Carbon fluxes from subducted carbonates revealed by U isotopes at Vesuvius

The fate of carbonate-rich sediments recycled at destructive plate margins is a key issue for constraining the budget of deep CO2 supplied to the atmosphere by volcanism. Experimental studies have demonstrated that metasomatic fluids/melts can be generated by decarbonation or partial melting of subducted carbonate-pelitic sediments, but signatures of the involvement of such components in erupted magmas are more elusive. We performed new U-Th disequilibria and high precision δ238U analyses on Vesuvius lavas showing that the measured 238U-excesses require a mantle source affected by the addition of U-rich carbonated melts generated by partial melting of subducted calcareous sediments in the presence of residual epidote. The presence of 238U-excess is anomalous in “sediment-dominated” arc magmas, but it is not unique of Vesuvius volcanoes, with similar characteristic occurring for example in the Lesser Antilles or Sunda arc. Accordingly, we argue that the occurrence of 238U-excesses in 'sediment-dominated' arc magmas represents unique evidence of addition of carbonate sediments via subduction, hence providing constraints on the deep carbon cycling within Earth. In addition, we combined a quantitative mantle enrichment model with published experimental results, in order to provide an estimate of the flux of CO2 resulting from the subducted carbonates to the mantle source of the Vesuvius and eventually back to the surface, yielding values ranging between 0.2 and 0.8 Mt/y.