Dynamics of pre-eruptive magmatic processes: evidences from volatiles

Comprehending the dynamics of pre-eruptive magmatic processes (e.g., magma residence, recharge events, final ascent to the surface) acting within a magmatic plumbing system is one of the primary challenges for volcano hazard assessment. These information can be revealed through multidisciplinary approaches, combining: (i) the measurements of volatiles (H2O, CO2, S, Cl) in melt inclusions, (ii) the definition of compositional zoning in phenocrysts, which can record the time elapsed at different magmatic conditions (T-P-X-fO2), and (iii) the instrumental geochemical monitoring of volcanic gases, which provide real-time information about the state of a volcano and its eruptive potential. This thesis focuses on Stromboli and Vulcano Islands (Aeolian Archipelago), each of these two volcanic systems being characterized by a different current state: Stromboli is an active, open-system volcano with a variable explosive activity, from mild to moderate Strombolian explosions, occurring regularly, to sudden and more energetic explosions, ranging from the major explosions to the more violent and rare paroxysm; Vulcano is instead a dormant, closed system volcano, whose last eruption occurred on 1880-90 and, since then, has been in a frequent state of volcanic unrest without culminating into new eruptions.Distinct objectives were defined for each case of study: for Stromboli (i) constraining the magma source, triggering mechanisms and timescales of the still not well-known major explosions, taking into account three events occurred on 3 May, 8 November and 24 November 2009; (ii) define and compare the magma source and trigger of the recent 11 July 2024 paroxysm with respect to the past paroxysmal activity; for Vulcano (iii) the focus was on the potential volatile source feeding the ongoing 2021-2023 hydrothermal unrest, using La Sommata basaltic eruption as a proxy. Olivine-hosted melt inclusions revealed that major explosions at Stromboli are fed by a more degassed, shallower (depth < 1 km below sea level; b.s.l.) magma source with respect to paroxysms depth (7-11 km b.s.l.), establishing that distinct triggering mechanisms explain major explosions characterized by different eruptive magnitude/intensity. The injection of variable amounts of volatile-rich, low-porphyritic (LP) magma and gas from depth was found to trigger the higher-intensity 24 November major explosion and (to a minor extent) the 8 November event, over timescales from hours to weeks. The lower-intensity 3 May event was instead triggered, as suggested by gas monitoring data, by the overpressurizzation of the shallow HP reservoir, induced by gas bubbles accumulation at some discontinuity within the magmatic plumbing system.The more violent 11 July 2024 paroxysmal explosion was preceded by an intense eruptive activity, including a major explosion and eight days of lava effusion. A deep mafic recharge was suggested to be responsible for the precursory escalation in the CO2 fluxes recorded in the months before the onset of the eruption. While, the top-down depressurization of the deep LP magma, induced by lava drainage, forced its ascent from the 5-6 km b.s.l. magma ponding zone (top of the deep magmatic system) to the shallower (2-4 km b.s.l.) levels, destabilizing the stagnant magma, and thus triggering the paroxysm.As regards Vulcano Island, high T-P rehomogenization experiments of melt inclusions revealed that La Sommata basaltic eruption contains moderate to high CO2 contents, in contrast to the low CO2 contents previously reported (< 200 ppm; Rose-Koga et al., 2012). Magma degassing models predicts a CO2-dominated free fluid phase co-existing with magma at rather high pressure, which could explain the CO2-rich and δ13C-CO2 isotopic composition with typical deep magmatic signature released from fumarolic gases during the 2021-2023 unrest. These findings support the injection of volatile-rich mafic magma (with composition similar to La Sommata magma) as a potential trigger of degassing unrest at Vulcano.