The hydrothermal system prior to a phreatic eruption: The case of the 1873 eruption at La Fossa (Vulcano Island, Italy)

Steam-driven eruptions, such as phreatic and hydrothermal explosions, likely represent some of the most frequent eruptive styles at stratovolcanoes. However, deciphering their timing, dynamics, and underlying mechanisms from eruptive deposits remains a significant challenge, even when, although such deposits are thin, altered, poorly dispersed, and strongly altered, they have not been removed by erosion. Lithic fragments from these eruptions can however preserve valuable information on lithology, pre-eruptive conditions, and the physico-chemical state of the disrupted aquifer. Here we examine the lithic fragments of the “Breccia De Fiore” deposit, emplaced by multiple explosions during a 44-day-long 1873 eruptive phase at La Fossa di Vulcano (Italy). The deposit consists of five poorly sorted lapilli tuff to tuff breccia beds, rich in hydrothermally altered lithics, lacking juvenile material. Petrographic, mineralogical, and geochemical analyses reveal a lithology dominated by silicic and advanced argillic altered particles (44–60 % vol%), together with 23–31 vol% devitrified quartz-bearing fragments hosting vapour-rich fluid inclusions–, and 7–27 vol% unaltered material. These features suggest the disruption of a shallow, acid-sulphate hydrothermal system, a rhyolitic plug occupying the shallow crater-conduit system, and unaltered adjacent lithologies. Quartz textures and fluid inclusions point to vapour-dominated conditions and explosions driven by episodic failure of sealed vapour-pockets within the shallow hydrothermal system, repeatedly recharged by magmatic gas flux. We interpret the 1873 steam-driven eruptions as a series of phreatic explosions occurring in a shallow hydrothermal system under conditions of alteration-driven permeability reduction and magmatic gas and heat input. These results underscore the role of alteration and sustained fluid supply into the hydrothermal system in driving prolonged, non-magmatic explosive activity at Vulcano.