Short residence times for alkaline Vesuvius magmas in a multi-depth supply system: Evidence from geochemical and textural studies

摘要

It is crucial to understand magma chamber chemico-physical conditions and residence times for high-risk volcanoes because these factors control the occurrence and size of future eruptions. In order to define magmatic pressure-temperature conditions and residence times at the Somma-Vesuvius volcano, we studied the geochemistry and texture of selected past eruptions that are representative of the entire volcanic history.Our petrological model indicates a multi-depth magma chamber composed of a deeper tephritic (350-400. Mpa) magma layer, which fed Strombolian and effusive eruptions during open-conduit activity, and an upper (200-250. Mpa) phonolitic level, which supplied the high explosive events that followed closed-conduit repose time. This upper reservoir matches the inferred transition between sedimentary sequences and metamorphic basement. At this level, the presence of a structural and lithological discontinuity favors magma storage during closed-conduit periods.The prevalent differentiation process was fractional crystallization during the magma cooling associated with upward migration of less dense, evolved liquids. Our results indicate that major steam exolution occurred during the late crystallization stage of phonolites, which accounts for the high Volcanic Explosivity Index (VEI) of eruptions supplied by these melts. Moreover, our phenocryst CSD data reveal the rapid crystallization and differentiation (decades to centuries) of alkaline Somma-Vesuvius magmas. This implies that the 400km~2 partial melting zone detected by tomography studies at 8-10km depth beneath Vesuvius should consist of differentiated magma that is already capable of generating a large-scale (plinian) explosive event if renewed activity develops out of the present closed-conduit state. Additionally, because our microlite CSD data indicate rapid magma migration from the chamber toward the surface, precursory activity could appear only short time before a major eruption.