Fluid and solid inclusions in ultrahigh-pressure metamorphic rocks can preserve information about processes that occurred during their formation and subsequent evolution. In this study, inclusions within garnets from the ultrahigh-pressure metapelites of the Pohorje Mountains were investigated, with particular emphasis on the origin and evolution of carbon-bearing phases. Raman spectroscopy and high-resolution transmission electron microscopy revealed that multiphase fluid inclusions from the outer parts of garnets are structurally heterogeneous and contain monocrystalline and polycrystalline diamond, polycrystalline diamond with graphitic domains, graphite with variable degrees of structural ordering, amorphous matter, C–O–H fluids, carbonates, moissanite, and pyrophyllite. Their complex mineral assemblage indicates a multistage evolution of the inclusions related to the interaction between the entrapped C–O–H fluid and the host garnet. The microstructural investigations enabled the reconstruction of the relationships between the individual phases and the sequence of their formation. In contrast to the multiphase fluid inclusions in the outer parts of garnets, solid inclusions in the inner parts of garnets mainly contain well-ordered graphite together with minerals also present in the metapelite matrix, indicating that they were trapped during garnet growth in the prograde stage of metamorphism. Thermodynamic modelling indicates that the entrapped fluid was initially CH4–H2O in composition. During exhumation, hydrogen diffused out of the inclusions due to differences in hydrogen fugacity between the inclusions and the surrounding rock. This process triggered a series of chemical reactions, producing the mineral assemblage observed today. These reactions resulted in the formation of different carbon allotropes, moissanite, carbonates, pyrophyllite, and amorphous matter; ultimately, only a CO2-rich fluid remained. The results demonstrate that multiphase fluid inclusions do not represent a direct record of equilibrium conditions at the metamorphic peak, but rather a record of kinetically controlled processes that occurred during exhumation.
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