Petrology of the 2020 2021 effusive to explosive eruption of La Soufriere volcano St Vincent: Insights into plumbling system architecture and magma assembly mechanism

The 2020-21 eruption of La Soufrière, St Vincent began with extrusion of a viscous lava dome, which was destroyed upon transition to a major explosive phase. Here we present petrological data to reconstruct the processes leading up to these events. Bulk-rock SiO2 contents range from 52.8 to 55.4 wt. %, classifying the lava and the subsequent scoria as basaltic andesite, the latter being slightly more mafic. Macrocrystal chemistry and modes (plag-cpx-opx-tmt-ol) and crystallinity (45–50 vol. %) are largely identical for both phases of the eruption. Pyroxenes are homogenous and precipitated mostly from andesitic melts. Conversely, plagioclase shows strong normal zonation resulting from magma ascent and stalling at multiple crustal levels. Clinopyroxene thermobarometry reveals that crystallisation predominantly took place between 8 and 13 km depth at temperatures of 997+18−35 °C. A lack of evidence for mafic recharge and changes in volatile content and the omnipresence of xenoliths, suggests pre-eruptive destabilisation of an andesitic-dacitic melt pocket that disrupted and entrained antecedent mush. Olivine diffusion profiles show that this interaction preceded the onset of eruption. Low dissolved sulfur contents (≤270 ppm S) place constraints on the total SO2 gas release. Melt-mush disruption appears to be a dominant driver of eruptions at La Soufrière. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6484877