New views on Somma Vesuvius subvolcanic system and on mechanism that could increase eruption explosivity by a review and immiscibility in melt inclusions

Somma-Vesuvius (SV) is an active volcanic system in southern Italy that has generated Plinian eruptions during its history as the infamous 79 CE (Pompeii) eruption. Plinian eruptions produced airfall pumice deposits characterized by white pumice clasts in the lower half part, and by grey pumice clasts in the other upper half part. Plinian pyroclastic deposits are also characterized by the presence of skarn xenoliths. Previous studies on the fluid (FI) and melt inclusions (MI) in the Somma-Vesuvius (SV) skarn xenoliths have widely documented immiscibility between silicate melt and hydrosaline melt (±aqueous chloride-rich liquid‑carbonate/sulfate melt). In this study, we recognize a new type of MI namely composite melt inclusions (CMI) hosted in minerals of SV skarn xenoliths previously studied in the literature. In addition to a review of studies about skarn at SV, we present new microthermometric observations of CMI based on heating experiments using a heating stage. We heated CMI to complete homogenization at 1080 °C, and we cooled the CMI to simulate a natural sequence of phase appearances during magma cooling. At T around 1060 °C the CMI unmixes forming two different silicate melts. Upon further cooling at around 700 °C, the CMI show droplets of chloride immiscible liquids instantly nucleating. The cooling experiments of CMI from high-T are assumed to reproduce on a small scale the sequence of magma cooling prior to eruption below SV. In this case, magma could unmix before eruption forming two different silicate melts producing a chemically stratified reservoir. In support of our heating experiments results of CMI, the reprocessing of SV bulk rock compositions data highlights that magma feeding interplinian eruptions is Fe-and P-rich and Si-poor. In contrast, magma feeding Plinian eruptions is Si-rich and Fe-and P-poor. As experimental studies have demonstrated, immiscibility can occur when one melt is Fe-and P-rich and Si-poor, and the other is Si-rich and Fe-and P-poor and the transition between the two end members is not gradual, resulting in a gap of homogeneous intermediate melt.