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

Abstract

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.This study presents compelling evidence for the petrological transition from carbon-free to carbon-bearing silicate melts by melt-marble interaction and therefore provides a new mechanism for the enrichment of carbon in slab-derived melts and an important pathway for the massive extraction of carbon from subducted slabs. Addition of CO2 and reduction of SiO2 as the interaction progress results in a lower viscosity and larger buoyancy of the reacted melts compared to the unreacted felsic melts, which facilitates the rapid transport and rising of these carbonated melts and thus helps a more efficient cycling of carbon in Earth's interior.