Similar pre-eruptive conditions behind contrasting explosive and effusive trachytic eruptions at Montagnone (Ischia volcano, Italy)

Abstract Most volcanoes on Earth display a range of eruptive styles, varying between effusive and explosive activity. Understanding the factors controlling these transitions is critical for volcanic hazard assessment but remains debated. While both pre-eruptive storage conditions and syn-eruptive conduit processes influence eruption style, their relative roles are often difficult to disentangle, particularly in alkaline volcanic systems. Here we investigate the transition between effusive and explosive behaviour recorded in the eruptive activity of the polygenetic Montagnone edifice on Ischia Island (Phlegrean Volcanic District, southern Italy), where trachytic magmas produced both lava domes and explosive eruptions of variable intensity. Magma storage conditions and pre-eruptive processes were investigated through feldspar and clinopyroxene zoning, apatite inclusions, and volatile content of melt inclusions. Phenocrysts assemblage and intracrystalline cation distribution exhibit only minor compositional variability among and within each eruption. They indicate a magmatic environment dominated by crystallisation and intermittent resorption, with minimal interaction with less evolved recharge magmas prior to eruption. Pre-eruptive temperatures and volatile contents are indistinguishable across events despite their contrasting eruptive styles. These results suggest that variations in eruption dynamics at Ischia were not primarily governed by pre-eruptive storage conditions. Instead, we propose that variations of the ascent dynamics, suggested by differences in groundmass crystallisation, have played the key role in eruptive style control. This study highlights the need to integrate the study of both pre-eruptive and syn-eruptive processes when assessing eruption style transitions in low-crystallinity alkaline magmatic systems.