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Carbon fluxes from subducted carbonates revealed by uranium excess at Mount Vesuvius, Italy

R. Avanzinelli,M. Casalini,T. Elliott,S. Conticelli

Published 2018 in Geology

ABSTRACT

The fate of carbonate-rich sediments recycled at destructive plate margins is a key issue for constraining the budget of deep CO 2 supplied to the atmosphere by volcanism. Experimental studies have demonstrated that metasomatic melts can be generated by partial melting of subducted carbonate-pelitic sediments, but signatures of the involvement of such components in erupted magmas are more elusive. We have made new U-Th disequilibria, Sr-Nd-Pb isotope, and high-precision δ 238 U analyses on lavas from Mount Vesuvius (Italy) and show that their measured 238 U 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. Accordingly, we argue that the occurrence of 238 U excesses in “sediment-dominated” arc magmas represents diagnostic evidence of addition of carbonate sediments via subduction, hence providing constraints on deep carbon cycling within Earth. Our quantitative enrichment model, combined with published experimental results, allows us to estimate a resulting flux of 0.15–0.8 Mt/yr CO 2 from the subducted carbonates to the mantle source of Mount Vesuvius.

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