Pre-Eruptive Conditions of La Sommata Basalt (Vulcano, Aeolian Islands): Constraints from Experimental Petrology.

One of the most primitive magma of Aeolian Arc is the basalt erupted (about 50 ka B.P.) by the La Sommata scoria cone at Vulcano; this is a silica-undersaturated (ne-normative) low-phyric (10-20 wt. %), Ca-rich (CaO = 13 wt. %) shoshonitic basalt (K2O = 2.28 wt.%) in which olivine phenocrysts host melt inclusions more primitive than whole rock composition and with an ultra-calcic character: CaO/Al2O3 > 1.4 [Métrich and Clocchiatti, 1996; Gioncada et al., 1998; Schiano et al., 2000]. Phenocrysts mainly include olivines (Fo92) and clinopyroxenes (Fs5). Plagioclase microlites (from An79 to An67) are the most abundant groundmass phase (45 wt. %), together with clinopyroxene (Fs11) and olivine (Fo85). Moreover, the groundmass is dotted by Fe-Ti oxides and rare Cr-spinel is included into olivine phenocrysts. This experimental petrology study was focused on the following main objectives: (1) to constrain the pressure-temperature-fluid activity during the ascent paths of La Sommata primitive magma, (2) to experimentally investigate the influence of H2O in this magma and the effects on phase equilibria, (3) to depict a petrological model and constrain the pre-eruptive conditions of this very peculiar eruption. Experiments were realized with an IHPV housed at Centre National de la Recherche Scientifique - Institut des Sciences de la Terre d’Orléans (CNRS-ISTO, France). In order to simulate the pre-eruptive conditions (P, T, aH2O) experienced by La Sommata magma all the experiments were run loading H2O+CO2 together with the chosen starting material, at different ΧH2O and carried out in the pressure range of 150 - 50 MPa and temperature of 1180-1050 °C, under two main guidelines: (i) a well characterized isobaric section at 150 MPa, the inferred maximum pressure of magma storage region (suggested by H2O-CO2 contents in MIs from Gioncada et al. [1998]; (ii) an isothermal section at 1150 °C (suggested by the homogenization temperature of MIs from Gioncada et al. [1998]) in the pressure range 150 - 50 MPa, in order to simulate the ascent path.