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Date Permissions Signed

8-1-2022

Date of Award

Summer 2022

Document Type

Masters Thesis

Department or Program Affiliation

Geology

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

DeBari, Susan M., 1962-

Second Advisor

Walowski, Kristina

Third Advisor

Shamloo, Hannah I.

Fourth Advisor

Sas, Mai

Abstract

Koma Kulshan (Mt. Baker) is classified as a high-threat volcano due to its past eruptive history and its proximity to populations, yet its eruptive products are understudied. Combining mineral chemistry from complexly zoned crystals with thermobarometry and thermodynamic modeling (MELTS) is a powerful way to provide constraints on magmatic processes beneath an active volcano. Up to four populations each of pyroxene, plagioclase, and olivine occur as phenocrysts and crystal clusters in the youngest (9.8 ka) lava flow at Koma Kulshan and represent four co-crystallizing assemblages derived from distinct magmas. These co-crystallizing assemblages are defined by petrologic observations and mineral chemistry (An, Fo, Mg#, trace elements). The host lava flow (Sulphur Creek) is composed of two segments: a basalt at the flow toe and a basaltic andesite towards the flow source. The basaltic segment carries three co-crystallizing assemblages; in order of abundance, they are derived from a) the host calc-alkaline basalt (designated as the B3 assemblage), b) a dacite (designated as the D1 assemblage), and c) a high-Mg basalt (designated as the B2 assemblage). The textures and compositions of the D1 and B2 co-crystallizing assemblages suggest entrainment in the host basalt. The up-flow basaltic andesite hosts a distinct native basaltic andesite assemblage (designated as the BA3 assemblage), plus cryptic evidence of the D1 and B2 assemblages. These assemblages, coupled with observed mingling textures in this basaltic andesite part of the lava flow, suggest more-extensive mixing of the magmas that generated the B3 and D1 assemblages to generate a hybridized basaltic andesite magma (parental to the BA3 assemblage). The D1 and B2 assemblages have been previously identified in lavas flows that span the last ~120 ka of eruptive history at Koma Kulshan, suggestive of the presence of long-lived crystal mushes beneath this volcano. Clinopyroxene thermobarometry and olivine thermometry combined with Rhyolite-MELTS models provide quantitative bounds for the storage condition of the native (to the host basalt) and entrained assemblages. The B3 assemblage, native to the basaltic segment, crystallized shallowly at 0-1.5 kbar. The BA3 assemblage, native to the basaltic andesite segment, displays evidence for three distinct episodes of cooling and crystallization within the crust at 0-1.5 kbar, ~2.5 kbar, and ~5 kbar. Storage conditions of the B2 entrained assemblage are constrained from Escobar (2016) and Escobar et al. (in prep.) at 5.8 kbar (22 km). Storage conditions of the D1 assemblage is also unconstrained; however, textural observations suggest the D1 crystal mush is at or below 18 km. These data require a two-stage history for eruption of the Sulphur Creek lava flow with open-system processes culminating in eruption of two distinct magma pulses. The first stage is an early pulse derived from a shallow basaltic system (3.4 km; B3), and the second stage is a later pulse derived from more complex multi-crustal-level mixing of the same basalt (B3) and a dacite (D1) at 3.4 km, 10 km, and 18 km to produce a hybridized basaltic andesite (BA3). This study elucidates magmatic processes occurring in the upper 18 km of a 45-km thick crust; however, the parental melts that drive these processes are generated deeper within and at the base of the crust. This study highlights the instability and reorientation of crystal mushes within arcs and the production of andesites through long lived dacitic reservoirs mixing with rejuvenating mafic magmas.

Type

Text

Keywords

Cascade Arc, Mt. Baker, Koma Kulshan, Volcanology, Thermobarometry

Publisher

Western Washington University

OCLC Number

1340048189

Subject – LCSH

Volcanology--Washington (State)--Baker, Mount; Atmospheric pressure--Washington (State)--Baker, Mount--Mathematical models; Atmospheric thermodynamics--Washington (State)--Baker, Mount--Mathematical models; Magmatism--Washington (State)--Baker, Mount; Crystallography

Geographic Coverage

Baker, Mount (Wash.)

Format

application/pdf

Genre/Form

masters theses

Language

English

Rights

Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.

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