Shallow factors controlling the explosivity of basaltic magmas: The 17-25 May 2016 eruption of Etna Volcano (Italy)

We present a detailed physical and geochemical analysis of a small-scale eruption at the type basaltic volcano, Mount Etna, Italy, that spanned 9 days in May 2016. A complexity rarely seen within the short timeframe was present with the style of activity manifesting as outgassing, strombolian explosions to weak fountaining, and lava flows, while the eruption migrated between most of the summit craters. Through microprobe analysis of phenocrysts, groundmass glasses and melt inclusions we define geochemical trends by differentiating the eruptive products into two groups - the explosive tephra produced by lava fountaining, and lava and scoria emitted during Strombolian explosions. We highlight plagioclase and olivine compositions and variations in glass compositions as evidence for the eruption of multiple magma bodies. The eruptive sequence was triggered and fed by a batch of magma reaching the surface after limited degassing and crystallisation. Despite its small mass (about 0.01% of the total erupted magma), it generated three lava fountains occurring within a four-day timespan, producing ash-rich plumes which reached heights of a few km above the vent. The same batch destabilised a much larger mass (the remaining 99.99%) of crystal-richer magma stored at shallow depth that then erupted as lava from the same vent system in temporal continuity with the initial batch. The eruption was concluded by minor spattering and final Strombolian activity. The compositions of the magma batches suggest that the shallow magma reservoir likely exists as a network of interconnected dykes, probably fed by the same that partly erupted in the years/months preceding the May 2016 eruption. Variation in explosivity of the eruption was directly controlled by pre-eruptive degassing histories of each magma batch.