Petrologic and geochemical constraints on the pre-eruptive storage conditions of magmas erupted during the ~12.5 ka flare up of Medicine Lake Volcano, CA

Author

Regina M. Khoury, Humboldt State UniversityFollow

Graduation Date

Fall 2024

Document Type

Thesis

Program

Master of Science degree with a major in Environmental Systems, option Geology

Committee Chair Name

Brandon Browne

Committee Chair Affiliation

HSU Faculty or Staff

Second Committee Member Name

Melanie Michalak

Second Committee Member Affiliation

HSU Faculty or Staff

Third Committee Member Name

Jacky Baughman

Third Committee Member Affiliation

HSU Faculty or Staff

Keywords

Geology, Medicine Lake Volcano, Petrology, Geochemistry, Volcanology, Cascades, Basin and Range, Shield volcano, Basalt, Magma

Subject Categories

Geology

Abstract

The Medicine Lake Volcano (MLV) system in the California Cascades is composed of a large shield volcano with numerous basaltic to rhyolitic vents on and around the flanks, many of which are aligned with N-S trending normal faults. A significant portion of the post-glacial volcanic activity at MLV is comprised of the ~12.5 ka “flare up” eruptions of basalt and basaltic andesite magmas from seven discrete flank vents. Prior work documented surface areas (0.02-198 km²), volumes (0.0001-4.35 km³), and eruption styles (Hawaiian to Strombolian; Donnelly-Nolan, 2010) of the ~12.5 ka deposits. This study uses new geologic mapping of near-vent features, whole-rock compositions of lavas, spatter and scoria, and in situ elemental concentrations of olivine and CPX phenocrysts to better understand the origins and pre-eruptive storage conditions of the flare-up magmas. Lavas from the ~12.5 ka eruptions are basalt and basaltic-andesite, with whole-rock compositions ranging from 49.3 to 54.2 wt% SiO₂ and 6.0 to 8.5 wt% MgO. Samples from eastern vents, Ribbon Flows and Valentine Cave, form trends that are parallel but offset from central and western lavas. Lavas from eastern vents also have higher concentrations of Na₂O, P₂O₅ and TiO₂ and lower concentrations of MgO, CaO, Al₂O₃ and K₂O/P₂O₅ compared to lavas erupted from vents on the central and western MLV flanks. Lavas and spatter generally contain 1-5% crystals with a mineral assemblage of olivine and plagioclase with less CPX and Fe-Ti oxides, although only a small proportion of olivine and CPX crystals are in chemical equilibrium with whole-rock compositions. Some crystals display disequilibrium textures, such as partially resorbed olivine and normal and reverse-zoned olivine as well as dusty-sieved and coarsely-sieved plagioclase. Most olivine crystals have core compositions of >Fo₈₀, but core regions of olivine crystals in deposits erupted from all flare-up vents range from to Fo₆₆ to Fo₈₈. Compositions of CPX crystals in lavas and spatter erupted from eastern vents are consistent with shallower crystallization pressures compared to lavas and spatter erupted from central and western vents. Geochemical and petrological results are consistent with a ~12.5 ka flare up that tapped multiple magma systems beneath MLV, whereas deeper TiO₂-rich magmas erupted from vents in the east compared to shallower MgO-rich magmas erupted by western and central vents.

Citation Style

GSA (Geologic Society of America)

Recommended Citation

Khoury, Regina M., "Petrologic and geochemical constraints on the pre-eruptive storage conditions of magmas erupted during the ~12.5 ka flare up of Medicine Lake Volcano, CA" (2024). Cal Poly Humboldt theses and projects. 792.
https://digitalcommons.humboldt.edu/etd/792