Deep Origin and Shallow Launch for the Etna 122 B.C. Mafic Plinian Eruption

Abstract Basaltic Plinian eruptions challenge our understanding of explosive volcanism. The 122 B.C. Plinian eruption of Etna ranks among the most powerful mafic explosive events known. Here, we combine volatile barometry of 122 B.C. from olivine‐hosted melt and fluid inclusions with comparative data from the sub‐Plinian Fall Stratified eruption at Etna (3930 BP) to assess the storage depths and degassing paths that govern explosive eruptive styles. Our results indicate that the 122 B.C. magma storage started deep (∼22 km) within Etna's plumbing system, had a complex ascent history, and was finally stored pre‐eruptively at shallow levels (∼2–5 km) for a minimum of ∼3 weeks, resulting in low equilibrated H 2 O (∼2 wt%) and CO 2 (≤500 ppm in bubble‐free melt inclusions) contents. The Fall Stratified event, in contrast, records deeper storage (∼24–30 km), high magmatic volatile contents (CO 2 concentrations up to 9600 ppm, H 2 O concentrations up to 6.3 wt%), and exceptionally rapid magma ascent rates (17.5 m/s). We propose that the 122 B.C. eruption likely resulted from multiple episodes of replenishment by basaltic magma with a much lower CO 2 concentration (<4,500 ppm) than that of the Fall Stratified event. The eruption was probably triggered by an increase in magma effective viscosity due to extensive microlite crystallization at shallow levels, which caused a secondary vesiculation event. This mechanism contrasts with mantle‐derived eruptions such as the Fall Stratified event, where deep volatile‐exsolution, controlled by CO 2 , is thought to drive the eruption.