Long-term precursors to large explosive eruptions of Vesuvius: evidence from the opening phase of the Avellino Plinian eruption

The onset of a large explosive volcanic eruption is generally characterised by a weak transient activity. The products of this opening phase(s) are confined to thin proximal beds of small volume and limited dispersal. The juvenile material erupted in the opening phase provides a unique record of pre- and syn-eruptive processes and preserves key information on eruption triggering and dynamics during magma ascent. The study of these products can provide insights into 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 as two sub-units of thin pumice lapilli fallout overlain by a fine ash bed. These deposits show textural and compositional variability of the juvenile material, depicting a complex process of magma extraction at the onset of the eruption. Geochemical, textural, and geobarometric data reveal the presence of a magma body at shallow depth that cooled and partially crystallised under different conditions compared to the main, deeper, and larger magma reservoir feeding the paroxysmal phases of the eruption. Elemental diffusion chronometry on clinopyroxene from juvenile material suggests that magma movement toward the shallow reservoir anticipated the eruption by a maximum of a hundred 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.