Petrological Evidence for Magma Mobilization Years Before the 2020/2021 Eruption of La Soufrière Volcano, St. Vincent

Anticipating the onset of the 2020/2021 effusive-explosive eruptive sequence at La Soufrière volcano, St. Vincent was challenging despite the established monitoring networks in operation. Here, we integrate petrological data to decipher retrospectively signs of imminent eruption from available pre-eruptive monitoring data. Using diffusion chronometry, we estimated the timescales over which magmas transported to the surface. We examined olivine crystals hosted in basaltic andesite scoria, categorizing them into four groups based on their textures (euhedral to anhedral) and core compositions (Fo_73–89). Multiply zoned olivine populations are tracked through a multi-stage journey from depth to surface corresponding to periods of magma ascent and accumulation years before eventual eruption. This correlates temporally with two phases of unrest from monitoring data: (a) a protracted priming phase (lasting more than a decade) manifesting in low-level seismicity, small crater transformations (rockfalls and new fumaroles) and an elevated CO₂ degassing signal; and (b) a subsequent transition phase, initiating just over a year before eruption with the onset of geophysical unrest in the form of discrete episodes of elevated seismicity and volcano inflation. Our findings provide new insight into the dynamics of magma mobilization at La Soufrière. We demonstrate that magmatic unrest in the roots of the sub-volcanic system precedes geophysical precursors by years, drawing connections between individually ambiguous surface signals over long timescales. Monitoring strategies optimized to detect early stages of magmatic unrest, such as identifying and locating rarer deep seismicity and routine sampling at the crater plume, could improve future responses to volcanic crises in St. Vincent.