Abiotic methane synthesis and serpentinization in olivine-hosted fluid inclusions.
basic_science · Level V
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- Record sourced from PubMed, PMID 31427518.
- Also identified by DOI 10.1073/pnas.1907871116 and PMC identifier 6731755.
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Abstract
The conditions of methane (CH<sub>4</sub>) 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 (H<sub>2</sub>) and CH<sub>4</sub> 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 CH<sub>4(<i>g</i>)</sub> and H<sub>2(<i>g</i>)</sub> 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 H<sub>2</sub>O. The generation of molecular H<sub>2</sub> through precipitation of Fe(III)-rich daughter minerals results in conditions that are conducive to the reduction of inorganic carbon and the formation of CH<sub>4</sub> Once formed, CH<sub>4(<i>g</i>)</sub> and H<sub>2(<i>g</i>)</sub> 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 CH<sub>4</sub> and H<sub>2</sub> in submarine and subaerial vent systems on Earth, and possibly elsewhere in the solar system.