Phenocrysts Crystallisation Pressures and Temperatures and Melts Evolution at La Fossa Volcano (Vulcano Island, Italy)

La Fossa Volcano (Vulcano Isl., Aeolian Arch., Italy) erupted last, explosively, in 1888-1890. Its eruptive history includes at least four eruptive cycles of mixed eruptions with strombolian and hydromagmatic phases followed generally by small lava flows ranging in composition from latites to trachytes and rhyolites. Crystallisation temperatures and pressures of phenocrysts and melts chemical evolution, have been modeled via thermochemical calculations and HP-HT laboratory experiments. The crystallisation temperatures and pressures of olivine and clinopyroxene phenocrysts of latitic and trachitic lavas (Punte Nere, Grotte dei Palizzi) were obtained via the empirical olivine-clinopyroxene-liquid thermobarometer of Sugawara (2000) and the olivine geothermometer of Ariskin et al. (1993) which gave consistent values of 1120°C - 60 MPa and 1080°C - 50 MPa. For the trachytic lava of Grotte dei Palizzi and the rhyolitic blocks of 1888-90 eruption, T - P of 1030°C - 50 MPa and 1000°C - 40 MPa were obtained using the two feldspars thermochemical equilibrium model of Green and Usdansky (1986). The %(H2O)m of trachytic and rhyolitic melts, in the range of 1.7 - 2.7% and 2.4 - 2.7% respectively, was obtained after the experimental calibration at P = 100 MPa and T = 1000-1020°C of X(An)plg against the %(H2O)m. The phase relations and melts composition under the above indicated conditions were finally investigated by the MELTS code (Ghiorso and Sack, 1995), allowing us to show how the more primitive latitic melt can evolve toward the more evolved trachytic and rhyolitic compositions.