Crystallization driven by decompression and water loss at Stromboli volcano (Aeolian Islands, Italy)

Stromboli, in. the Aeolian Archipelago, is famous for its persistent volcan ic activity. ne 'normal' activity, consisting of rhythmic explosions ejecti ng crystal-rich scoriae, is periodically, interspersed with more energetic explosions during which, in addition to crystal-rich scoriae, crystal-poor pumices are also emitted. The scoriae contain similar to 50 vol. % crystals (Plag similar to 65; Cpx similar to 25: Ol similar to 10) whereas the pumi ces display < 10 vol. % crystals (Plag similar to 42-50: Cpx similar to 47 30, Ol similar to 11-20). The bulk rocks, mainly ranging between K-rich bas alts and shoshonitic basalts, surprisingly exhibit only, slight variations in major and trace element contents, and rare earth element patterns. Syste matic studies of melt inclusions (MI) and their host minerals were performe d on three scoria pumice pairs erupted together during the violent explosio ns. The MI cover a compositional range (CaO/Al2O3 = 0.99 - 0.29) far wider than that of the whole rocks (CaO/Al2O3 = 0.69 - 0.52) and attest to the pr esence of rather primitive melts not yet identified as erupted magmas. On t he basis of MI analyses, the crystal-poor magmas contain between 2.3 and 2. 8 wt % H2O, 894 1689 ppm CO2. 2250 1660 ppm S and 2030-1660 ppm Cl, with th e S/Cl ratio close to unity. In contrast, the quasi-rich magina is extensiv ely degassed. We propose that this degassed magina, which sustains the 'nor mal' activity, results from the crystallization of volatile-rich magmas wit hin the cone itself, driven by decompression and H2O exsolution at low pres sure. The crystallization is accompanied by S and Cl fractionation into the gas phase, consistent with partition coefficients D-S and D-Cl between flu id and melt of 40 and 10, respectively. The most violent explosions appear to result from the uprising and emission of volatile-rich magma blobs.