Dissertation/Thesis Abstract

Compositional and Physical Gradients in the Magmas of the Devine Canyon Tuff, Eastern Oregon: Constraints for Evolution Models of Voluminous High-silica Rhyolites
by Isom, Shelby Lee, M.S., Portland State University, 2017, 160; 10606618
Abstract (Summary)

Large-volume silicic ignimbrites erupt from reservoirs that vary in composition, temperature, volatile content and crystallinity. The 9.7 Ma Devine Canyon Tuff (DCT) of eastern Oregon is a large-volume (>250 km3), compositionally zoned and variably welded ignimbrite. The ignimbrite exhibits heterogeneous trace element compositions, variable volatile content and crystallinity. These observations were utilized in the investigation into the generation, accumulation and evolution of the magmas composing the DCT. Building off previous research, pumices were selected from the range of trace element compositions and analyzed with respect to crystallinity, mineral abundances and assemblages. The DCT displays a gradational trace element enrichment and decrease in crystallinity from least evolved, dacite, at ∼22% crystals to most evolved high-silica rhyolite at 3% crystals. Two distinct mineral populations of feldspar and clinopyroxene were identified in previous work, one belonging to the rhyolitic magma and the other to the dacitic magma. Volatile content derived from melt inclusion Fourier Transform Infrared (FTIR) spectrometer analysis revealed an increase in water content from 1.2 to 3.7 wt.% in the most evolved rhyolite. The DCT exhibits low and variable δ18 O signatures, 4.52‰ to 5.76‰, based on δ 18O values measured on quartz and sanidine. Low δ18O signatures of all DCT rhyolites suggest the incorporation of hydrothermally altered crust into the melt. Furthermore, quartz phenocrysts from all high-silica rhyolite groups display dark oscillatory zoned cores and Ti-rich bright rims.

These data provide insight into how these magmas were generated and subsequently stored in the crust. Commonalities of petrographic and compositional features among rhyolites, especially the zoning characteristics of quartz phenocrysts, exclude the possibility of storage and evolution in multiple reservoirs. Envisioning a scenario where all magmas are stored within a single reservoir prior to eruption and assuming rhyolites A and D are the product of partial melting. The mixing of A and D rhyolites produced rhyolite B, and subsequent mixing of intermediate rhyolite B and end-member rhyolite D generated rhyolite C. However, some trace element inconsistencies, between mixing model and observed intermediate rhyolites suggest a secondary process. Post mixing, rhyolites B and C require some modification by fractional crystallization to account for LREE and other inconsistencies between mixed models and observed rhyolites. Finally, the origin of the dacite is likely through mixing of group D rhyolite and an intrusive fractionated basalt, which could have led to the eruption of the Devine Canyon Tuff.

Indexing (document details)
Advisor: Streck, Martin J.
Commitee: Bershaw, John, Grunder, Anita L., Streig, Ashley R.
School: Portland State University
Department: Geology
School Location: United States -- Oregon
Source: MAI 57/01M(E), Masters Abstracts International
Subjects: Geology
Keywords: Devine canyon tuff, Eastern oregon, High lava plains, High-silica rhyolite, Magma model, Zoned ash-flow tuff
Publication Number: 10606618
ISBN: 978-0-355-36788-1
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