Methane (CH 4 ), the chief constituent of natural gas, is one of the most widely used “clean” fuels. 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 cold subduction zone, evidenced by the massive CH 4 -rich fluid inclusions in eclogites from Western Tianshan, China. Based on their calculation, the potential CH 4 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 by high-pressure serpentinization.
Massive CH 4 -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 abiotic origin and formed by water-rocks reactions during the prograde high-pressure to ultrahigh-pressure metamorphism.
Phase equilibrium and DEW simulations showed that the favorable temperature, pressure and oxygen fugacity conditions for abiotic methane formation were 450-560℃, 1.5-3.5 GPa, and FMQ-1 to FMQ-3.5 respectively. During the cold subduction and exhumation of the oceanic crust:
It can be inferred that the cold subduction zone is the factory of abiotic CH 4 gas, which can form enormous methane gas.
“We report a large yet previously overlooked source of methane gas”, Prof. Zhang said, “the released abiotic CH 4 might contribute to natural gas deposits at shallow basins. Otherwise, if it goes into the atmosphere by degassing through arc volcanoes, an impact on climate can be expected given its large potential volume.”
This study was funded by the National Key Research and Development Program of China (2019YFA0708501) and the National Natural Science Foundation (Nos. 42172060 and 41702052).
Link to the paper: https://doi.org/10.1093/nsr/nwac207
Correspondence: Prof. Lifei Zhang, Lfzhang@pku.edu.cn
National Science Review