Thesis
The Umatilla Member of the Columbia River Basalt Group: a chemically unusual flood basalt
Washington State University
Master of Science (MS), Washington State University
01/2021
DOI:
https://doi.org/10.7273/000002443
Handle:
https://hdl.handle.net/2376/119252
Abstract
The Umatilla Member of the Columbia River Basalt Group is chemically unusual for a flood basalt. It is classified as a basaltic-trachyandesite to trachyandesite, is enriched in incompatible trace elements, and has the highest Ba concentrations of any unit in the CRBG (2600 – 4700 ppm). New trace element data on the Umatilla Member has identified previously unrecognized unusual ratios of Zr/Hf (~47), Nb/Ta (~17), and Eu/Eu* (1.2 – 1.5). New electron microprobe analysis has revealed Ba-rich plagioclase and alkali feldspars. Additionally, electron microprobe and laser ablation analyses show the glass is not homogeneous and that Ba content is split between plagioclase and glass/groundmass, rather than being found only in alkali feldspar. Wavelength dispersive spectroscopy maps of several plagioclase crystals show Ba concentration decreasing from core to rim of plagioclase feldspars, suggesting Ba decline in the melt during feldspar growth, perhaps due to dilution of a high-Ba contaminant by continuing addition of magma. The combination of high Ba, positive Eu anomalies, Zr and Hf enrichment, high Zr/Hf, and alkaline character suggest the parental magma of the Umatilla Member was assimilating contaminant material enriched in alkali feldspar and zircon, likely before the onset of plagioclase growth. Such material could be a zircon-saturated feldspathic cumulate magma with granitic or syenitic composition. A heterogenous contaminant, and/or a mix of multiple similar contaminants, could be responsible for the contrast between the Umatilla and Sillusi flow chemistries of the Umatilla Member. Possible contamination sources could be linked to nearby silicic volcanism contemporaneous with the Columbia River Basalt Group. Feldspathic cumulates associated with Dinner Creek Tuff rhyolites is a plausible candidate based on Ba-rich feldspars, possible zircon saturation, and cumulates that likely have positive Eu anomalies. Possible parental magma compositions are similar to the Grande Ronde and/or Wanapum Basalts. However, additional modeling of possible contaminants and isotopic analysis of the Umatilla Member could add to further understanding of the Umatilla Member’s unusual nature.
Metrics
26 File views/ downloads
94 Record Views
Details
- Title
- The Umatilla Member of the Columbia River Basalt Group: a chemically unusual flood basalt
- Creators
- Ramsey Nersinger
- Contributors
- John A Wolff (Advisor)Arron Steiner (Committee Member)Scott Boroughs (Committee Member)Victor Camp (Committee Member)Kent Keller (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Environment, School of the (CAHNRS)
- Theses and Dissertations
- Master of Science (MS), Washington State University
- Publisher
- Washington State University
- Number of pages
- 109
- Identifiers
- 99900606854801842
- Language
- English
- Resource Type
- Thesis