Comparison of the 2021 and 1971-72 lava domes of La Soufriere, St. Vincent and potential for transition to explosive behavior

The northern part of St. Vincent in the Lesser Antilles volcanic arc is dominated by La Soufriere volcano. This stratovolcano has had seven major eruptions in the historical record, including a Plinian eruption in April 2021 that was preceded by a lava dome extrusion. The effusive-explosive transition in 2021 was the fourth eruption in the last 125 years. In 2021, 16,000 residents were successfully evacuated <24 hours before explosive activity began, highlighting the need to understand why volcanoes may change eruptive styles during active periods. In contrast to the 2021 eruption, from October 1971 to March 1972, a lava dome emerged from the crater floor, but never transitioned to an explosive phase. To understand this contrasting behavior, we are characterizing the petrology of the lava domes to assess potential differences in magma source, storage conditions, and/or ascent. Both the 1971-72 and 2020-21 lava domes are basaltic-andesite. The plagioclase in both samples is mostly normally zoned, which means that the crystals cooled uninterrupted. Cores are Ca-rich, and the rims are Na-rich. Plagioclase microlites, an extremely small crystal found in the matrix of the rock usually smaller than 20 microns that forms during magma ascent, are more sodic and similar to the rim compositions. In contrast to the 2021 dome, some plagioclase microlites in the 1972 dome are more potassic. Cpx compositions are unzoned and broadly similar between the samples, but the 2021 cpx are slightly more Mg-rich. Opx compositions are nearly indistinguishable. However, there is a difference in the microlites of the pyroxenes. The microlites within the 1972 samples are more Fe-rich meaning they may have formed at lower temperatures. The olivine textures are distinctive between the domes. Olivine in the 1971-72 dome has a fairly restricted composition and is unzoned, whereas the 2021 dome has some normally zoned olivines in addition to an unzoned population and one primitive crystal (Fo95). Olivine in the modern dome has a symplectite exsolution texture, typically associated with more oxidizing conditions and higher temperatures. My future work is to ascertain crystal size distribution, which defines the number, volume, and/or mass of crystals within a given volume. This is being done to understand if there is a difference in ascent rates and 3D crystal structures, which may help us understand the contrasting eruptive styles.