Deep recycling of carbonate-rich sediments in the mantle source of Vesuvius volcano

The fate of carbonate-rich sediments recycled at destructive plate margins in a key issue for constraining the budget of deep CO2 supplied to the atmosphere by volcanism. Experimental studies have demonstrated that carbonate-like metasomatic liquids (fluids and/or melts) can be generated during subduction by decarbonation and partial melting of carbonate-pelitic sediments (e.g., Poli, 2015). These liquids may modify the mantle sources of subduction-related magmas that eventually recycle carbon and other elements back to the surface thorough magmatism. Evidence of such a process in erupted magmas is, however, elusive and much debated. Magmas erupted at Vesuvius volcano show geochemical evidence of involvement with carbonate-rich lithologies, either deeply subjected and recycled into the mantle (Avanzinelli et al., 2008) or acquired through shallow level contamination with limestone (Iacono Marziano et al., 2009). Here we present new U-Th disequilibria data couples with high precision 238U on Vesuvius lavas, demonstrating that the 238U-excess of Vesuvius lavas is anomalous with respect to other subduction-related magmas and it requires a deep addition of U-rich carbonated melts generated by partial melting of calcareous sediments in the presence of residual epidote (Skora et al., 2015). The combined used of U-series disequilibria and 238U also shown that the isotope composition of Vesuvius lavas cannot be explained by shallow level crustal contamination. Accordingly, we argue that the occurrence of 238U-excess in trace element-enriched, "sediment-dominated" arc magmas, represent a unique evidence of deep recycling of carbonate sediments, hence providing constraints on the deep carbon cycling of Earth.