HR: 14:10h
AN: GP32A-03 INVITED     [PDF]
TI: Paleointensity Distributions and the Early Geodynamo
AU: * Selkin, P A
EM: pselkin@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0220 United States
AU: Gee, J S
EM: jsgee@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0220 United States
AU: Meurer, W P
EM: wpmeurer@mail.uh.edu
AF: Department of Geosciences, University of Houston, 312 Science & Research Bldg. 1, Houston, TX 77204-5007 United States
AU: Tauxe, L
EM: ltauxe@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0220 United States
AU: Constable, C G
EM: cconstable@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0220 United States
AB: Sharp increases in geomagnetic intensity are sometimes cited as evidence for the onset of ancient geodynamo activity. High-quality paleointensity measurements, however, do not reveal any drastic increases in the average intensity of the geomagnetic field over the past $\sim$2.8 Ga. It is therefore necessary to examine other properties of the Precambrian geomagnetic field. We present Thellier paleointensity results (with pTRM checks; corrected for the effects of remanence anisotropy and slow cooling) and stable remanence directions from several Precambian intrusions. We focus on cumulate rocks from 35 sites in the 2.7 Ga Stillwater Complex (Montana, USA) and from nearly that many sites in the 1.4 Ga Laramie Anorthosite. These data, along with the 1.1 Ga Tudor Gabbro paleointensities of \textit{Yu and Dunlop} [2001] are not only reliable records of the Precambrian Earth's mean dipole moment, but also provide information about paleointensity distributions and directional paleosecular variation. For example, the Stillwater mean virtual dipole moment (VDM, 5.1$\times$10$^{22}$ Am$^2$) is similar to the average VDM of the past 300 Myr, but the distributions of VDMs and of characteristic remanence directions are not what one would expect from the Phanerozoic geomagnetic field. This suggests that, although the Earth had a magnetic field as early as 2.7 Ga, the planet did not develop a "modern" geodynamo until later. Yu, Y., and D. J. Dunlop, Paleointensity determination on the Late Precambrian Tudor Gabbro, Ontario, \textit{J. Geophys Res.}, \textit{106}, 26331-26343, 2001
DE: 1507 Core processes (8115)
DE: 1521 Paleointensity
DE: 1522 Paleomagnetic secular variation
DE: 1560 Time variations--secular and long term
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting