Characterization of the initial explosive phases of the 2021 La Soufriere eruption, St. Vincent

The Soufriere Volcano on St. Vincent Island, in the Lesser Antilles volcanic arc, is one of the most active subaerial volcanos in the Caribbean, erupting both explosively and effusively at least 7 times in the last 500 years. In December of 2020, the volcano began erupting effusively, showing no evidence of degassing and on April 9^th of 2021, it transitioned to an explosive eruption that continued until April 22, with more than 30 eruptions classified as Vulcanian and sub-Plinian. Scoria samples from three units (U1-U3) erupted in the first 10 hours of the explosive eruptive phase were analyzed by SEM and EMPA in order to determine any petrologic differences and similarities that would give insights into what triggered the transition in eruptive style. The scoria was classified as basaltic andesite (52.8-54.2 wt% SiO₂), similar to the compositions of scoria from previous eruptions at the volcano, with a common mineral assemblage of plagioclase + orthopyroxene + clinopyroxene + olivine + titanomagnetite. Crystal sizes range from microlites (<10 um) to >1mm phenocrysts. Phenocrysts in all samples from U1, U2, and U3 are in textural equilibrium, however they are in chemical disequilibrium with the groundmass glass. Plagioclase is either un-zoned (An_56-96) or displays normal zoning with calcic cores (An_80-95) and thick (20-50μm) sodic rims (An_58-65). Plagioclase microlites compositions in U3 are more sodic than the phenocryst rims- suggestive of different crystallization depths. Pyroxene phenocrysts are un-zoned (Mg# 0.84-0.58) and microlites are more Fe-rich, with opx more abundant to cpx in U3, whereas their frequency is approximately equal in U1 and U2. Olivine is present in all samples as un-zoned phenocrysts (Fo_59-87) or zoned phenocrysts with thin (~5 um) Fo₆₅ rims. Olivine microlites are abundant in U1 and U2 and are rare in U3. One distinction between the units is the vesicle size, which appears to become progressively larger over the course of the eruptive sequence, providing evidence of over-pressurization preceding the initial explosive eruption. The lack of reversely zoned phenocrysts suggests that a mafic injection did not trigger the explosive eruption. However, based on the broad range of microlite compositions, which become more sodic during the eruptive sequence and mineral cores that are not in equilibrium with the melt, there is evidence for cryptic magma mingling or entrainment of crystals that occurred rapidly upon ascent.