Defining rates of magma ascent and degassing at open-conduit volcanoes: clues to re-evaluating the hazard associated to violent paroxysmal eruptions

Magma ascent velocity during explosive eruptions is often too rapid to be recorded by diffusion of most major and trace elements. Only elements with very high diffusivity, such as Li in plagioclase or the melt-bubble exchange of fast-diffusing volatile species can provide direct measurements of such short-lived processes. We are using two different modeling approaches that benefit from the diffusion of volatiles (H2O, CO2 and S) in partially-opened melt inclusions and tubes in olivine and the fast diffusive relaxation of Li zoning in plagioclase to recover timescales of magma ascent and degassing during unusual explosive eruptions at open-conduit volcanoes.Some relevant explosive episodes with different intensity, which occurred at Mt. Etna between 2011 and 2016, have been individuated and selected as case study. Modeling the diffusion of volatile species along olivine melt tubes leads us to recover timescales over which volatiles have been exchanged by diffusion with the external melt, and to determine the duration of magma ascent over a wide range of depth. Open-system separation of the gas phase upon eruption produces Li depletion from the melt, due to preferential and rapid transport of Li into the exsolved fluid phase. This process induces outward decrease of Li concentration in plagioclase, enabling to derive timescales of magma ascent and degassing just before the emission. Results evidence that processes of gas flushing are responsible for the re-activation of the magmatic system and the generation of some highly energetic eruptions, which occurred over unusually short timescales. The choice of eruptions characterized by different intensity of the explosive activity leads us to produce an array of ascent rates for a wide range of possible eruptions at open-conduit systems erupting low-viscosity basic magmas, which revealed to have important hazard associated.