The campanian Ignmibrite cataclysmic eruption that segregated and discharged a huge amount of volatiles and generated extremely diluted pyroclastic density currents

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 (CFc). 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). With respect to compositionally similar but also smaller CFc eruptions (e.g. Agnano-Monte Spina, A-MS), the large amount of volatiles discharged by IC was likely due to fractional crystallization and longer residence times of volatiles sourced by the subducting Appennininc slab. This yielded high-water contents (up to 6-7 wt%), as well as high void fractions (~70-80% for IC, ~45% for A-MS) and also produced 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.