Thesis
The central Snake River Plain low [symbol]18O rhyolite province: crustal melting processess
Washington State University
Master of Science (MS), Washington State University
2017
Handle:
https://hdl.handle.net/2376/100642
Abstract
The Snake River Plain (SRP) is a topographic low stretching across southern ID, recording time transgressive Yellowstone hotspot volcanism to the northeast. The discovery of the central SRP low [symbol]18O rhyolite province has exposed a fundamental gap in understanding of crustal melting processes in the region. A low [symbol]18O signature in rhyolite is not unusual, however the large quantity (~1.4 x 104 km3) in the central SRP is abnormal. Previous studies have proposed two formation models for the rhyolite: recycling of intracaldera hydrothermally-altered material and melting of previously altered protolith. This study characterizes potential low [symbol]18O protolith chemistry and proposes melting conditions to produce the rhyolite. Proposed protolith includes the Idaho batholith (Cretaceous) and associated volcanic rocks (Eocene) that stretch north-south across, and presumably beneath, the SRP. New sampling and analysis was conducted south of the SRP to complement prior data available from the northern side. Geochemical results were used to compare proposed protolith and low [symbol]18O central SRP rhyolite. Trace and REE geochemistry comparisons determined that Eocene and Cretaceous rocks are both potentially viable protoliths. Batch melting and fractional crystallization calculations using these results suggest protolith melting of ~40-80% could match central SRP rhyolite chemistry. Major element chemistry comparisons were conducted using the rhyolite-MELTS software package to model protolith melting. Melting regimes between 2-10 Kbar, 600-1200oC, and variable magma water contents (1-3 wt%) at f02 = QFM (i.e. a full range of plausible conditions) were conducted. Modeling results suggest incomplete melting of either Eocene or Cretaceous rock at pressures of 3-4 Kbar, 800-900oC, and medium to high water contents (2-3 wt%) can reproduce the bulk chemistry of the rhyolite. Oxygen isotope measurements reveal low [symbol]18O values for protolith north of the central SRP, but mostly normal [symbol]18O values to the south. However, evidence suggesting high temperature alteration also exists in the southern rocks. In all assessments, Eocene rock proved a better match with the central SRP rhyolite than Cretaceous rock. This study suggests Eocene rock is a plausible protolith for the pre-existing protolith model, but both formation models are still pertinent.
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Details
- Title
- The central Snake River Plain low [symbol]18O rhyolite province
- Creators
- Caleb Evans Birchard
- Contributors
- John Wolff (Chair)Scott Boroughs (Committee Member) - Washington State University, Environment, School of the (CAS)Peter B. Larson (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; [Pullman, Washington] :
- Identifiers
- 99900525161401842
- Language
- English
- Resource Type
- Thesis