HR: 0800h
AN: GP11B-0829 INVITED     [Abstracts]
TI: Structural and Temporal Requirements for Geomagnetic Field Reversal Deduced From 40Ar/39Ar Dated Lava Flows
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: Hoffman, K A
EM: khoffman@calpoly.edu
AF: Physics Department, California Polytechnic University, San Luis Obispo, CA 93407 United States
AU: Coe, R S
EM: rcoe@es.ucsc.edu
AF: Earth Sciences Department, University of California, Santa Cruz, CA 95064 United States
AU: Brown, L L
EM: lbrown@geo.umass.edu
AF: Dept. of Geosciences, University of Massachussetts, Amherst, MA 01003 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: Pringle, M S
EM: mpringle@MIT.EDU
AF: Dept. of Earth Atmosphere, and Planetary Sciences, MIT, Cambridge, MA 02139 United States
AU: Chauvin, A
EM: Annick.Chauvin@univ-rennes1.fr
AF: Geosciences Rennes UMR-CNRS, Campus Beaulieu, Rennes, 35042 France
AB: 40Ar/39Ar dating of lavas on Tahiti, long thought to record the primary part of the Matuyama-Bruhnes (M-B) reversa1, gives an age of 795+/- 7 ka, indistinguishable from that of transitional lavas in Chile and La Palma, but older than the accepted age for the reversal. Only the transitional lavas on Maui and one from La Palma (dated at 776 +/- 2 ka), agree with the astronomical age for the M-B reversal. Virtual geomagnetic poles (VGPs) associated with the Tahitian and Chilean lavas cluster near Australia, as do VGPs recorded on Tahiti during the Big Lost and Punaruu events, two apparently unsuccessful reversals. These findings, suggestive of a recurring, mantle-held flux pattern at the outer core surface during reversal attempts, are also theoretically equivalent to the situation that would arise today if the axial dipole were to continue to weaken and vanish. Hence, we propose that the 795 ka lavas record the onset of a dynamo process--one which only on occasion would result in polarity change. This initial instability, associated with the first of two decreases in field intensity, began 18 kyrs prior to the actual polarity switch. These data may provide the first observational support to the claim that complete reversals require a significant interval of time for magnetic flux to escape from the solid inner core and sufficiently weaken its stabilizing effect.
DE: 1035 Geochronology
DE: 1507 Core processes (8115)
DE: 1521 Paleointensity
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting