Crystallization Driven by Decompression and Water Loss at Stromboli Volcano (Aeolian Islands, Italy)

Stromboli, in the Aeolian Archipelago, is famous for its persistent volcanic activity. The ‘normal’ activity, consisting of rhythmic explosions ejecting 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 ∼50 vol. % crystals (Plag ∼65; Cpx ∼25; Ol ∼10) whereas the pumices display <10 vol. % crystals (Plag ∼42–50; Cpx ∼47–30; Ol ∼11–20). The bulk rocks, mainly ranging between K-rich basalts and shoshonitic basalts, surprisingly exhibit only slight variations in major and trace element contents, and rare earth element patterns. Systematic studies of melt inclusions (MI) and their host minerals were performed on three scoria–pumice pairs erupted together during the violent explosions. 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 presence of rather primitive melts not yet identified as erupted magmas. On the 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 the S/Cl ratio close to unity. In contrast, the crystal-rich magma is extensively degassed. We propose that this degassed magma, which sustains the ‘normal’ activity, results from the crystallization of volatile-rich magmas within the cone itself, driven by decompression and H2O exsolution at low pressure. The crystallization is accompanied by S and Cl fractionation into the gas phase, consistent with partition coefficients DS and DCl between fluid and melt of 40 and 10, respectively. The most violent explosions appear to result from the uprising and emission of volatile-rich magma blobs.