The plumbing system of the Ischia island: A physico-chemical window on the fluid-saturated and CO2- sustained Neapolitan volcanism (Southern Italy)

Ischia, a small island located 18 miles NW offshore Naples (Southern Italy), is a densely populated active caldera (last eruption 1302 A.D.). Melt inclusions in phenocrysts of poorly differentiated eruptive products constrain structure and nature of the Ischia deep magmatic feeding system. Volcanic products bear clear evidence for CO2-dominated gas fluxing, under very oxidized conditions, and CO2 enrichment in magma portions stagnating at major crustal discontinuities. Volatile concentrations require gas-melt equilibria between 3 and 18 km depth. At Ischia there is much less magma than that needed to directly supply the amount of released magmatic fluid. Comparison with data from the other nearby Neapolitan volcanoes (Procida, Campi Flegrei –CF-, and Somma- Vesuvius –SV-) highlights the pivotal role of deep fluids in originating the volcanism. Despite the compositional and eruptive style differences observed within the small extension of the Neapolitan Volcanic District, and the variable occurrence of mixing, crustal assimilation and fractional crystallization, the different kinds of volcanism are mostly linked by supercritical CO2 fluids produced by the devolatilization of subducted terrigenous-carbonatic metasediments. Geochemical and isotopic differences among Ischia, CF and SV from one side, and Procida from the other one, reflect the tectonically controlled slab-derived fluids release and upraise trough the mantle wedge, that, in turns, control magma generation.