The active plumbing system of La Fossa (Vulcano, Italy): clues to mafic-silicic magma interactions and the link with the magmatic-hydrothermal environment

La Fossa is the active volcanic center of Vulcano Island, the southernmost of Aeolian Islands (Italy). Although the volcano has been the site of many volcanological, petrological and geochemical studies since decades, some questions are still debated. Examples are the genetic relationships between the mafic and rhyolitic magmas, often involved in the same eruptions, and the depth of their storage. An additional limitation to the present knowledge comes from the fact that much of the deposits of La Fossa are ash-sized and, therefore, explosive phases of La Fossa volcanic history are less studied by a petrological point of view with respect to lava and coarse pyroclastic material, exception made for the famous AD 1888–90 last eruption. Further interest in this volcano comes from the fact that La Fossa hosts an acid-sulfate hydrothermal system which has been involved in phreatic eruptions, whose fine ashes are enriched in metals carried by hydrothermal fluids, and metal-bearing sulfide accessories have been found in some recent products. Thus, the plumbing system of this volcano may be an ideal site where studying the evolution of chalcophile metal concentration in magmas. In this PhD thesis, a study integrating petrography, whole-rock geochemistry, compositional analyses of mineral phases, melt inclusions and high pressure high temperature experimental petrology has been carried out on the products representative of the whole magmatic differentiation path (from basalt to rhyolite) of Vulcano, mostly focusing on the explosive products of the last 1000 years erupted at La Fossa. The main aim was to improve the knowledge of the plumbing system of La Fossa by defining the genetic relationships between the mafic and felsic magmas and their crystallization conditions, modeling their geochemical evolution including the concentration of chalcophile metals, and identifying an appropriate approach for reconstructing magma dynamics along the ash-dominated pyroclastic sequences. Whole-rock major and trace element analyses, major and trace element compositions of mineral and glass phases of natural and experimental products (temperature gradients experiments that simulate differentiation in a thermal-zoned reservoir), together with thermobarometric calculations and geochemical modelling allowed to depict the plumbing system of La Fossa as a complex polybaric system where rhyolitic magmas can be generated by extraction from crystal mush regions. The partial melting of the crystal mush by mafic magma inputs is accountable for the chemical and textural variability of the erupted intermediate and evolved products including the genesis of K- and Ba-rich trachytic magma. The stratigraphic record of La Fossa is mostly characterized by ash-sized deposits, originated by long-lasting low-intensity eruptive phases, that have not been fully employed to gain insights into La Fossa plumbing system dynamics. In this thesis, clinopyroxene phenocrysts were analyzed along the complex explosive sequence of the Palizzi Eruptive Unit (12th-13th century) that represents one of the most important eruptive period of the last 1000 years activity at La Fossa. Major element core to rim chemical profiles of clinopyroxene phenocrysts have been employed to perform a hierarchical cluster analysis that allows to recognize four chemical clusters. The compositional differences between the clusters have been reconciled with variations in the degree of undercooling and in the silicate melt composition towards both a mafic and an evolved end-members. The clusters distribution in the different portions of the crystals (cores, mantles and rims) and thermobarometric calculations, allowed to recognize several cycles of mafic magma input followed by ascent and eruption. The polybaric nature of the plumbing system together with the evidence of sulfide-silicate melt immiscibility in intermediate and evolved products erupted at La Fossa allowed to investigate the evolution of chalcophile metal concentration in crustal magma reservoirs, thus shedding light on mechanisms at the base of mineralization processes beneath arc volcanoes. Indeed, 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 this thesis, the major and trace element analyses of silicate melt and sulfide inclusions, for the basalt to rhyolite compositional spectrum, have been carried out to understand the evolution of chalcophile metals in the plumbing system and to give insights into magma fertility potential. The obtained results suggest that in case of sulfide-undersaturated arc basalts, metals reach the highest abundances in intermediate lati-trachytic magmas. At the point of sulfide saturation the chalcophile metal contents in the silicate melt dramatic decrease after fractionation of only 0.2-0.3 wt.% of sulfide in the solid assemblage. In this thesis the whole-rock Platinum-group elements (PGE) concentrations of the Vulcano magmatic suite have been measured with a Ni-sulfide fire assay method. The PGE are sensitive indicators of sulfide saturation because of their very high partition coefficients into a sulfide phase. The decrease in PGE constrained the onset of the sulfide saturation during magma differentiation at ~ 4 wt.% MgO; by this point magma evolution takes place under sulfide-saturated conditions. The obtained results, together with the high content of Pd comparable to other arc- related igneous suites associated with mineralized deposits, suggest that magmatic evolution in crustal magma reservoirs at an arc-related magmatic system similar to La Fossa, fed by sulfide-undersaturated shoshonitic basalts, has the potential to produce fertile magmas.