HR: 1340h
AN: V53B-1321    [Abstracts]
TI: New ⁴⁰Ar/³⁹Ar age Determinations of Basalts From the Yellowstone Plateau Volcanic Field
AU: * Abedini, A A
EM: aabedini@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025,
AU: Calvert, A T
EM: acalvert@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025,
AU: Hurwitz, S
EM: shaulh@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025,
AB: The Yellowstone Plateau volcanic field is one of the largest and most active silicic volcanic systems in the world and is the youngest manifestation of the Snake River Plain-Yellowstone hotspot track of bimodal basaltic-rhyolitic volcanism. Yellowstone's volcanic history is marked by three cataclysmic caldera-forming eruptions at 2.05, 1.29, and 0.64 Ma. Each volcanic cycle has been interpreted to result from the accumulation, storage, and differentiation of discrete silicic magma reservoirs. The latest cycle, forming the 2900-km² Yellowstone caldera was followed by at least 30 large rhyolite flows as young as 70,000 years old [Christiansen, 2001]. Coeval basalts are exposed around the margins of the Yellowstone Plateau, mainly north of Yellowstone caldera and in the Island Park area southwest of Yellowstone caldera. The total estimated volume of basalts in the volcanic field is about 250 km³. The basalts of the field are generally low-potassium olivine tholeiites; most have plagioclase and olivine phenocrysts and all contain olivine, clinopyroxene, plagioclase, and iron-titanium oxides in the groundmass. Basalt chemistry ranges from 46 to 50 wt% SiO2 and 0.3 to 0.5 wt% K2O. A more quantitative understanding of the relation between basaltic volcanism and Yellowstone's rhyolitic cycles is hampered by a lack of high-precision geochronology. Previous age determinations of Yellowstone basalts were based on stratigraphy, paleomagnetic polarity, and K/Ar dating, and two ⁴⁰Ar/³⁹Ar ages of basalt flows coeval with the third cycle (Smith and Bennett, 2006). We present 12 new ⁴⁰Ar/³⁹Ar ages representing seven different basalt units (Christiansen, 2001). Samples were selected based on their lack of alteration, groundmass texture, and potassium concentrations. Age determinations were made on separated crystalline groundmass and are a mixture of plateau and isochron ages. The new ⁴⁰Ar/³⁹Ar ages (1 σ errors) are 835±17 ka for basalt of Warm River, 650±9 ka for Undine Falls basalt, 743±5 ka and 725±25 ka for Snake River basalt, 380±30 ka for Madison River basalt, 260±12 ka for Osprey basalt, 254±12 ka to 108±15 ka (n=4) for Swan Lake Flat Basalt, and 151±7 ka for Gerrit basalt. These new age determinations provide more rigorous constraints on Yellowstone's volcanic history, and can be incorporated into assessments of Yellowstone's volcanic hazards. Christiansen, U.S. Geol. Surv. Prof. Pap. 729-G, 2001, 145 p. Smith and Bennett, Yellowstone Science, 2006, 14, 5-12.
DE: 1105 Quaternary geochronology
DE: 3615 Intra-plate processes (1033, 8415)
DE: 3641 Extrusive structures and rocks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting