Magmatic stoping during the caldera-forming Pomici di Base eruption (Somma-Vesuvius, Italy) as a fuel of eruption explosivity

Magma-carbonate interactions and the subsequent CO2 release can occur before and during an eruption, critically affecting magma storage and ascent processes. However, the mechanisms and timescales of those interactions are unclear, particularly during the fast magma withdrawal that feeds high-intensity eruptions. In order to better understand magma‑carbonate interactions, we selected the caldera-forming Pomici di Base plinian eruption, the oldest (22 ka) and largest (> 4.4 km3) explosive event in the history of the Somma-Vesuvius volcanic system, as case study. During this event the emission of trachytic and latitic-shoshonitic (~25% and ~75% of the erupted magma volume respectively) magmas generated a long-lasting plinian column, hypothetically driven by CO2 liberation during magma‑carbonate interaction. In this study, we reconstruct in detail the evolution of the plumbing system during the Pomici di Base eruption combining geochemical (major/minor elements and radiogenic/stable isotopes) analyses of juvenile products with thermodynamic and kinetic calculations. Our results demonstrate that magmatic stoping (i.e., the formation and transport of host-rock pieces into a magma body) during caldera collapse evolution can promote rapid magma assimilation of carbonate blocks and CO2-rich fluids resulting from destabilization of the carbonate bedrock, thus triggering CO2 release and acting as a fuel for the eruption explosivity, especially when residual hot mafic magmas are involved. Our findings suggest that the accurate knowledge of these processes and their influence on eruptive dynamics are critical for improving the hazard assessments of volcanoes with plumbing systems located in carbonate bedrocks.