Dacite origins during a compositional shift in eruptive products at Middle Sister volcano, Oregon Cascades

The Three Sisters Volcanic Complex (TSVC) in the Oregon Cascades provides a unique framework for investigating trans-crustal magmatic systems and intermediate magma petrogenesis at arc systems, due to its compositional variability (erupting the full suite of lavas, from basalt to rhyolite). Middle Sister, one of the central stratovolcanoes within the TSVC, is particularly noteworthy due to its extensive production of intermediate magmas, specifically andesites and dacites. This study focuses on two dacitic lava flows from Middle Sister, the dacite of Sister Spring (dss) and the dacite of Lane Plateau (dlp), which erupted following a 10,000-year eruptive lull at the volcano ca. 22-14 ka and mark the onset of a bimodal basaltic andesite-dacite eruption sequence. Through detailed petrologic and geochemical analyses, this study examines the petrogenesis of dss and dlp to determine (1) the occurrence of magma mixing, recharge, or fractional crystallization within these dacites, (2) the compositional similarity of post-lull dacites at Middle Sister, and (3) the petrogenetic implications for the post-lull dacites Middle Sister. Textural observations and detailed mineral-focused geochemical analyses of mineral phases reveal that both dacites are sourced from single-component magmatic reservoirs that underwent at least two episodes of magma recharge from a compositionally related, more primitive magma prior to eruption. Comparison with other post-lull dacites from Middle Sister, including the dacite north of Separation Creek (dsn) and the dacite of Irving Glacier (dig), demonstrates compositional homogeneity among these units, reinforcing a shared petrogenetic pathway. Calculated formational parameters such as oxygen fugacity and temperature estimates indicate similar pre-eruptive conditions across all four dacites. Findings of this study enhance our understanding of the processes governing intermediate magma formation (dacites) at Middle Sister and provide valuable insights into the magmatic plumbing system beneath the TSVC, contributing to both hazard assessment and petrogenesis models in the region.