Formation of carbon-bearing silicate melts by melt-metacarbonate interaction at convergent plate margins

The incorporation of carbon into slab-derived silicate melts is a vital process that boosts carbon release and transfer at convergent plate margins. The available models mostly suppose the generation of carbonated silicate melts through partial melting of carbon-rich rocks. Here we show that the carbon-bearing silicate melts can also form by the interaction of slab-derived, carbon-free, silicate melts with eclogite-facies metacarbonate rocks in a subduction-collision environment. We examine the marble – reaction zone – migmatite systems from the Sulu ultra-high pressure (UHP) terrane (China). Field, petrological, geochemical, and Sr-O-C isotopic studies demonstrate that carbon-free silicate melts produced by the anatexis of gneisses infiltrate the marbles, leading to the formation of silicate reaction zones around the melt channels and the synchronous release of CO2 at ∼820–860°C. The occurrence and textural characteristics of calcite-bearing leucosomes in the reaction zones indicate an enrichment of CO2 in silicate melts after the melt-marble interaction. Compositional analyses and mapping of carbonates indicate that these carbonated silicate melts contain >0.8–1.6 wt.% CO2, in accordance with the solubility of carbon in silicate melts at similar conditions. In addition to the variation in carbon contents, the interaction causes a decrease in SiO2 (from 72–73 to 58–63 wt.%) and an increase of CaO (from 0.6–0.8 to 3–8 wt.%) contents in the infiltrating melts. Estimation of carbon budget suggests that less than 14% of released carbon was retained in the carbonated rocks in the form of calcite, whereas the rest was removed from the system through the long-distance transport of the melts or other processes.