Explosive eruptions at Stromboli volcano (Italy): a comprehensive geochemical view on magma sources and intensity range

A comprehensive understanding of the processes that occur during magmatic storage and pre-eruptive ascent-and of their associated timescales-is critical to identifying potential precursory signals, and to developing robust volcano early-warning systems. Stromboli's persistent activity comprises continuous degassing and explosive activity that ranges from hourly, low-intensity "normal" activity to occasional, more violent, paroxysmal activity. While the magma source processes that drive normal and paroxysmal activity are reasonably constrained, eruptive activity intermediate in magnitude and intensity (i.e., major explosions) remains elusive in terms of classification, source region, and pre-eruptive timescales. Here, we investigate the 19 July 2020 major explosion that geophysical parameters place at the upper limit of the major explosions field, close to small-scale paroxysms such as the 2003 and 2007 events. The geochemical signatures of matrix glass, olivine, melt inclusions, and embayments-integrated with gas measurements-highlight important differences in eruption source, ascent behaviour, and pre-eruptive timescales of the studied event when compared to paroxysms. Melt inclusion volatile contents identify that magma rise begins from a slightly shallower source (similar to 9.5 km below sea level, b.s.l.) than for paroxysms (11.4 km b.s.l.), with the activation of a shallower ponding zone at 5-6 km b.s.l.. This, in combination with intermediate matrix glass compositions, suggests complex ascent behaviour, characterised by CO2 buffering in the deep ponding region and magma self-mixing in the shallower zone. Fe-Mg-diffusion modelling in olivine indicates a system perturbation starting similar to 20-25 days before eruption onset, in agreement with the timescale of volcanic gas CO2/SO2 ratio changes observed in the plume, and significantly shorter than that observed prior to paroxysms (similar to 4 months). The geochemical dataset provides insights into the processes controlling the steady-state conditions and the broad spectra in eruption magnitude and intensity at Stromboli and bears important implications for eruption forecasting.