The rheology of magma feeding the February-September 2021 lava fountains at Etna volcano (Italy)

All magmas form crystals upon cooling and volatile loss, following shifts in the melt liquidus during both decompressive ascent and eruption. Crystallisation modifies the melt chemistry and adds solid particles (crystals) in suspension in a magma. Both changes increase the bulk viscosity of the magma, potentially affecting the final eruptive style. Hence, it is crucial to understand the complete evolution of viscosity in a magmatic system in order to properly constrain its role in the evolution of a magma from depth to surface.

Here we measured the viscosity of magma feeding five events of lava fountaining at Etna volcano, Italy, during a period of seven months in 2021. All measurements were performed using remelted lapilli samples of trachybasalt composition (47-48 wt.% SiO₂, 5.37-5.78 wt.% Na₂O+K₂O), originally collected during or immediately after each explosive event. Rheology analyses were performed at superliquidus conditions between 1198-1490°C in a concentric cylinder rheometer. The results show low viscosity variations - up to 0.14 log units at 1198°C - in time among the explosive events. These results are consistent with calorimetric analyses performed in both the remelted and natural samples. Additionally, pre-eruptive crystal contents vary between 21-53% of mainly clinopyroxene, plagioclase, olivine and orthopyroxene, as well as some oxide minerals, while interstitial melt compositions show an enrichment of up to 7 wt.% in SiO₂ and more than 2 wt.% in Na₂O+K₂O after microlite crystallisation, which affects the magma bulk viscosity. Further analyses will help constrain such evolution of magma bulk viscosity. These results help to better constrain the minimum viscosity for the pre-eruptive magmas, as well as the shifts in bulk viscosity that the magmas experienced during ascent and eruption before quenching in air, potentially affecting the eruptive behaviour.