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Cascade Arc, Mount Baker, Koma Kulshan, Volcanology, Igneous Petrology, Geochemistry, Volcanic ash


Koma Kulshan (Mount Baker) is an active stratovolcano in the northern Washington Cascades. Kulshan’s most recent magmatic eruption at 6.7 ka was explosive, producing the ~0.2 km3 BA tephra (black ash) from the edifice (Scott et al. 2019). Comprehensive geochemical data for the BA tephra were previously limited to major elements from one whole rock lapillus (silicic andesite) and several in situ glass analyses (dacite), despite being Kulshan’s most voluminous Holocene tephra. Here, I present the first extensive major and trace element study of the pyroxene- and plagioclase-bearing BA tephra glass to determine magma source and eruption processes. My goal was to test whether the BA tephra magma was generated by rejuvenation of the source of the older, ~9.8 ka flank eruption that produced the 10-km-long zoned Sulphur Creek basalt/basaltic andesite lava flow. That zoned flow was derived from low-Mg basaltic magmas that mixed with a long-lived and laterally extensive upper crustal dacite mush (Garvey, 2022). I used 15 widely spaced BA tephra samples to first characterize the major and trace element variation of tephra glass (via SEM and LAICPMS) and compared those data with the silicic endmember of the Sulphur Creek lava flow. I found that the BA tephra glass is dacitic with little compositional variation (66-70% SiO2), is high-K calc-alkaline, and has relatively steep REE patterns (La/Yb 8.5-11) with a strong negative Eu-anomaly. These characteristics and other trace element ratios such as Ba/La and Sr/Y suggest that the BA tephra is more closely related to Koma Kulshan’s late Pleistocene H2O-rich high-Mg basaltic andesite (HMBA) series than the low-Mg Holocene zoned lava flow. These results are important because they suggest that potentially explosive HMBA is the dominant magma composition that feeds the main Kulshan edifice, though it is not found in similar-aged flank eruptions. This suggests that two distinct but contemporaneous magma systems are feeding the volcanic field in close proximity. Further work will use phenocryst barometry and phase equilibria to constrain the depth of the BA tephra magma source which is critical for interpreting subvolcanic architecture of the magmatic plumbing system.


This paper is Stone Machel's Senior Thesis for the Department of Geology, Western Washington University. Stone's advisor was Dr. Susan DeBari.






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