What could be the largest source of abiotic methane on Earth

4 in the eclogite of the WesternTianshan subduction zone. Credit: Science China Press” width=”800″ height=”464″/>

CH4– Eclogite-rich fluid inclusions from the WesternTianshan subduction zone. Credit: Science China Press

Methane (CH4), the main constituent of natural gas, is one of the most widely used “clean” fuels. Although methane is generally thought to originate from organic matter, recently there is growing evidence that methane can be produced by abiotic processes.

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

According to their calculation, the potential CH4 the flux from modern subduction zones around the world is estimated to be about 10.8 Mt/yr. Therefore, subducted cold oceanic crust may produce the greatest amount of abiotic methane, along with other sources of abiotic methane such as that from mid-ocean ridges or that from high-pressure serpentinization.

massive CH4Rich fluid inclusions were 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 subduction Cold). Isotope analyzes and petrological studies have both demonstrated that this methane was of abiotic origin and formed by water-rock reactions during high pressure to ultra high pressure prograde metamorphism.

La plus grande source de méthane abiotique sur Terre

Stable compositions of the C and H isotopes of CH4 in the eclogites of the West Tianshan subduction zone. Credit: Science China Press

Phase equilibrium and DEW simulations showed that the favorable conditions of temperature, pressure and oxygen fugacity for the formation of abiotic methane were 450-560℃, 1.5-3.5 GPa and FMQ -1 to FMQ-3.5 respectively. During cold subduction and exhumation of oceanic crust:

  • When the oceanic crust subducted at ∼50 km, carbon species in the fluids were dominated by reduced CH4and its proportion is about 61%;
  • When oceanic crust is subducted at ∼80 km, CH4 in fluids peaks at ~97%;
  • When the oceanic crust is subducted at 80-120 km, CH4 in fluids remained at a maximum of ~97%;
  • During the exhumation, CH4 in fluids decreases drastically, while oxidized carbon species, such as CO2 and H2CO3gradually increase.
  • The largest source of abiotic methane on Earth

    Variation in the composition of aqueous carbonaceous species in the ecclogitic fluid along the P–T–fO2– fluid development trajectory. Credit: Science China Press

  • The largest source of abiotic methane on Earth

    Illustration of abiotic CH4 production during HP-UHP prograde metamorphism during cold subduction. Credit: Science China Press

It can be inferred that the cold subduction zone is the abiotic CH factory4 gas, which can form huge methane gas.

“We report a significant but hitherto overlooked source of methane gas,” Professor Zhang said, “the released abiotic CH4 could contribute to natural gas deposits in shallow basins. Otherwise, if it passes into the atmosphere by outgassing through arc volcanoes, an impact on climate can be expected given its large potential volume. »

More information:
Lijuan Zhang et al, Massive abiotic methane production in eclogite during cold subduction, National Science Review (2022). DOI: 10.1093/nsr/nwac207

Provided by Science China Press

Quote: What might be the largest source of abiotic methane on Earth (2022, November 28) Retrieved November 28, 2022 from https://phys.org/news/2022-11-largest-source-abiotic-methane-gas.html

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