Magma degassing and crystallization processes during eruptions of high-risk Neapolitan-volcanoes: Evidence of common equilibrium rising processes in alkaline magmas

Compositional, textural and experimental data on products from explosive and effusive eruptions of Neapolitan volcanoes (Campi
Flegrei and Somma-Vesuvio) allow us to constrain degassing and fragmentation conditions during eruptions of alkaline magmas.
Significant differences in compositional and textural features have been recognized between lavas, scoria and pumice resulting
respectively from effusive, moderately and extremely explosive eruptions. Pumice samples have highly-vesicular glassy matrix, low
microlite number density and moderate to high water content. Crystal Size Distributions (CSD) are steep with high intercept values; the
narrow microlite size range indicates single nucleation event. Scoria products are characterized by moderate vesicularity and water
content. They have high number density of microlites which are bimodal in size. CSD show distinct inflections that are explained as two
crystal populations growing in distinct time. Lava samples generally have low vesicularities, moderate to high microcrystalline
groundmass and low glass water content. The comparison between textural and compositional features of natural rocks with samples
obtained by decompression experiments allows us to conclude that degassing processes during magma ascent occurs in near-equilibrium
conditions even at high decompression rate. Moderate to long magma rise times, calculated in the order of a few days, produce opendegassing
responsible formoderately explosive to effusive activity. Shortmagma rise times, calculated in the order of a fewhours, result in
closed-system degassing that allow explosive fragmentation when the volume of growing bubble reaches a fixed threshold. Vesicularity
and water content measured on matrix glass of pumice indicate that this process occurs at pressure of 10–30 MPa. In these conditions,
degassing, fragmentation and in turn the eruptive style is strongly influenced by initial conditions in themagma chamber (volatile content,
temperature, pressure) instead of decompression rate, in contrast with that observed for rhyolitic melts. These differences have important
consequences in terms of volcanic hazards and risk. The low-viscosity alkaline magma is able tomaintain efficient degassing even during
the final stage of magma ascent, favoring, in the case of closed-system, fragmentation and explosive activity.

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