HR: 1330h
AN: V32D-1051    [PDF]
TI: 31$\pm$17 ka $^{40}$Ar/$^{39}$Ar Plateau Age on the Very Low-Potassium Hat Creek Basalt Fits Stratigraphic and Geochronologic Constraints of Contiguous Units
AU: * Clynne, M A
EM: mclynne@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025-3591 United States
AU: Turrin, B D
EM: bturrin@rci.rutgers.edu
AF: Rutgers University, Department of Geological Sciences, Piscataway, NJ 08854 United States
AU: Champion, D E
EM: dchamp@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025-3591 United States
AU: Muffler, L P
EM: pmuffler@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025-3591 United States
AB: The Hat Creek Basalt is a late Pleistocene low-K olivine tholeiite that floors much of the Hat Creek Valley, north of the Lassen volcanic highland in NE California. The flow erupted from a fissure trending N 10$\deg$ W located 0.5 to 4.0 km SSE of Old Station and flowed north nearly 30 km, mainly through lava tubes. Paleomagnetic data indicate that the Hat Creek Basalt is a single eruptive unit with an inclination of 64.3$\deg$ and a declination of 1.0$\deg$. The Hat Creek Basalt is overlain by gravel related to the last major glaciation, which ended about 15 to 17 ky ago. The Hat Creek Basalt overlies numerous andesite and basaltic andesite lava flows that are closely related geographically, stratigraphically, petrographically and geochemically and probably represent the same eruptive episode (although not necessarily the same year or century). Three of these flows have been dated by the $^{40}$Ar/$^{39}$Ar method and yield concordant plateau, integrated (total fusion), and isochron ages: Little Potato Butte (1.42% K$_{2}$O; 67$\pm$4 ka), Sugarloaf (1.85% K$_{2}$O; 55$\pm$7 ka), and Big Potato Butte (1.62% K$_{2}$O; 73$\pm$14 ka). The Hat Creek Basalt also overlies the basaltic andesite of Cinder Butte (1.65% K$_{2}$O) with a $^{40}$Ar/$^{39}$Ar plateau age of 38$\pm$7 ka. Given that the age of the Hat Creek Basalt is thus firmly bracketed between $\sim$15 ka and 38$\pm$7 ka, we attempted to date this low-potassium tholeiitic basalt directly. One sample produced $^{40}$Ar/$^{39}$Ar data that cannot be interpreted, but a second sample (0.17% K$_{2}$O) yielded concordant plateau, integrated (total fusion), and isochron ages of 31$\pm$17 ka, 30$\pm$20 ka, and 24$\pm$6 ka; within the time bracket determined by stratigraphic relations. These data indicate that samples of latest Pleistocene tholeiitic basalt with very low K$_{2}$O can give useful $^{40}$Ar/$^{39}$Ar ages. As with all isotopically determined ages, the confidence of the results are significantly enhanced when additional constraints imposed by other isotopic ages within a stratigraphic context are taken into account.
DE: 8494 Instruments and techniques
DE: 9350 North America
DE: 9604 Cenozoic
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting