Ischia, a volcano island located 18 miles NW offshore Naples (Southern Italy), is a densely populated active caldera (last eruption 1302 A.D.). Melt inclusions (MIs) in phenocrysts of the Vateliero and Cava Nocelle shoshonite-latite eruptive products (VI-IV century B.C.) constrain structure and nature of the Ischia deep magmatic feeding system. The volcanic rocks of Ischia bear clear evidence for CO2 dominated gas fluxing, under very oxidized conditions, and consequent dehydration of magma portions stagnating at major crustal discontinuities. The geochemical features of the studied products make Ischia a natural borehole probing the deep physico-chemical conditions of magmas generated in a mantle contaminated by slab derived fluids largely dominated by CO2.Volatile concentrations in MIs require gas-melt equilibria between 3 and 18 km depth. MI-based evidence for CO2 fluxing is further supported by selective enrichment of K and related trace elements during magma differentiation. Accordingly, the budgets of magma degassing show that at Ischia there is much less magma than that needed to directly supply the amount of released magmatic fluids, thus constraining the role of CO2 rich deep fluids in originating the volcanism and generating caldera resurgence. The acquired data, together with those from the other Neapolitan volcanoes (Procida, Campi Flegrei –CF-, and Somma-Vesuvius –SV- ) show that, despite the compositional and eruptive style differences within the poorly extended Neapolitan Volcanic area, the different kinds of volcanism are mostly linked by supercritical CO2 fluids produced by devolatilization of subducted terrigenous-carbonatic metasediment, that infiltrate the mantle wedge, generate magmas and control their ascent up to eruption. Geochemical features of Ischia and the other volcanoes reveal that the ,extent of fluid contamination of the pre- subduction asthenospheric-mantle was similar at Ischia, Procida, CF and SV. However, the different isotopic composition (more enriched in radiogenic Sr at Ischia, CF, SV with respect to Procida) and water amount in the source (almost double at Ischia, CFc and SV than at Procida), reflect the different flow-rate of deep slab-derived fluids through the mantle wedge that, in turns, controls the amount of magma generation. The high bulk permeability below Ischia, CFc and SV, determined by the intersection of regional NW-SE normal and NE-SW transfer fault systems, favours fluid upraise and accumulation at crustal levels, with consequent magma production and storage larger than at Procida, that is located along the NE-SW system.

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

MORETTI, Roberto;
2011

Abstract

Ischia, a volcano island located 18 miles NW offshore Naples (Southern Italy), is a densely populated active caldera (last eruption 1302 A.D.). Melt inclusions (MIs) in phenocrysts of the Vateliero and Cava Nocelle shoshonite-latite eruptive products (VI-IV century B.C.) constrain structure and nature of the Ischia deep magmatic feeding system. The volcanic rocks of Ischia bear clear evidence for CO2 dominated gas fluxing, under very oxidized conditions, and consequent dehydration of magma portions stagnating at major crustal discontinuities. The geochemical features of the studied products make Ischia a natural borehole probing the deep physico-chemical conditions of magmas generated in a mantle contaminated by slab derived fluids largely dominated by CO2.Volatile concentrations in MIs require gas-melt equilibria between 3 and 18 km depth. MI-based evidence for CO2 fluxing is further supported by selective enrichment of K and related trace elements during magma differentiation. Accordingly, the budgets of magma degassing show that at Ischia there is much less magma than that needed to directly supply the amount of released magmatic fluids, thus constraining the role of CO2 rich deep fluids in originating the volcanism and generating caldera resurgence. The acquired data, together with those from the other Neapolitan volcanoes (Procida, Campi Flegrei –CF-, and Somma-Vesuvius –SV- ) show that, despite the compositional and eruptive style differences within the poorly extended Neapolitan Volcanic area, the different kinds of volcanism are mostly linked by supercritical CO2 fluids produced by devolatilization of subducted terrigenous-carbonatic metasediment, that infiltrate the mantle wedge, generate magmas and control their ascent up to eruption. Geochemical features of Ischia and the other volcanoes reveal that the ,extent of fluid contamination of the pre- subduction asthenospheric-mantle was similar at Ischia, Procida, CF and SV. However, the different isotopic composition (more enriched in radiogenic Sr at Ischia, CF, SV with respect to Procida) and water amount in the source (almost double at Ischia, CFc and SV than at Procida), reflect the different flow-rate of deep slab-derived fluids through the mantle wedge that, in turns, controls the amount of magma generation. The high bulk permeability below Ischia, CFc and SV, determined by the intersection of regional NW-SE normal and NE-SW transfer fault systems, favours fluid upraise and accumulation at crustal levels, with consequent magma production and storage larger than at Procida, that is located along the NE-SW system.
File in questo prodotto:
Non ci sono file associati a questo prodotto.

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11591/211633
Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus ND
  • ???jsp.display-item.citation.isi??? ND
social impact