Constraints on magma source, pre-eruptive dynamics and timescales of the 2009 major explosions at Stromboli volcano (Italy)

Stromboli is one of the best studied and monitored open-vent volcanoes worldwide, but still our understanding of the magma source, triggering mechanisms and ascent timescales of larger explosive events than the normal Strombolian activity remains incomplete. Within this variability, major explosions are relatively frequent (1-4 events/year) explosive eruptions of intermediate intensity between the recurrent (10 events/hour) and low-intensity regular Strombolian eruption and the more violent and rarer eruptive paroxysms (with the most recent occurred in 2003, 2007 and 2019). While regular explosions are fed by a crystal-rich (HP, high porphyricity) magma residing in a degassed shallow reservoir (< 3 km), and paroxysms by volatile-rich (LP, low porphyricity) magma rapidly ascending from the deep portions of the plumbing system (6-9 km b.s.l. down to the crust-mantle interface), much less is known about the magma source of major explosions. Here we present a comprehensive dataset on dissolved volatile contents (H2O, CO2, S and Cl) in olivine-hosted melt inclusions (MI), embayments and glassy matrix, coupled to major element compositions and Fe-Mg diffusion profiles in olivine phenocrysts from three major explosions which occurred on 3 May, 8 November and 24 November 2009. Each of these events consisted of a brief explosive sequence that erupted coarser and more widely dispersed tephra than ordinary eruptive activity, with the 24 November 2009 event being the most energetic one. Olivine-hosted melt inclusions from the 24 November 2009 event have dissolved H2O ranging from ca. 0.22 to 2.37 wt% and CO2 generally below the detection limit, with a maximum recorded of 576 ppm. Major elements composition of melt inclusions varies from the compositional field of HP products toward the LP compositional field. Two distinct compositional clusters (one overlapping with the HP compositional field and one overlapping the LP one) are observed in the glass wetting the phenocrysts, with no intermediate compositions. Excluding the rare occurrence of homogeneous evolved olivines (Fo70), the majority are variably reversely zoned, with Mg-richer (Fo77-83) reaction rims (50-100 µm thick). Melt inclusions from the 8 November 2009 event have H2O ranging from ca. 0.15 to 0.55 wt% and CO2 always below the detection limit. Melt inclusions from the 3 May 2009 event have H2O ranging from ca. 0.15 to 0.75 wt% and CO2 generally below the detection limit and max. 291 ppm, major elements composition overlaps the HP compositional field. The host olivines are compositionally homogeneous and evolved (Fo70-72). Volatile contents of MI (EMPA and FTIR) suggest that these explosions are fed by more degassed, shallow stored magma if compared to paroxysms. Preliminary data suggest a positive correlation between eruption magnitude and volatile content. Volatile elements are higher in the most energetic major explosion of 24 November 2009, which erupted larger volumes of coarser pyroclasts of both HP and LP magma that eventually reached the coast of the island. In contrast, the 8 November and 3 May events ejected a smaller volume of pyroclasts (mostly confined to the summit area of the volcano) with a minor LP component, suggesting a shallower source origin.