PETROLOGICAL AND EXPERIMENTAL CONSTRAINTS OF THE PRE-ERUPTIVE CONDITIONS OF LA SOMMATA BASALT AND BROWN TUFFS TEPHRA (AEOLIAN ISLANDS)

The principal focus of this thesis was the reconstruction of pre-eruptive conditions of two key eruptions at Vulcano island, the Brown Tuffs and the La Sommata, by combining the melt inclusion approach with phase equilibria derived by crystallization experiments. Brown Tuffs (the most energetic and widely distributed eruption of the Archipelago) were produced by several eruptions that occurred in a large age interval, 78-7.7 ka B.P. At Vulcano BT deposits crop out as proximal facies. La Sommata centre (age ca. 50 ka B.P.), is instead a basaltic scoria cone fed by one of the most primitive magmas of Aeolian Arc with ankaramitic affinity. Pyroxene-hosted melt inclusions on Lower and Intermediate Brown Tuffs (78- 22 ka) gave dissolved H2O and CO2 of 0.2-1.7 wt.% and up to 1280 ppm, respectively, constraining the magma saturation pressure between 20 and 80 MPa. In order to constrain the storage conditions of La Sommata and Brown Tuff magmas, low pressures crystallization experiments with H2O+CO2 mixture were performed. Brown Tuffs composition was experimentally investigated at 150 MPa, in 1000-1080 °C region for a variable H2Omelt in a total of 12 charges. Phase assemblage was characterized by plagioclase as a near liquidus-phase, followed by orthopyroxene, pigeonite, amphibole, augite and phlogopite. Potassium enrichment of experimental melts (K2O = 3.7 wt.%; SiO2 = 56.5 wt.%) is correlated with water contents decrease. Experimental melts partly reproduced the lower alkali range of the Vulcano UBT, Punta di Mastro Minico and Quadrara Formations. Lower and Intermediate Brown Tuffs pre-eruptive conditions were thus constrained at T= 1020- 1030 °C, P>80 MPa and H2Omelt ≥ 2.0 wt. %. Experiments on La Sommata were performed at the 150-50 MPa, 1150-1050 °C region for the 24 charges studied. Whatever the pressure, clinopyroxene was the liquidus phase followed by olivine; plagioclase crystallizes at H2O-poor at near-solidus conditions. We have found that the Ca in clinopyroxene was sensitive to the H2O thus providing a sensitive geohygrometric tool. Experimental melts are characterized by a potassium enrichment (K2O = 5.4 wt.%; SiO2 = 54.6 wt.%) with H2O decrease and progressive crystallization. Experimental melt compositions were similar to mafic K-rich magmas at Vulcano (e.g. Saraceno and Vulcanello). Pre-eruptive conditions for La Sommata magma were constrained at P=100-150 MPa, T= 1120 °C and H2O 2.6 - 2.9 wt%. The magma ascent path was characterized by two discrete stages of crystallization: 1) at 100-150 MPa, with clinopyroxene and olivine the main crystallizing phases and 2) at ca. 50 MPa with the crystallization of plagioclase in a very small shallow crystallization level, reached after abundant water exsolution.