Tracing the pre- to post-eruptive crystallization of trachybasaltic melts: insights into the 1651–1654 CE lavas of Mount Etna (Sicily, Italy)

Understanding the crystallization of silicate melts is key to reconstructing the processes occurring from magma rising to lava emplacement, the latter representing a major hazard for human settlements during effusive eruptions. Crystal growth, along with melt H₂O degassing, strongly influences lava rheology and surface flow behaviour. This study investigates the pre- to post-eruptive crystallization dynamics of trachybasaltic melts from the 1651–1654 CE eruption on Mount Etna's western flank (Sicily, Italy), one of the 17th century's most significant events due to its duration, lava field extent, and reach into inhabited areas. Investigation on different layers of a fractured pressure ridge allowed to reconstruct the crystallization history of a single flow unit, revealing significant textural differences between the inner and outer (crust) portion of the lava, allowing to quantify the extent of crystallization at subaerial conditions. By combining 2D and 3D textural analyses with chemical and mineralogical investigations, the pre-eruptive pressure-temperature (P–T) conditions of crystal formation were constrained. Phenocrysts nucleated in a vertically extended feeding system (down to 23 km below the sea level) at almost stationary condition of T = 1070–1060 °C. In the glass-rich crust, detailed chemical and textural analyses revealed chemical boundary layers around plagioclase microlites, which was used to model a subaerial growth rate of the outermost plagioclase rim in the order of 0.2–4.5 μm/s. These findings enhance our understanding of lava behaviour during flow, offering key insights for improving hazard models, monitoring, and response during effusive volcanic events similar to the 1651–1654 CE eruption.