The late Pleistocene trachytic Campanian Ignimbrite (CI; 300 km3 DRE) covers the Campanian Plain near Naples, and is found behind ridges more than 1,000 m high at 80 km from source, the Campi Flegrei caldera. Distal ignimbrite deposits reveal downhill and/or downvalley flow directions prior to deposition, whereas in the absence of significant topography, deposition came from a flow moving in a roughly radial direction. These features point to very dilute currents, that together with the huge amount of discharged magmatic material, suggest a magma reservoir highly enriched in volatiles, rather than fluid entrainment from hydrothermal bodies or seawater. Petrologic and geochemical modelling of erupted products and their chemical and textural zoning, together with MI-based studies of gas-melt saturation, corroborate this view and show that the CI huge volume differentiated and mixed at shallow depth (6-3 km). The progress of crystallization yielded high-water contents (up to 6-7 wt%), thus producing an overpressurized CO2-dominated gas cap (about 150 km3), uniformly distributed at the top of the magma chamber. The onset of the eruption tapped this cap, with consequent depressurization and fast volume decrease that facilitated or even drove the caldera collapse, and allowed the water-rich magma to be discharged during the pyroclastic current phase. The gas saturation-based estimates of the tapped foamy magma are compatible with the extent of magma chamber roof collapse, strong expansion revealed by textural data and transport and deposition mechanisms, reflecting depressurization and magma inflation within the collapsed and laterally confined caldera.
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