Lorenzo Cappelli, Paul A Wallace, Evelyne Mbede, Shimba Kwelwa, Edista Abdallah, Gerald G.J. Ernst, Karen Fontijn

The style and explosivity of volcanic eruptions are primarily influenced by interrelated factors: conduit dynamics (such as magma ascent rate and degassing efficiency) and pre-eruptive magmatic conditions, both of which control the magma's rheological behaviour. In the case of highly alkaline magmas (i.e., agpaitic index > 1), the depolymerisation of silica bonds exerted by alkaline elements promotes a relatively low-viscosity rheological response for a given temperature and therefore theoretically less explosive eruptive behaviour, even for silica-rich magmas. However, several well-studied eruptions show that peralkaline magmas (e.g., trachytes or phonolites) can experience highly explosive Plinian events, suggesting other parameters influence eruption style. In the East African Rift, several such volcanoes with peralkaline magma compositions have erupted both explosively and effusively in the past. We investigated the pre-eruptive magmatic system of the Plinian eruption that produced the Rungwe Pumice (RP) deposit in southern Tanzania. The late chemical evolution of the plumbing system was modelled by analysing haüyne-hosted melt inclusions (MIs) which suggests that an evolved magmatic body of deeper origin was stored at shallow depths shortly before the eruption. Water concentrations in the MIs, measured using transmitted Fourier-transformed infrared spectroscopy, revealed water-poor conditions (e.g., 2-3 wt.%) and shallow depths (2-5 km). Thus, volatile concentration alone is not sufficient to explain the explosive behaviour, but the degree of water undersaturation and conduit dynamics must also be considered.