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

Abstract The compositional variation of individual volcanic centres over time 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 re-exposed crater walls at La Soufrière reveals that sequentially emplaced crater lavas, feeder dykes and lavas on the volcano’s western flank are low-magnesium basaltic andesites with little variation in major and trace element concentrations. A single La Soufrière crater lava unit is more mafic (8 wt.% MgO, 51–52 wt.% SiO 2 ) and is compositionally similar to rare low-magnesium basaltic tephra from large explosive eruptions on St. Vincent, suggesting more primitive melts are present in the underlying crust but rarely reach the surface. New 40 Ar/ 39 Ar ages fill in a period of previously undocumented eruptive history and highlight an unconformity within the crater: a crater lava dated at 25.2 ± 3.8 ka is overlain by considerably younger 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. Combining 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. The compositional uniformity of La Soufrière basaltic andesites is consistent with buffering of magma compositions by a crystal mush in the underlying crust. Commonly erupted plutonic xenoliths at St. Vincent likely represent fragments of such mushes.