Carbon mineralization and abiotic methane synthesis within fluid inclusions in mafic minerals from postcollisional pyroxenite

Carbon mineralization in mafic-ultramafic rocks is not only a natural process that can form significant carbon sinks in the lithosphere, but also a promising procedure for artificial CO2 storage. In this study, we report unique records of in situ carbon mineralization within fluid inclusions in postcollisional pyroxenite from the Dabie orogen, central China. Olivine, orthopyroxene, clinopyroxene, and amphibole in the pyroxenite trapped CO2-rich magmatic fluids as fluid inclusions, which were subjected to internal carbon mineralization due to interaction between trapped fluids and host minerals during cooling of the pyroxenite. In fluid inclusions, carbonation of olivine produced magnesite, talc, magnetite, and CH4; carbonation of orthopyroxene generated magnesite, talc, cristobalite, and CH4; carbonation of clinopyroxene formed calcite, dolomite, actinolite, talc, cristobalite/quartz, and CH4; carbonation of amphibole created calcite, dolomite, chlorite, cristobalite/quartz, talc, mica, a TiO2 phase, a NaAlSi3O8 phase, and CH4. Carbonation reactions within the fluid inclusions have general implications for reaction pathways of carbon mineralization in mafic-ultramafic rocks. Moreover, it is shown that iron oxidation is thermodynamically favored during high-extents carbonation of diverse mafic minerals in the presence of CO2-rich fluids, facilitating abiotic synthesis of CH4 that is a crucial gas in both natural carbon cycling and engineered carbon storage.