Magma evolution at La Fossa volcano (Vulcano Island, Italy) in the last 1000 years: evidence from eruptive products and temperature gradient experiments

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 petro-chemical 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 SiO2/K2O ratio with the increasing H2O in the system (from 0 to 4 wt.%). This is consistent with the origin of K-rich trachytes at lower pressure and lower H2O 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.