Stuck in the Middle – Dacitic Eruptions through Time at Three Sisters, Oregon

Author

Alex Newsom, Western Washington UniversityFollow

Date of Award

Fall 2024

Document Type

Masters Thesis

Department or Program Affiliation

Geology

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Sas, Mai

Second Advisor

Andersen, Nathan L.

Third Advisor

Mulcahy, Sean

Abstract

The Three Sisters Volcanic Complex (TSVC) is a locus of mafic-to-silicic volcanism located in the central Oregon Cascades (USA). Amongst the many eruptive conduits of the TSVC, the three main stratocones are North Sister (primarily mafic, >50 ka), Middle Sister (primarily mafic-intermediate, 50-14 ka), and South Sister (primarily intermediate-felsic, 40-2 ka). Middle Sister initially erupted a suite of mafic to silicic lavas before experiencing a 10-ka lull, during which South Sister entered a period of frequent eruptive activity. As South Sister eruptions declined, Middle Sister experienced renewed volcanism, erupting dominantly andesites before shifting to a bimodal distribution of dacites and basaltic andesites. To address the current knowledge gap regarding in-situ mineral geochemical investigation of Middle Sister dacites, this study focuses on three dacites that span the entire eruptive history of Middle Sister: dacite of Linton Creek (dlc, ~37 ka); dacite north of Separation Creek (dsn, ~19 ka); and dacite of Irving Glacier (dig, ~14 ka). Compositional data, textural observations, and estimated intensive parameters suggest that the oldest known TSVC dacite, dlc, comprises multiple magmatic components while the younger dacites (dsn and dig) are largely homogeneous and may have formed via partial melting and fractional crystallization. The crystal cargo of dlc comprises multiple populations of plagioclase, clinopyroxene, and orthopyroxene, while also hosting olivine crystals that share South Sister compositional trends and amphibole pseudomorphs indicative of a South Sister magmatic origin. Both dsn and dig have singular populations of plagioclase, clinopyroxene, and orthopyroxene, and follow Middle Sister compositional trends.

Type

Text

Keywords

Middle Sister, igneous petrology, plagioclase, pyroxene, geochemistry, trace element, geothermobarometry, transcrustal magmatic system, petrogenesis

Publisher

Western Washington University

OCLC Number

1478427790

Subject – LCSH

Volcanic eruptions--Oregon--Three Sisters

Geographic Coverage

Three Sisters (Or.)

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.

Recommended Citation

Newsom, Alex, "Stuck in the Middle – Dacitic Eruptions through Time at Three Sisters, Oregon" (2024). WWU Graduate School Collection. 1346.
https://cedar.wwu.edu/wwuet/1346