Pre-eruptive reservoir conditions of the peralkaline Rungwe Pumice Plinian eruption (Tanzania) from haüyne-hosted melt inclusions

Abstract The style and explosivity of volcanic eruptions are primarily governed by the interplay between conduit dynamics (e.g., magma ascent rate and volatile outgassing) and pre-eruptive magmatic conditions (e.g., P-T-X), both of which influence magma rheology and degassing processes. To better understand the controls on explosivity in peralkaline magmas (e.g., trachytes and phonolites), we investigated the pre-eruptive magmatic reservoir that fed the ~4 ka Rungwe Pumice (RP) Plinian eruption in southern Tanzania. Evolved peralkaline magmas (yielding agpaitic index 1) are typically volatile-rich (up to 8 wt.% H2O) and exhibit relatively low viscosities (e.g., ~109-10 dry rhyolite at ~900 K). Despite their rheological properties, which might suggest moderate explosivity, such magmas have produced both highly explosive and effusive eruptions across the East African Rift. To reconstruct the final stages of the RP magmatic plumbing system, we analysed haüyne-hosted melt inclusions (MIs), revealing the presence of an evolved magma body stored at shallow depths prior to eruption. Water concentrations in MIs, measured via transmitted Fourier-transform infrared spectroscopy, indicate up to ~4.8 wt.% H2O and shallow water saturation depths (~3.5 km). Our findings suggest that volatile concentration alone does not fully explain the explosive behaviour; rather, the degree of water undersaturation and conduit dynamics must also be considered as key factors influencing eruption style.