Exploring the compositional variability of magmas erupted at La Soufrière volcano, St Vincent, using chemostratigraphy and new 40Ar/39Ar ages

Beitris Morrison Evans, Elena Melekhova, Richard Robertson, Brian Jicha, George Cooper, Holli Frey, Jon Blundy

The temporal compositional variation of individual volcanic centres is key to understanding magmatic processes in the underlying crust. La Soufrière volcano, St. Vincent, Lesser Antilles, has erupted predominantly basaltic andesite magmas for hundreds of thousands of years. Sampling of the recently exposed crater walls at La Soufrière reveals that sequentially emplaced crater lavas, feeder dykes, and western flank lavas are low-magnesium basaltic andesites with little variation in major and trace element concentrations, suggesting the system is buffered by the underlying crystal-mush. A single basaltic crater lava unit is more mafic (8 wt. % MgO, 51 – 52 wt. % SiO2) and is compositionally similar to rare low-magnesium basaltic tephra from large explosive eruptions, suggesting more primitive melts are present in the underlying crust but rarely erupt at the surface. New 40Ar/39Ar ages fill a period of previously unaccounted for eruptive history and highlight an unconformity in the crater. A crater lava unit dated at 25.2 ± 3.8 ka is overlain by considerably younger crater lavas dated at 7.9 ± 7.7 and 5.7 ± 4.4 ka. A hornblende-gabbro xenolith is dated at 475.6 ± 28.3 ka, in agreement with early pre-Somma magmatism at La Soufrière volcano, and possibly representing an earlier, now inactive, crystal-mush. Combing new and published data, we show that hiatuses in the geochronological record coincide with major summit collapse events and that the system appears to have stabilised compositionally after the most recent Somma collapse.