Effusive and explosive eruptions of trachytic magmas shared common pre-eruptive storage conditions and processes at Ischia volcano, Italy

Most volcanoes on Earth display a range of eruptive styles, alternating 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 historical eruptive activity of Ischia Island (Phlegraean Volcanic District, southern Italy), where trachytic magmas produced both small volume lava domes and explosive eruptions of variable intensity. We focus on three eruptions from the Montagnone volcanic complex whose activity started in the 6^th century B.C.: the Cretaio Tephra (sub-Plinian), the Bosco di Conti Tephra (sub-Plinian to lava fountaining activity), and Montagnone–Maschiata lava dome. We investigated magma storage conditions and processes through phenocrysts’ zoning patterns and compositions, including clinopyroxenes, plagioclases, alkali-feldspars, and apatites. Volatile content (H₂O, CO₂, F, Cl) was measured in clinopyroxene-hosted trachytic melt inclusions and groundmass glass. Pre-eruptive temperature, pressures, and water content were additionally calculated through clinopyroxene-liquid thermobarometry and alkali feldspar-liquid hygrometry. The phenocrysts content is low in both explosive (pumice clasts) and effusive (lava) products. The mineral assemblage is identical across the different eruptions, characterised by unzoned sanidine phenocrysts with plagioclase crystals as core, normal-zoned plagioclase crystals and oscillatory sector-zoned clinopyroxenes. We also observe the presence of sanidine microlites in the lava, whilst pumice clasts are characterised by a microlite-free groundmass. Clinopyroxene-liquid geothermometer indicates a pre-eruptive temperature of 915±18ºC. Water and CO₂ contents in melt inclusions and their solubility in the Montagnone trachytic magmas calculated using the MagmaSat model indicate minimum pre-eruptive pressures between 100 and 200 MPa. Our results suggest crystallisation of a trachytic magma as the dominant process in the magma reservoir prior to the eruption, and extraction of melt from the resulting crystal-rich environment. The three eruptions from the Montagnone complex were fed by those extracted magma batches, with low crystal content and indistinguishable pre-eruptive state from one eruption to another. Our petrological and geochemical data indicate that magma mixing or mingling with a more mafic magma prior to eruptions is unlikely. The study of Montagnone eruptions give new constraints for models of Ischia’s plumbing system, trachytic magmas existing at larger depths than previously thought. Our study highlights that trachytes can erupt effusively or explosively with similar magma reservoir conditions, and that the observed differences in eruptive styles will be induced by conduit dynamics. This must be considered for volcanic risk management in trachyte-dominated volcanic areas like Ischia or the nearby Campi Flegrei.