GEOCHEMICAL AND MINERALOGICAL INVESTIGATION INTO MAGMA RESERVOIR PROCESSES AND CONNECTIVITY AT MIDDLE SISTER AND SOUTH SISTER VOLCANOES, OREGON, USA

The Three Sisters (Klah Klahnee) are part of a volcanic complex located in the central Cascade Arc near Bend, Oregon. Widespread volcanism dominates this region of the arc and includes three stratovolcanoes: North Sister, Middle Sister, and South Sister. Middle Sister and South Sister (<50 ka) share a coeval eruptive history characterized by alternating episodic activity as well as temporally related eruptions. Middle Sister and South Sister lavas range from basaltic andesite to rhyolites, and the diverse eruptive style associated with these compositions lead to complex eruptive hazards. Therefore, understanding magma reservoir processes and whether the two volcanoes are connected at depth has important implications for improved hazard assessment at this currently inflating volcanic complex. This study focuses on two temporally related (ca. 27 ka) andesites erupted on the west flanks of their respective edifices: the Middle Sister unit andesite of Linton Creek (alc_MS) and South Sister unit andesite of Lost Creek Glacier (alg_SS). These andesites have indistinguishable whole rock chemistry, similar mineral cargoes and abundances, and consistent major element concentrations across mineral phases. Minor differences in plagioclase and pyroxene trace element concentrations suggest similar parental sources but discrete transcrustal processes beneath Middle Sister and South Sister, indicating these andesites were not derived from the same reservoir. Overall, compositional trends in alc_MS suggest fractionation as the dominant process. Although, enclaves/lithics have been identified in this unit, there is no indication of mixing or mingling, indicating lithic entrapment shortly before eruption. In contrast, textural and compositional trends of crystals in alg_SS indicate mixing or a recharge event, followed by a period of homogenization prior to eruption. Overall, divisions in trace elements suggest no interactions between Middle Sister and South Sister on the west flanks ca. 27 ka. Additionally, unit alg_SS suggests there is a more complex magma mush system at South Sister ca. 27 ka.