HR: 11:55h
AN: V52C-07    [Abstracts]
TI: Distribution of Hydrothermal Mineral Assemblages in the Sevenmile Hole Area, Grand Canyon of the Yellowstone River, Yellowstone National Park, Wyoming
AU: * Phillips, A
EM: alliephillips5@gmail.com
AF: Washington State University, School of Earth and Environmental Sciences, Pullman, WA 99164-2812, United States
AU: Larson, P
EM: plarson@wsu.edu
AF: Washington State University, School of Earth and Environmental Sciences, Pullman, WA 99164-2812, United States
AU: John, D
EM: djohn@usgs.gov
AF: United States Geological Survey, MS-901 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Cosca, M
EM: Michael.Cosca@unil.ch
AF: University of Lausanne, Institute of Mineralogy and Geochemistry BFSH-2, Lausanne, 1015, Switzerland
AU: Pauley, B
EM: bmpauley@mail.wsu.edu
AF: Washington State University, School of Earth and Environmental Sciences, Pullman, WA 99164-2812, United States
AU: Manion, J
EM: paleojen@uga.edu
AF: Washington State University, School of Earth and Environmental Sciences, Pullman, WA 99164-2812, United States
AU: Pritchard, C
EM: cpritchard@wsu.edu
AF: Washington State University, School of Earth and Environmental Sciences, Pullman, WA 99164-2812, United States
AU: Andersen, A
EM: a@ndersenonline.com
AF: Washington State University, School of Earth and Environmental Sciences, Pullman, WA 99164-2812, United States
AB: Incision of the Grand Canyon of the Yellowstone River in Yellowstone National Park has exposed approximately 350 vertical meters of hydrothermally altered rhyolites. This older alteration formed in the shallow portion of a hydrothermal system that was most likely similar to the modern Yellowstone hydrothermal environment. Hydrothermal fluid circulation is related to the ongoing rhyolitic magmatism that produced the Yellowstone caldera at 640 ka. The rhyolitic magmatism and hydrothermal system are shallow expressions of deeper mantle- derived basalts. The older alteration is well exposed in the Sevenmile Hole area, near the northeastern margin of the caldera. Here, the alteration protolith is the high silica, low-¹⁸O, rhyolitic Tuff of Sulfur Creek. The tuff erupted at about 480 ka after resurgent doming associated with the third cycle collapse of the Yellowstone caldera. The tuff is a rheomorphically deformed densely welded agglutinate fallout ash that was deposited along the caldera wall. It contains phenocrysts of quartz, sodic plagioclase, and potassium feldspar. The tuff is exposed from the rim of the canyon, which is very close to the pre-alteration paleosurface, to the river bottom where it is covered by detrital sediments and actively forming hot spring deposits. Rocks exposed within the field area are pervasively hydrothermally altered. Mineral phases in approximately 90 samples were determined in the field using a Portable Infrared Mineral Analyser (PIMA). Subsequently, more precise mineral determinations were made using standard petrographic and powder XRD techniques. The alteration mineralogy consists of variable assemblages that include zones of kaolinite + opal; kaolinite + alunite with local dickite and typically high opal and/or quartz concentrations; highly silicified zones containing illite with or without smectite; and weakly silicified zones containing mostly illite. Minor (less than 1 percent) fine-grained disseminated pyrite is ubiquitous. The alteration is predominantly an advanced argillic, acid sulfate assemblage. Distribution of clay minerals may indicate a vertical temperature gradient related to depth below the paleosurface. Kaolinite and opal tend to occur along and just below the canyon rim. Montmorillonite (smectite) is the dominant clay mineral at intermediate depths or in areas suspected to be near zones of higher heat flow. Illite is the most common clay in the deepest exposures, and also at intermediate depths along a strongly silicified ridge that locally contains vuggy quartz alteration and hydrothermal breccias. The ridge is believed to be a local center of higher temperature fluid upwelling. This hydrothermal environment was most likely produced by deeper seated vapors that rose and mixed with shallow surface meteoric waters. A single sample of alunite yielded an ⁴⁰Ar/³⁹Ar age of 150 ka for the Sevenmile Hole altered area. It is unknown if the hydrothermal system in this area has been continuously or intermittently active since caldera collapse. The distribution of clay and other hydrothermal minerals preserved in the Grand Canyon walls suggest patterns in temperature and zonation that can be applied to the interpretation of the temporal evolution of the active hot spring systems in Yellowstone National Park.
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 8415 Intra-plate processes (1033, 3615)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting