Evolution of inorganic and organic carbon in the slab during subduction indicated by continental volcanic rock

Continental volcanic rocks provide critical insights into carbon recycling processes within the mantle transition zone. Here I present the carbon isotope compositions of carbonates in the Cenozoic continental volcanic rocks from Eastern China. δ¹³C of carbonates in these rocks, which represent carbon isotope composition of the involved magma before carbonate precipitation, vary from −15.0 ‰ to −2.4 ‰. After the correction of magma degassing assuming the initial CO₂ content of the magma is 1500 ppm, the initial δ¹³C of the magma is between −13.6 ‰ to +6.2 ‰. Combining with degassing corrected carbon isotope compositions and whole rock trace element and SrNd isotope compositions, these rocks can be divided into two groups, which includes samples with δ¹³C higher than DM and within or lower than DM. The distinction of carbon isotope compositions for the two groups can be attributed to the different proportions of high δ¹³C inorganic carbon and low δ¹³C organic carbon in the mantle sources. δ¹³C of these continental volcanic rocks are lower than arc emissions. This marks the consumption of inorganic carbon in the slab during decarbonation at sub-arc depth. The petrogenesis of continental volcanic rocks is related to the decarbonation in the mantle transition zone. However, δ¹³C of these rocks are comparable with OIB. This observation indicates the proportions of inorganic and organic carbon in the slab may not be significantly changed after decarbonation in the mantle transition zone.