Dissertation
Petrogenesis of the Columbia River Basalts: Crustal Processing and Mantle Source Lithologies
Washington State University
Doctor of Philosophy (PhD), Washington State University
05/2025
DOI:
https://doi.org/10.7273/000007472
Abstract
The mid-Miocene Columbia River Basalt Group (CRBG), a part of the broader Yellowstone Hotspot province in the northwestern United States, is the best studied continental flood basalt province in the world. Ongoing study of the CRBG nonetheless continues to offer a more thorough understanding of the origin and evolution of flood basalt provinces worldwide. To that end, I focus on the buildup and peak of eruptive activity for the group, which is represented by the Imnaha and Grande Ronde Basalts. These two formations share close physical and geochemical associations, suggesting closely related magma systems or different phases of a shared magma system. I investigate the mantle source lithologies for lavas from the Imnaha, Steens and Picture Gorge Basalts, which contain the most primitive lavas in the CRBG. These lavas exhibit trace element enrichments and depletions, correlated with highly variable Ni contents in olivine phenocrysts, that vary systematically by formation and lava chemical type. The variations are attributed to a range of mantle source lithologies from peridotite to pyroxenite. Individual Imnaha lavas each contain distinct Ni signatures, recorded in olivine, that preserve signals imparted at the source, indicating that the Imnaha magma system existed as a network of largely isolated dikes and sills. I carry out thermodynamic simulations which confirm an earlier hypothesis for the origin of the Grande Ronde Basalt by assimilation and fractional crystallization while imposing additional conditions, particularly wallrock temperatures elevated well above a normal geothermal gradient, and a major role for bulk assimilation (stoping). The conditions fit well within a model of a dike-and-sill network that, after repeated additions of heat and basalt melt, coalesces into an integrated magma system. I estimate the magmatic conditions for the Imanha and early Grande Ronde Basalts as recorded by olivine, pyroxene, and plagioclase using an array of mineral-liquid thermometers, barometers and hygrometers in conjunction with a large geochemical database for the CRBG. Pyroxene-based crystallization pressures are dominantly <4 kbar, suggesting mid- to shallow-crustal storage conditions, consistent with earlier estimates and the thermodynamic simulations. Plagioclase-based estimates suggest Imnaha and Grande Ronde magmas were relatively water poor (<1 wt. %), whereas olivine-based estimates suggest higher water contents, up to 4.72 wt. %. While primitive water rich magmas may have existed deeper in the system, consistency between pyroxene and
plagioclase results argue for water-poor magmas during pre-eruptive storage.
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Details
- Title
- Petrogenesis of the Columbia River Basalts
- Creators
- Evan Soderberg
- Contributors
- John A. Wolff (Chair)Victor E. Camp (Committee Member)Johannes Hämmerli (Committee Member)Sean P. Long (Committee Member)Arron Steiner (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- School of the Environment (CAHNRS)
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Publisher
- Washington State University
- Number of pages
- 227
- Identifiers
- 99901221151101842
- Language
- English
- Resource Type
- Dissertation