulation of seawater-derived hydrothermal vent fluids in response to the highly reducing (H 2 -rich) conditions that result from extensive fluid-mineral interactions during serpentinization of ultramafic rock (Charlou et al, 2002;Proskurowski et al, 2008).…”

“…Cooling of these rocks along an f O 2 trajectory parallel to those of typical oxygen buffers may allow for respeciation of mantle-derived CO 2 to CH 4 to occur in the presence of mafic minerals (olivine and orthopyroxene) deep within the oceanic crust (Mathez et al, 1989;Kelley and Früh-Green, 1999). Serpentinization occurring distal to the rocks from which CH 4 -rich fluids are extracted may explain why CH 4 and H 2 concentrations are not tightly correlated across seafloor hydrothermal systems (Keir, 2010;Kawagucci et al, 2013).Models of convective hydrothermal circulation at Lost City indicate that the bulk of actively-venting fluids have migrated along a narrow range of flow paths that are surrounded by already fully-serpentinized rock with little additional potential for H 2 generation, suggesting that H 2 in vent fluids may have instead formed by serpentinization occurring in meandering flow paths away from the main flow channels, and later diffused or mixed into the ascending fluid (Titarenko and McCaig, 2016). Assuming that equilibration of the methane isotopologues proceeds through CH 4 -H 2 O exchange, ∆ 13 CH 3 D temperatures for CH 4 from the present study (red bar in Fig.…”

“…Calculations assume that molalities of species other than CH 4 have a negligible effect on mole fractions in the high-CH 4 fluid. The black and white bars show CH 4 concentrations in vent fluids from this study (Table 3.2) and from mid-ocean ridge hydrothermal systems globally (Keir, 2010). fluids discharging at the Medea and Isabel vents is thermodynamically disfavored at in situ temperatures (270-292°C; Table 3.2 and Fig.…”

“…mid-ocean ridge hydrothermal fields, whereas basalt-hosted fields tend to have lower CH 4 contents (~0.1 mM; McCollom and Seewald, 2007;Keir, 2010). In this respect, concentrations of CH 4 in fluids at the Lucky Strike field (~0.9 mM, Table 3.2), as well as at a similar basalt-hosted field, Menez Gwen on the Mid-Atlantic Ridge (~1.7 mM, Charlou et al, 2000), appear to be anomalously elevated relative to those in other basalt-hosted settings such as those on the fast-spreading East Pacific Rise, where CH 4 concentrations of~0.1 mM are more typical (McCollom and Seewald, 2007;Keir, 2010).Contents of low-molecular weight hydrocarbons are also similar between the studied fields, with high C 1 /C 2 ratios (Fig. 3.3B) observed in fluids from Rainbow (~2300, Charlou et al, 2002), Von Damm (~4500, McDermott et al, 2015), Lost City (~1100, Proskurowski et al, 2008), and Lucky Strike (>3000, Charlou et al, 2000).…”