Abiotic methane generation through reduction of serpentinite-hosted dolomite: implications for carbon mobility in subduction zones

Abiotic methane has been increasingly detected at the surface of Earth and other terrestrial planets, exerting a strong effect on the study of chemolithoautotrophic life and thus astrobiology. In contrast, abiotic methane generation in subduction zones, which is intimately linked to questions such as the mechanisms of deep carbon mobility, has received scarce attention. Experiments elucidated the significant production of abiotic methane through reduction of carbonate minerals under subduction zone conditions, whereas detailed geological conditions and processes for the reduction in natural rocks are hitherto poorly understood. Here, we report carbonate reduction and genesis of abiotic methane in dolomitized serpentinites (referred to as ophidolomites) from a fossil subduction zone (SW Tianshan, China). Detailed petrological, Raman spectroscopic, strontium and carbon isotopic, and thermodynamic results provide evidence for dolomite reduction into the phase assemblage of calcite + brucite + methane, likely associated with retrograde serpentinization starting at 7−9 kbar and 410−430 oC in the subduction zone. Microthermometric data for dolomite-hosted fluid inclusions are consistent with petrographic observations, indicative of fluid entrapment postdating the onset of dolomite reduction during exhumation. Model calculations suggest that water-rich fluids characterized by relatively high hydrogen fugacities can create favorable conditions for the reduction process, which, however, do not exclude the possibility of carbonate methanation by hydrogen-rich fluids as reported in previous studies. The widespread occurrence of methane in these rocks gives credence to the intricate redox transformations of subducted carbon, implying that the elevated hydrogen fugacities may facilitate abiotic synthesis of methane through dolomite reduction at convergent plate boundaries. Our work shows that alteration of dolomite-bearing lithologies represents a potential source for abiotic methane in subduction zones, which may have implications for the transfer of subducted carbon.