The intense explosive and effusive volcanic activity of the last 1000 years at La Fossa volcano (Vulcano Island, Italy) was characterized by the eruption of magmas ranging in composition from latites to trachytes and rhyolites, as well as K-rich trachytes. Evidence of syn-eruptive mixing among these magmas is frequently observed in the form of magmatic enclaves and bands in lava flows and pyroclastic products. The petrological and volcanological diversity of the erupted materials suggests that complex differentiation processes occurred in the shallow part of the plumbing system. With the aim to reconstruct the magmatic feeding system and to identify the differentiation processes behind such a petrologic complexity, we analysed lavas and pyroclastic products representative of the recent eruptive sequences at La Fossa and combined the petrochemical features with thermo-barometric calculations, geochemical modelling and temperature gradient experiments. Thermo-barometric calculations indicate that the K-rich trachytic magma crystallized at lower pressure (160 ± 54 MPa) compared to the latitic (307 ± 47 MPa) and trachytic (208 ± 30 MPa) magmas. Differentiation modelling suggests that both trachytic and rhyolitic compositions can be obtained through differentiation of a common latitic magma, essentially by varying the plagioclase/ sanidine ratio. Temperature gradient experiments, performed at the conditions inferred for the shallow plumbing system of La Fossa volcano (150 MPa and 1050-900 °C), indicate different paths of melt differentiation that overall produce an increase of the SiO 2 /K 2 O ratio with the increasing H 2 O in the system (from 0 to 4 wt.%). This is consistent with the origin of K-rich trachytes at lower pressure and lower H 2 O content. In turn, the formation of crystal-poor rhyolites is explained by the segregation of the interstitial melt formed in a latitic-trachytic crystal mush, favoured by the second boiling of the melt and consequent exsolution of a fluid phase.

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The most frequent volcanic eruptions are of low-intensity and small magnitude. They produce abundant ash-sized (< 2 mm) clasts, which are too small to establish quantitative links between magmatic processes and eruptive dynamics using classic approaches. This inhibits our ability to study the past behaviour of frequently erupting volcanoes, essential to predict their future activity and mitigate their impact. The Palizzi unit (10–13th century, Vulcano, Italy) includes a prototype sequence of ash deposits resulting from prolonged Vulcanian eruptions punctuated by those of two larger sub-Plinian events. We apply Hierarchical Clustering to chemical analyses of clinopyroxene collected along the stratigraphy to decipher magma dynamics during this eruptive period. We identify periods of magma input and we link deep magmatic processes to eruptive dynamics, also showing that our approach can be used to connect magma and eruptive dynamics in any volcanic sequence. This is essential to track the processes occurring during frequent eruptions and to identify the build-up to larger explosive events.

The eruptive products of the last 1000 years at La Fossa volcano on the island of Vulcano (Italy) are characterized by abrupt changes of chemical composition that span from latite to rhyolite. The wide variety of textural features of these products has given rise to several petrological models dealing with the mingling/mixing processes involving mafic-intermediate and rhyolitic magmas. In this paper, we use published whole-rock data for the erupted products of La Fossa and combine them in geochemical and thermodynamic modelling in order to provide new constrains for the interpretations of the dynamics of the active magmatic system. The obtained results allow us to picture a polybaric plumbing system characterized by multiple magma reservoirs and related crystal mushes, formed from time to time during the differentiation of shoshonitic magmas, to produce latites, trachytes and rhyolites. The residing crystal mushes are periodically perturbated by new, fresh magma injections that, on one hand, induce the partial melting of the mush and, on the other hand, favor the extraction of highly differentiated interstitial melts. The subsequent mixing and mingling of mush-derived melts ultimately determine the formation of magmas erupted at La Fossa, whose textural and chemical features are otherwise not explained by simple assimilation and fractional crystallization models. In such a system, the compositional variability of the erupted products reflects the complexity of the physical and chemical interactions among recharging magmas and the crystal mushes.

The mineralization potential of arc magmas depends, among other factors, on the timing of sulfide melt saturation relative to magma differentiation and to exsolution of a magmatic fluid phase. In fossil mineralized or barren systems, understanding the evolution of metals along the magma differentiation path is often hindered by late magmatic processes and hydrothermal alteration. To better understand the process of metal evolution "caught in the act" in crustal reservoirs, we analyzed magmatic sulfides and melt inclusions found within eruptive products from the active arc volcano, La Fossa (Vulcano Island, Italy), for the basalt to rhyolite compositional spectrum. We found that, in case of sulfide-undersaturated and volatile-rich arc basalts, metals are scarcely subtracted by degassing during ascent to shallow crustal reservoirs and reach the highest abundances in intermediate magmas (250 ppm Cu). At sulfide saturation the sulfide melt has 34-66 wt% Cu, leading to a dramatic decrease in chalcophile metals dissolved in the silicate melt. After fractionation of only 0.2-0.3 wt% of sulfide in the solid assemblage, the exsolved sulfide is a monosulfide solid solution (pyrrhotite) containing <3 wt% Cu. Metals that do not partition in sulfides (Pb, Zn) increase their concentrations during magmatic evolution until they are sequestered by a Cl-rich aqueous fluid phase exsolved at the rhyolitic stage. The absolute and Cu-normalized concentrations of metals in sulfide inclusions are similar to sulfide accessories in magmatic rocks associated with world-class porphyry Cu systems. Our results demonstrate that the mechanisms governing metal evolution inferred for the magmatic stage in porphyry Cu environments can be also tracked at an active arc volcano, using eruptive products as snapshots of the magmatic evolution. Arc volcanoes can thus be viewed as ideal active analogues when studying these crucial processes for the formation of porphyry Cu deposits.

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