Thesis
An oxygen isotope, fluid inclusion, and mineralogy study of the ancient hydrothermal alteration in the Grand Canyon of the Yellowstone River, Yellowstone National Park, Wyoming
Washington State University
Master of Science (MS), Washington State University
2010
Handle:
https://hdl.handle.net/2376/105925
Abstract
The Grand Canyon of the Yellowstone River displays regions of pervasive hydrothermal alteration, formed in the shallow parts of an ancient hydrothermal system. The altered protolith, the 480 ka post-collapse Tuff of Sulphur Creek, is a high silica, low δ18O rhyolite tuff. The localized alteration is controlled by an underlying caldera ring fault from the 640 ka caldera collapse. Incision of the canyon exposed 350 vertical meters of altered rock in the Sevenmile Hole vicinity. The alteration shows evidence of both acid-sulfate and neutral-pH fluid chemistry. There is a kaolinite to illite transition at a depth of ~100 m below the current canyon rim elevation. Boiling of groundwater at depth creates a neutral-pH environment (illite is precipitated), and condensation of rising sulfuric vapors above 100 m creates an acid-sulfate environment (kaolinite is precipitated). At ~50 m, the silica phase precipitated changes from quartz at depth to opal at shallow elevations. Mineralization in the canyon can be divided into seven alteration assemblages based on vertical transitions and proximity to a high heat and fluid upwelling zone. Fluid inclusion homogenization temperatures in quartz samples range from 160° to 350°C. Homogenization temperatures generally increase with depth and are higher than reference boiling point curve temperatures. Additional hydrostatic pressure from a glacial ice sheet with a thickness of <460 m, could account for anomalously high temperatures at shallow depths. Freezing of inclusions yield salinities of 0.35-0.71 wt % NaCl eq. δ18O values of magmatic and hydrothermal quartz were measured for 50 samples using laser fluorination techniques. Values ranged from -5.7‰ to 1.3‰, all of which are more depleted than magmatic quartz in the fresh TSC at 1.7‰. Low salinities and negative δ18O values indicate a dominantly meteoric water source for ancient hydrothermal fluids. The δ18O values of quartz are controlled by the intensity of alteration and the water-rock ratio. The high heat and fluid upwelling zone had the most evolved water-rock ratios and lowest δ18O values, whereas less altered zones had higher δ18O values. Calculated fractionation between quartz and water yields δ18O values ranging from -10.8 to -19.6 ‰ for ancient hydrothermal waters.
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Details
- Title
- An oxygen isotope, fluid inclusion, and mineralogy study of the ancient hydrothermal alteration in the Grand Canyon of the Yellowstone River, Yellowstone National Park, Wyoming
- Creators
- Allison R. Phillips
- Contributors
- Peter B. Larson (Degree Supervisor)
- 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, Wash. :
- Identifiers
- 99900525103801842
- Language
- English
- Resource Type
- Thesis