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Abstract The onset of a large explosive volcanic eruption is generally characterized by a weak transient activity. The products of this opening phase(s) are confined to thin proximal beds of small volume and limited dispersal, and thus generally not well-studied. The juvenile material erupted in these phases provide a unique record of pre- and syn-eruptive processes and preserve key information on eruption triggering processes on the onset of magma ascent. The study of these products can provide insights into short-term precursory phenomena and assumes a key role for the interpretation of monitoring signals at high-risk volcanoes. In this work, we present a detailed study of the products from the opening phase of the Avellino Plinian eruption of Somma-Vesuvius (Italy), well preserved in proximal deposits mainly formed by two sub-units of thin pumice lapilli fallout followed by a fine ash bed. These deposits show an unusual textural and compositional variability of the juvenile material, depicting a complex process of magma extraction during the first phases of the eruption. Geochemical, textural and geobarometric data reveal the presence of a magma body at shallow depth, that cooled and partially crystallised at different conditions respect to the main, deeper, magma reservoir. Elemental diffusion chronometry on clinopyroxene from the dense juvenile material suggests that magma movement toward the shallow reservoir anticipated the eruption by a maximum of about 50 years, while the arrival of a last mafic input into the shallow magma body can be confined to only a few years before the eruption.
Falasconi et al. (Thu,) studied this question.