GNGTS 2019 - Atti del 38° Convegno Nazionale

192 GNGTS 2019 S essione 1.3 As evidenced by the studies carried out by Nicotra et al. (2018), several magmatic reservoirs constitute the plumbing system of Vulcano. The deepest storage level is located at depth of 18.5-21 km (Moho is ca. 21 km deep) and is thought to bear magma basaltic in composition. Geochemical and petrological data support the idea that this reservoir fed volcanism either at the La Fossa crater either at Vulcanello. The second reservoir (B2 in Nicotra et al., 2018) is located at 12-16.8 km of depth and could be a zoned volume of magma shoshonitic to latitic in composition. The third reservoir (B3) is shallower, being located 3.6-5.5 km beneath La Fossa cone. The last storage zone is the rhyolitic chamber (1.5-2 km of depth), which is completely exhausted from fluids. Nicotra et al. (2018) proposed that the degassing magmatic body is deeper than the nucleation depth of plagioclases. Geochemical compositions clearly suggest correspondence with the lowest storage zone characterized by basaltic compositions. The volcanism of La Fossa has been fed by basic magma stored in the lowest part of the feeding system, and it is controlled by regional tectonics. Tectonic movements control the uprising process of basic magma, leading to a reaction chain reactivating the shallower magma batches through gas flushing process, mixing and mingling of different magmatic fluids. Cintorrino et al. (2019) have modelled the shallow and continuous process of subsidence at Vulcano during the last 20 years. The average value of negative vertical movement is -10.0 mm/y to the northern area of Vulcano, whereas -4.8 mm/y to the south. Furthermore, the area between Vulcano and Lipari is affected by horizontal contraction, mostly located in the northern part of the island of Vulcano. Cintorrino et al. (2019) proposed a model (M3) in which the pressure source has been integrated with a tectonic structure (Fig. 2). The analysed features clearly point out that the overall observed surface deformation field results from the joint contribution of a regional (tectonic) and a local (magmatic) sources. However, such a deformation field as well as the geodetic network geometry of the northern sector of Vulcano is unable to adequately constrain the magmatic source. Moreover, a magmatic source placed beneath the Vulcanello cone should be the most plausible. Fig. 2 - Sketch model of the inferred shallow portion (< 5 km b.s.l.) of the present plumbing system configuration at Vulcano island (source: Cintorrino et al. , 2019). As reported by Mandarano et al. (2016), the 3 He/ 4 He ratio in fluid inclusions at Vulcano shows lower values than those of other Italian volcanoes (e.g., Etna, Stromboli, Vesuvius). This ad been described as a process of crustal assimilation from a deeper magmatic source of fluids (basaltic to shoshonitic in composition), which is able to reactivate the shallower magmatic reservoirs that undergo to gas flushing processes, changing the original 3 He/ 4 He ratio. As evidenced by the comparison between He and Sr ratios, there is a negative correlation between He and Sr, as evidence of a crustal assimilation process ongoing through ascent of deeper fluids. As previously evidenced, we can conclude that the positive deformations recorded during the 1988 unrest can be interpreted as a cause/effect relationship between a source of deep warm fluids (triggered by regional tectonic phenomena), which has in turn led to the genesis of seismic swarms of low magnitudes and at shallow depths due to effects of new fracture opening