127-6 Controls on eruptive style in W-set Tephra Deposits at Mount St. Helens

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session

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Pre-eruptive H₂O content reflecting shallow magma ascent impacts eruptive behavior as it is associated with shifts in eruption styles and intensity. This study uses plagioclase-liquid hygrometry to examine pre-eruptive H₂O of W-set tephra deposits from three explosive eruptions (Wn, Wb, and Wd; 1479-1482 CE) during the Kalama period at Mount St. Helens, WA. We aim to assess the role of volatiles and shallow conduit storage pressures in driving shifts in eruption styles within the sequence. The initial Wn eruption was large, followed by a dacitic dome which was later destroyed and preserved in block and ash flow and dense clasts in tephra deposits. Wb and Wd were smaller explosive eruptions. Differences in pre-eruptive H₂O content from final shallow stalling could explain these shifts in eruption style and intensity preserved by the W-set tephra deposits, which are otherwise chemically similar.

Electron probe micro-analysis of zoned plagioclase crystals across all three W-set tephra eruptions shows a wide range of anorthite compositions in cores (An_29-79), and narrower anorthite range (An_35-49) in rims, indicating that W-set tephra eruptions were sourced from similar final reservoir compositions. Melt inclusion and glass data reveal no SiO₂, K₂O, Na₂O trends with eruption style, indicating geochemistry is not a main driver of style shifts. Preliminary hygrometer calculations show an overlap in H₂O content during final stalling between the first major explosive (Wn) and the second smaller explosive eruption (Wb). The third and final smaller explosive eruption (Wd) had a distinctly lower H₂O content. H₂O solubility indicates that magma from the Wn and Wb eruptions equilibrated ~0.5 km deeper in the shallow crust than the Wd eruption. These trends are supported by the prevalence of amphibole reaction rims in Wd samples, which are missing in Wn and Wb samples. Wn and Wb pumice instead contain dissolved amphiboles, which could indicate a deeper equilibration depth. These results confirm that differences in pre-eruptive H₂O content from final stalling can only partially explain shifts in eruption behavior. Additional W-set tephra samples will be studied from the second major explosive eruption (We) and the first smaller explosive eruption (Wa) to confirm these preliminary trends and quantify more of their volatile contents. These results have implications for forecasting changes in eruption style at dome-forming volcanoes and at Mount St. Helens.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10495

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