What may be the largest source of abiotic methane gas on Earth

abiotic — Not associated with or derived from living organisms. Calling methane, aka natural gas, a ‘fossil fuel’ is shown by geological evidence to be inaccurate.
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Methane (CH4), the chief constituent of natural gas, is one of the most widely used “clean” fuels, says Phys.org.

Although methane is usually considered to originate from organic matter, recently, more and more evidence shows that methane can be produced by abiotic processes.

In a recent paper published in National Science Review (NSR), Professor Lifei Zhang’s team from Peking University demonstrated that large amounts of methane gas can form during prograde metamorphism in a cold subduction zone, evidenced by the massive CH4-rich fluid inclusions in eclogites from Western Tianshan, China.

Based on their calculation, the potential CH4 flux from worldwide modern subduction zones is estimated to be as much as ~10.8 Mt/y.

Consequently, the subducted cold oceanic crust may produce the largest amount of abiotic methane, along with other abiotic methane sources such as that from mid-ocean ridges or that from high-pressure serpentinization.

Massive CH4-rich fluid inclusions have been found in garnet and omphcite, which are the main constituent minerals of eclogite in the West Tianshan subduction zone (eclogite is the most important high grade metamorphic rock during cold subduction).

Isotopic analyses and petrological studies both demonstrated that this methane was of abiotic origin and formed by water-rocks reactions during the prograde high-pressure to ultrahigh-pressure metamorphism.

Full article here.
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JOURNAL ARTICLE — Massive abiotic methane production in eclogite during cold subduction