Crystal-poor rhyolite plagioclase textures at South Sister volcanic field in central Cascade arc, Oregon

South Sister volcanic field is unique among other volcanoes in the Cascade arc due to routinely erupting rhyolitic lava. Rock Mesa (71-73% SiO₂) a rhyolitic lava dome, and Devil's Chain (70-72.2% SiO₂), a series of 20 rhyolitic lava domes, are the two most recent eruptions, having last erupted both effusively and explosively at 2.2 ka and 2 ka, respectively. The objective of this research was to characterize these similar young rhyolite deposits by looking at the compositions and textures of the most abundant phenocryst phase, plagioclase (35%). Plagioclase crystals analyzed from lava and pumice rhyolite samples from Rock Mesa and Devil's Chain show three distinct textures. These textures, categorized as euhedral, melt inclusion, and zoned can give us insight into the conditions of the magma chamber where the plagioclase was formed or was introduced to. While there is little bulk chemistry variability between Rock Mesa and Devil's Chain, the plagioclase crystal textures do show contrasts between the two domes. The Rock Mesa lava had 70% melt inclusion crystals, 17% euhedral, and 13% zoned, whereas the Rock Mesa pumice had 56% melt inclusions and 44% zoned crystals. In contrast, the Devil's Chain pumice contained 12% euhedral crystals. Melt inclusion crystals comprised 59% of the sample while zoned made up 29%. The Devil's Chain lava had 42% melt inclusion, 32% zoned, and 26% euhedral crystals. This sample was also the one which had plagioclase microlites throughout the matrix. Glass chemistry results showed that matrix glass varied from 74.6 to 80.3 SiO₂ (wt %), overlapping with inclusions, which varied from 73.4 to 79.6 SiO₂ (wt %). An content results showed that euhedral texture crystals had the widest range of An values, from An-25-80, zoned crystals had a range of 30-70, and melt inclusions had a range of An-30-50. Lava rim An values were most constrained, with lava rims being the most constrained from An-40-50. Devil's Chain and Rock Mesa had similarly constrained rims, but Rock Mesa had a larger variation of cores An30-70. The euhedral crystals are likely young autocrysts, which formed quickly and under close to ideal conditions. Due to the melt composition and the grungy appearance of the melt inclusion crystals and glass inclusion composition, we suggest that these crystals are likely xenocrystic, and, once introduced to the melt in the magma chamber, began to resorb under the less than ideal conditions. In general, the data shows that Rock Mesa crystals, with an abundance of melt inclusion plagioclase, lack of microlites, and most variation of An core values, had a magma chamber that was more hostile to xenocrysts compared to Devil's Chain's magma chamber, which had microlites and overall more restricted An values for rims and cores.