HR: 0800h
AN: GP21A-0019    [Abstracts]
TI: An ⁴⁰Ar/³⁹Ar age for Geomagnetic Instability Recorded at the Albuquerque Volcanoes and Pringle Falls, Oregon
AU: * Singer, B S
EM: bsinger@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison 1215 W. Dayton St., Madison, WI 53706 United States
AU: Jicha, B R
EM: bjicha@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison 1215 W. Dayton St., Madison, WI 53706 United States
AU: Kirby, B T
EM: btkirby@students.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison 1215 W. Dayton St., Madison, WI 53706 United States
AU: Zhang, X
EM: xzhang@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison 1215 W. Dayton St., Madison, WI 53706 United States
AU: Geissman, J W
EM: jgeiss@unm.edu
AF: Department of Earth and Planetary Sciences, University of New Mexico 203 Northrup Hall, Albuquerque, NM 87131 United States
AU: Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: Hawaii Institute of Geophysics, University of Hawaii at Manoa 2525 Correa Rd., Honolulu, HI 96822 United States
AB: The timing and frequency of short-lived geomagnetic events, including excursions and aborted reversal attempts, provide important observational constraints on models of geodynamo behavior as well as calibration points for stratigraphic and paleoclimatic age models derived from marine sediments. The number of potential geomagnetic events during the Quaternary period has proliferated as more detailed paleodirectional and paleointensity data have emerged from sediments worldwide. Yet, determining ages for these events remains a challenge because astronomical dating of sediment cores is subject to assumptions and non-systematic errors that are difficult to quantify and vary from core to core. ⁴⁰Ar/³⁹Ar geochronology applied to lava flows or ash falls that record geomagnetic excursions can yield radioisotopic ages sufficiently precise to verify individual events, test correlations among seemingly disparate events, and quantify periods between events. Seventeen furnace incremental heating experiments on 100-200 mg groundmass samples from six sites in the transitionally magnetized basalt of the Albuquerque Volcanoes yield an isochron of 211 ± 22 ka* that is within error of previous K-Ar (155 ± 94 ka) and U-Th isochron (156 ± 58 ka) age determinations, but is 3 to 4 times more precise. At Pringle Falls, Oregon, Ash D was deposited during the onset of an excursion recorded by a lacustrine sediment sequence. Sixteen laser incremental heating experiments on 20-40 mg samples of plagioclase crystals from Ash D gave 64 concordant plateau age points that define an ⁴⁰Ar/³⁹Ar isochron of 211 ± 13 ka which is an order of magnitude more precise than the isochron (198 ± 118 ka) associated with the published plateau age from a single plagioclase age spectrum (221 ± 20 ka). Although the Virtual Geomagnetic Pole (VGP) recorded by the Albuquerque Volcanoes lies near, bot not on, the VGP path of the Pringle Falls excursion, these two sites are 15 arc degrees apart and need not record identical directions for the same high-amplitude, short-lived event. The common and precise ⁴⁰Ar/³⁹Ar isochron age from the two sites (weighted mean 211 ± 11 ka) provides strong evidence that a single period of geodynamo instability is recorded. Marine sediments at several localities world wide reveal a prominent set of anomalous inclinations or low intensities variously designated as the Jamaica or Iceland Basin event. Astronomical ages for this event vary, but are mainly between about 190 and 200 ka. Thus, we infer that the new radioisotopic age of 211 ± 11 ka marks the onset of a globally synchronous period of geodynamo instability including directional shifts and weakening of the field that, depending on the veracity of the astronomical dating, persisted until about 190 ka. *all uncertainties ± 2 sigma; argon ages relative to 28.34 Ma Taylor Creek Rhyolite sanidine.
DE: 1115 Radioisotope geochronology
DE: 1520 Magnetostratigraphy
DE: 1535 Reversals: process, timescale, magnetostratigraphy
DE: 1560 Time variations: secular and longer
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005