Senior Project Advisor
igneous petrology, plagioclase, olivine, pyroxene, Cascade Arc
The Middle Sister and South Sister volcanoes, near Bend, Oregon, are overlapping, active Cascade Arc stratovolcanoes which share a complex, contemporaneous eruptive history. This history is characterized by an extreme compositional diversity of lavas erupted in alternating phases of high activity from one neighboring volcano to the other, with both vents producing material ranging from basaltic andesite to rhyolite. This system is understood to be predominantly fed by basaltic andesites fractionated from partial mantle melts within the lower crust, but magma compositions are additionally impacted by mixing, assimilation, and crustal contamination while in transit to the surface. Thus, the subterranean relationship between these volcanoes is, at present, poorly understood, with relatively little work having been done to identify and constrain the extent of reservoir interconnectivity and magma sharing which may influence eruptive behaviors in the past and inform understandings of present-day activity. Here, we investigate chemical zoning in minerals from three basaltic andesite lavas–two from Middle Sister and one from South Sister–as a record of magmatic interactions impacting the more primitive, mafic material supplied to each vent. By identifying and correlating similar mineral populations found in each lava, we identify interactions between three discrete magmatic components, recorded as distinctive chemical bands in plagioclase, olivine, and pyroxene phenocrysts. Two of these magmatic components are observed in all three units of study, shown to represent consistent recharge of basaltic andesite magma via a two-part mixing process during the interval from 48 ka to 21 ka. At Middle Sister, the studied basaltic andesites are interpreted as chemically consistent eruptions of this two-part hybrid magma from 48 ka to 22 ka. At South Sister, mineral zones record a mixing event between a similar hybrid magma and a third, chemically distinct magmatic component, suggestive of independent processes and discrete crustal reservoirs beneath Middle Sister and South Sister around 22 to 21 ka. Future work is proposed to investigate the origins of each of these components via trace element analysis and thermobarometry, in order to assess the similarities and differences in melt sourcing, magma transit, and crustal contamination at Middle Sister and South Sister.
Halstead, Sean Francis, "Shared Roots: A geochemical investigation of basaltic andesites to understand magmatic cogenesis at the Middle Sister and South Sister volcanoes, central Oregon" (2023). WWU Honors College Senior Projects. 751.
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