Petrology of the explosive deposits from the April 2021 eruption of La Soufrière volcano, St Vincent: a time-series analysis of microlites

After more than three months of lava dome extrusion, La Soufrière (St. Vincent) transitioned to a series of explosive eruptions in April 2021. Here we present a time-series petrologic analysis of the phenocryst and microlite populations during the first ∼48 hours of explosivity to constrain ascent conditions and processes that drove changes in behavior. Primary eruptive products were crystal-rich (45-50 vol%) basaltic andesites with similar phenocryst phase assemblages and compositions. The change in eruptive style is consistent with overpresurization as a consequence of second boiling from anhydrous microlite crystallization. The microlites display variation between the explosive phases, with two populations: 1) “inherited” - normally zoned high-An plagioclase (>An70) + olivine (Fo62-79) + clinopyroxene + titanomagnetite, inferred to have crystallized at depths >15 km and high water pressures; 2) “juvenile” - unzoned plagioclase (An45-65) + clinopyroxene + orthopyroxene + intermediate pyroxene (Wo12-38) + titanomagnetite, inferred to have crystallized upon ascent due to decompression and degassing. Scoria from the first explosions featured extensive groundmass crystallization and a significant “inherited” microlite population. Later explosions had a more abundant “juvenile” microlite population and lower crystallinity, consistent with more rapid ascent from depth, initiated by decompression following initial blasts and destruction of the lava dome.