HR: 10:35h
AN: V22A-02 INVITED    [Abstracts]
TI: Pumice textures, magma mingling and fragmentation processes at Stromboli volcano (Italy)
AU: * Pioli, L
EM: lpioli@uoregon.edu
AF: University of Oregon, Department of Geological sciences, Eugene, OR 97403-1272, United States
AU: Marco, P
EM: pistolesi@dst.unipi.it
AF: Universita' di Pisa, Dipartimento di Scenze della Terra, Pisa, 56126, Italy
AU: Rosi, M
EM: rosi@dst.unipi.it
AF: Universita' di Pisa, Dipartimento di Scenze della Terra, Pisa, 56126, Italy
AB: A paroxysmal eruption of Stromboli volcano on April 5, 2003, lasted about ten minutes, and comprised 4 different phases. The eruption climax was reached during phase 2, where the average magma eruption rate was 2.6*10⁶ kg/s (Rosi et al. 2006). The erupted juvenile material included both low porphyritic (LP) and highly porphyritic (HP) magmas (Metrich et al., 2005; Corsaro et al., 2004). LP magma was predominant during the climactic phase, strongly decreased in the successive explosions of Phase 3, and was negligible in the final ash emitted during Phase 4. Textural and physical features of the juvenile clasts emitted during the climactic phase were analyzed to obtain information about eruption dynamics and fragmentation processes. We studied a population of pumices (diameter size 2-3 cm) deposited about 1.5 km from the vent, on the Punta Lena coast. The clasts are equant to slightly elongated with vesicularity ranging from 40 to 85%, and phenocryst content ranging from <5 to 50%. They have glassy groundmass, with sparse microlites. Vesicularity and mean vesicles size increase from the rim to the inner portion of the clasts, the result of postfragmentation expansion. Some exceptionally expanded clasts display a central main cavity surrounded by a microvesicular glassy rim less than 1 cm thick. The quenched rims contain a high number density of small uniform vesicles suggesting very fast rates of magma ascent and bubble nucleation. Most clasts show intimate mingling between the two, compositionally distinct, magma types. In LP glass vesicles are mostly spherical and display homogenous features, sub mm diameters, and show little coalescence. In HP magma the vesicle population is heterogeneous: groundmass vesicles have sub mm size, spherical shapes and little coalescence; larger, mm sized vesicles generally have elongated or convoluted shapes often adapting to the surrounding phenocrysts.As a conclusion, our data indicate that the dynamics of the April 5 2003 Stromboli paroxysm was likely originated by magmatic fragmentation of of volatile-rich LP magma subjected to fast ascent and decompression eventually rising through, and effectively mingling with a superficial reservoir constituted by degassed, more crystalline, texturally mature HP magma.
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting