Magma degassing and eruption dynamics of the Avellino pumice Plinian eruption of Somma–Vesuvius (Italy). Comparison with the Pompeii eruption

The eruptive history of Mt. Somma–Vesuvius is characterised by large explosive events: Pomici di Base eruption
(22,030±175 yr cal BP), Mercato (8890±90 yr cal BP), Avellino (3945±10 yr cal BP) and Pompeii
(79 AD). Pre-eruptive conditions and sin-eruptive degassing processes of the Avellino eruption, the
highest-magnitude Plinian event, have been investigated, using volatile contents (F, Cl, H2O) in melt inclusions
and residual glass, and textural characteristics of pumice clasts of the 9 fallout layers sampled in detail
in a representative sequence. The sequence displays an up-section sharp colour change from white to grey,
corresponding to variations in both magma composition and textural characteristics. The pre-eruptive conditions
have been constrained by systematic measurements of Cl content in both melt inclusions and matrix
glass of pumice clasts. The pumice glass composition varies from Na-rich phonolite (white pumice) to Krich
phonolite (grey pumice). The measured Cl values constantly cluster at 5200±400 ppm (buffer value),
whatever the composition of the melt, suggesting that the entire magma body was saturated with subcritical
fluids. This Cl saturation constrains the pre-eruptive pressures and maximum H2O contents at
200±10 MPa and 6.3±0.2 wt.% H2O for the white pumice melt and 195±15 MPa and 5.2±0.2 wt.% H2O
for the grey pumice melt. The fluid phase, mainly composed of a H2O-rich vapour phase and brine, probably
accumulated at the top of the reservoir and generated an overpressure able to trigger the onset of the eruption.
Magma degassing was rather homogeneous for the white and grey eruptive units, mostly occurring
through closed-system processes, leading to a typical Plinian eruptive style. A steady-state withdrawal of
an H2O-saturated magma may explain the establishment of a sustained Plinian column. Variation from
white to grey pumice is accompanied by decrease of mean vesicularity and increase of mean microcrystallinity
and permeability related to significant vesicle coalescence. Despite this, the ascending magma column still
evolves under closed-system degassing, without significant gas loss through conduit walls. The Avellino
eruption shows numerous similarities with the 79 AD Pompeii eruption in pre-eruptive conditions, degassing
processes and eruptive style which are discussed here.