Crystal clots record stable mush conditions and rapid pre-eruptive heating beneath Unzen volcano (1991–1995)

The condition of a crystal-rich, mushy magma reservoir plays a crucial role in governing both the processes leading up to volcanic eruptions and the style of eruption that follows. However, constraining its pre-eruptive state remains a significant challenge. In this study, we use crystal clots as a petrological tool to investigate the thermal and compositional evolution of the mushy magma reservoir beneath Unzen volcano prior to the 1991–1995 eruption. We analyzed the major-element compositions of crystals and interstitial glass within the crystal clots, with particular focus on amphibole and plagioclase, to estimate the depth of formation, temperature, and chemical composition of the magma reservoir. Our results indicate that the physicochemical conditions of the mushy reservoir remained largely stable throughout the eruption sequence. Furthermore, textural evidence of amphibole breakdown suggests that heating of the reservoir prior to magma mixing occurred on a timescale of approximately between > 24 h and <  ~ 48 h.