CHARACTERIZATION OF THE INITIAL EXPLOSIVE PHASES OF THE 2021 ERUPTION OF LA SOUFRIÈRE, ST. VINCENT

La Soufrière Volcano on St. Vincent, in the Lesser Antilles volcanic arc, is one of the most active subaerial volcanoes in the Caribbean. In December, 2020, it began erupting effusively. On April 9^th of 2021, it transitioned to a series of explosive eruptions that continued until April 22, with more than 30 individual events. Scoria sampled from three units (U1-U3), inferred to have been erupted in the first 48 hours, were analyzed by SEM and EMPA. This helped to determine any petrologic differences and similarities that would give insights into what triggered the transition from effusive to explosive. The scoria was basaltic andesite (52.8-54.2 wt% SiO₂), with a mineral assemblage of plagioclase + orthopyroxene + clinopyroxene + olivine + titanomagnetite. Crystals range in size from microlites to >1mm phenocrysts and appear as isolated phenocrysts or glomerocrysts of multiple phases. The compositions of the minerals across the different units is similar. Plagioclase is either unzoned (An_56-96) or displays normal zoning with calcic cores (An₈₀-₉₅) and thick (20-50μm) sodic rims (An_58-65). Plagioclase microlites span an even broader compositional range than the phenocrysts, with compositions of An_44-93 and a peak at An_55-65. Pyroxene phenocrysts are largely unzoned (Mg# 0.84-0.58) and pyx microlites are more Fe-rich, with cpx subordinate to opx. The abundance of olivine varies across the samples and occurs as unzoned phenocrysts (Fo_59-87), with more forsteritic (>Fo₇₅) phenocrysts sometimes displaying thin (~5 um) Fo₆₅ rims. The composition of the groundmass glass is andesite-dacite (62-64 wt% SiO₂), whereas a preponderance of the melt inclusions are significantly less evolved (SiO₂ <53 wt%). The lack of reversely zoned phenocrysts suggests that the eruption was not triggered by the influence of a hotter or mafic material . However, based on the broad range of microlite compositions, differences in groundmass glass and melt inclusion compositions, and mineral cores that are not in equilibrium with the melt composition, there is evidence for magma mingling or entrainment of crystals that occurred upon ascent. This is further supported by the presence of rare glomerocrysts that exhibit evidence of fluid alteration and disequilibrium textures, similar to those observed in samples from the 2020-21 and 1979 lava domes.