Immiscible metamorphic water and methane fluids preserved in carbonated eclogite

Subduction zones metamorphic fluids are pivotal in geological events such as volcanic eruptions, seismic activity, mineralization, and the deep carbon cycle. However, the mechanisms governing carbon mobility in subduction zones remain largely unresolved. Here we present the first observations of immiscible H₂O-CH₄ fluids coexisting in retrograde carbonated eclogite from the Western Tianshan subduction zone, China. We identified two types of fluid inclusions in host ankerite and amphibole, as well as in garnet and omphacite. Type-1 inclusions are water-rich with CH₄ vapor, whereas Type-2 are CH₄-rich, with minimal or no H₂O. The coexistence of these fluid types indicates the presence of immiscible fluid phases under high-pressure conditions (P = 1.3-2.1 GPa). Carbonates in subduction zones can effectively decompose through reactions with silicates, leading to the generation of abiotic CH₄. Our findings suggest that substantial amounts of carbon could be transferred from the slab to mantle wedge as immiscible CH₄ fluids. This process significantly enhances decarbonation efficiency and may contribute to the formation of natural gas deposits.