Using Olivine-Hosted Melt Inclusions to Better Understand the Transcrustal Architecture Beneath Mount Baker (Koma Kulshan),…

The Schriebers Meadow cinder cone is located on the southern flank of Mount Baker and produced the Sulphur Creek lava flows and SC tephra deposits ~9.8 ka. Mount Baker, a Cascade volcano, is located ~50 km east of Bellingham, Washington. Previous work on the Sulphur Creek lava flows show that the eruption comprised two primary bulk rock compositions, ranging from basalt at the toe to basaltic andesite closer to the vent. However, little is known about the relative composition and eruption timing of the SC tephra from the same vent. In this thesis, I quantify compositional variations through the explosive portion of the eruption and determine depths of magma storage not captured by previous work. I collected 12 new tephra samples (named Units 1-12 from base to top, respectively) from a ~4 m thick tephra section ~3 km north of the cinder cone to assess the variation in tephra physical and chemical characteristics over time. I present grain size, textural, and geochemical analyses of the SC tephra, including major and minor element variations in matrix glass, olivine phenocrysts, and olivine-hosted melt inclusions collected by EPMA and volatile concentrations in melt inclusions collected by FTIR. I find that the dominant grain size throughout the section is >4 mm, however two units have a smaller average grain size. Even with these variations, grain size distributions and sorting are consistent with a Hawaiian to Strombolian style eruption. Matrix glass compositions in the lower Units 1 through 11 range from 50 to 54 wt.% SiO2, and 3.6 to 5.7 wt.% MgO. Matrix glass at the top of the section, Unit 12, ranges from 54 to 58 wt.% SiO2, and 2.9 to 3.2 wt.% MgO. Based on modeling fractionation of plagioclase, olivine, and clinopyroxene from the bulk basaltic and basaltic andesite lavas, Units 1 through 11 are associated with the basaltic portion of the eruption and Unit 12 is associated with the eruption of the basaltic andesite. Major element and volatile compositions of olivine-hosted melt inclusions are used to determine the evolution of magma prior to eruption and melt inclusion entrapment depths. Melt inclusions in Units 1 through 11 preserve basaltic, basaltic andesite, and andesitic liquids. These liquid variations, mineral textures and chemistry, as well as H2O/K2O models of melt inclusion glass suggest mixing between a long lived dacite mush and a basalt, resulting in a hybrid basaltic andesite. Importantly, this hybrid is preserved in melt inclusions from the opening phase of the eruption, and support a model where mixing and development of a basaltic andesite magma in the upper crust precedes the eruption, which is initiated by a subsequent injection of basaltic magma. Melt inclusion saturation pressures range from 0.77 to 5.9 kbar, suggesting crystallization in the middle to upper crust prior to mixing, during mixing, and during ascent. Using these data, I improve existing models and place new constraints on magma storage for some of the most recently erupted basaltic and basaltic andesite magmas beneath Mount Baker and provide context for future eruptions.