Whether carbonate melting can take place and its role in recycling carbon in deep subduction zones remain ambiguous in many aspects. Studies on ultrahigh-pressure (UHP) rocks exhumed back from great depths can help resolve these ambiguities, but evidence for hydrous carbonatitic liquids has been rarely documented in such rocks. Here, we report primary polyphase solid inclusions (PSIs) of calcite + chlorite in garnet from an UHP diopside-garnet rock enclosed in impure marbles from the Dabieshan, China. These PSIs display typical fluid-mediated microstructures, such as inclusion-garnet interfaces that reflect the control of the crystallographic structure of garnet. In addition, garnet was partially re-equilibrated around the PSIs to form new patches with higher Ca, Fe3+, and Mn contents. In combination with phase equilibria modelling and analogue melting experiments on the calcite-chlorite system, the PSIs are inferred to have resulted from reactions between UHP hydrous carbonatitic liquid inclusions and host garnet during exhumation. The PSIs contain higher contents of light rare earth elements (LREE) and large ion lithophile elements (e.g., Pb and Sr) than garnet but display a LREE-depleted distribution pattern. The latter feature is comparable to the bulk-rock composition, implying that the carbonatitic liquid resulted from partial melting of the host diopside-garnet rock. In addition, our experiments reveal that carbonated chlorite-rich rocks, as a common metasomatic rock in subduction channels, have a solidus temperature between 800 and 850 °C at 4.5 GPa and can thus partially melt to form hydrous carbonatitic liquids at sub-arc depths in warm and hot subduction zones. To sum up, both our natural and experimental studies suggest that the formation of carbonatitic liquid could be more common than expected in subducted slabs. Carbonate melting, being an efficient mechanism for carbon release, thus deserves more attention in deep carbon recycling in subduction zones.
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