Crater Gas Emissions and the Magma Feeding System of Stromboli Volcano

Quiescent and explosive magma degassing at Stromboli volcano sustains high-temperature crater gas venting and a permanent volcanic plume which constitute key sources of information on the magma supply and dynamics, the physical processes controlling the explosive activity and, more broadly, the volcano feeding system. The chemical composition and the mass output of these crater emissions (gases, trace metals, radioactive isotopes) were measured using different methodologies: within-plume airborne measurements, ground-based plume filtering, and/or in situ analysis, remote UV and open-path Fourier transform infrared absorption spectroscopy. The results obtained, summarized in this paper, demonstrate a primary control of the magmatic gas phase on the eruptive regime and the budget of the volcano. The large excess gas discharge, compared with the lava extrusion rate, and the source depth of slug-driven Strombolian explosions evidence extensive separate gas transfer across the volcano conduits, promoted by the high gas content (vesicularity) and then permeability of the shallow basaltic magma. Combined with data for volatiles dissolved in olivine-hosted melt inclusions, the results provide updated constraints for the magma supply rate (similar to 0.3 m(3) s(-1) average), the ratio of intrusive versus extrusive magma degassing (similar to 15), and the amount of unerupted degassed magma that should be convectively cycled back in conduits and accumulated beneath the volcano over time (similar to 0.25 km(3) in the last three decades). The results also provide insight into the possible triggering mechanism of intermittent paroxysmal explosions and the geochemical signals that might allow forecasting these events in the future.