Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions

Abstract

The conditions of methane (CH4) formation in olivine-hosted secondary fluid inclusions and their prevalence in peridotite and gabbroic rocks from a wide range of geological settings were assessed using confocal Raman spectroscopy, optical and scanning electron microscopy, electron microprobe analysis, and thermodynamic modeling. Detailed examination of 160 samples from ultraslow- to fast-spreading midocean ridges, subduction zones, and ophiolites revealed that hydrogen (H2) and CH4 formation linked to serpentinization within olivine-hosted secondary fluid inclusions is a widespread process. Fluid inclusion contents are dominated by serpentine, brucite, and magnetite, as well as CH4(g) and H2(g) in varying proportions, consistent with serpentinization under strongly reducing, closed-system conditions. Thermodynamic constraints indicate that aqueous fluids entering the upper mantle or lower oceanic crust are trapped in olivine as secondary fluid inclusions at temperatures higher than ∼400 °C. When temperatures decrease below ∼340 °C, serpentinization of olivine lining the walls of the fluid inclusions leads to a near-quantitative consumption of trapped liquid H2O. The generation of molecular H2 through precipitation of Fe(III)-rich daughter minerals results in conditions that are conducive to the reduction of inorganic carbon and the formation of CH4. Once formed, CH4(g) and H2(g) can be stored over geological timescales until extracted by dissolution or fracturing of the olivine host. Fluid inclusions represent a widespread and significant source of abiotic CH4 and H2 in submarine and subaerial vent systems on Earth, and possibly elsewhere in the solar system.

Pulsated Global Hydrogen and Methane Flux at Mid‐Ocean Ridges Driven by Pangea Breakup

“…The lower limit of methane flux suggested from this ratio is slightly higher than that of Emmanuel and Ague (2007) (hereafter, EA07) (~2.7 Mt/a or 1.6 • 10 11 mol/a) and than that estimated by Cannat et al (2010) of 0.4 Mt/a (2.4 • 10 10 mol/a). EA07 assumed that methane was generated as a result of hydrogen production through the stoichiometric Sabatier equation; however, recent work has suggested more strongly that abiotic methane sampled at mid‐ocean ridges has a deeper, mantle origin, rather than produced in the crustal column using hydrogen generated from serpentinization (e.g., Grozeva et al, 2020; Klein et al, 2019; McDermott et al, 2015; Wang et al, 2018).…”

“…Proposed sources include Fischer‐Tropsch‐type or Sabatier reactions (see review by McCollom, 2013) or degassing of deep fluid inclusions (e.g. Grozeva et al, 2020; Klein et al, 2019; McDermott et al, 2015; Wang et al, 2018). In the Phanerozoic, the most common and regular exposure of the mantle minerals to water, which lead to these reactions, occurs at mid‐ocean ridges (MORs).…”

Methane release on Early Mars by atmospheric collapse and atmospheric reinflation

“…This is not surprising, because serpentinization may be mostly restricted to rocks too deep for subsequent exhumation to the surface (Carter et al 2013, Sun & Milliken 2015. Moreover, radiolysis+fluid-inclusion-scale serpentinization can produce H2+CH4 and not leave behind a mineralogic byproduct that would be detectable by CRISM (Klein et al 2019). Regardless of production depth and process, CH4-charged waters may be swept to the near-surface by hydrothermal circulation (Parmentier & Zuber 2007).…”

“…1) Noachian-age hydrothermal minerals record Noachian hydrothermal reactions (e.g., Carter et al 2013, Ehlmann et al 2010, Parmentier & Zuber 2007, Sun & Milliken 2015. Hydrothermal reactions between Mars' mafic/ultramafic crust and waters charged with magmatic and/or atmospheric C should yield both H2 and CH4 (e.g., Lyons et al 2005, Oze & Sharma 2005, Klein et al 2019. CH4 is emphasized here (Etiope & Sherwood Lollar 2013), due to its stability in clathrate hydrates under lower pressures and higher temperatures relative to H2.…”

“…Because of the lack of recharge by this mechanism, repeated ice unloading will only yield CH4 gas from a given latitude-longitude-depth volume element for the first time that volume element is unloaded (considering only the hydrothermal-circulation mechanism for CH4 production; other mechanisms are possible, e.g. Klein et al 2019). We found that the fraction of runs with a strong CH4 burst increases steeply when the difference between the obliquity for collapse and the obliquity for re-inflation increases (i.e.…”