HR: 0800h
AN: GP11C-0845    [Abstracts]
TI: Cooling Rate Effects on Paleointensity Estimates in Submarine Basaltic Glass and Implications for Dating of Young Flows
AU: * Bowles, J
EM: jbowles@ucsd.edu
AF: Scripps Institution of Oceanography, Univ. of California, San Diego 9500 Gilman Dr., MC 0208, La Jolla, CA 92093 United States
AU: Gee, J
EM: jsgee@ucsd.edu
AF: Scripps Institution of Oceanography, Univ. of California, San Diego 9500 Gilman Dr., MC 0208, La Jolla, CA 92093 United States
AU: Kent, D
EM: dvk@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Paleomagnetics Lab 61 Route 9W - PO Box 1000, Palisades, NY 10964 United States
AU: Bergmanis, E
EM: bergmani@hawaii.edu
AF: University of Hawai'i, 1680 East-West Road, Honolulu, HI 96822 United States
AU: Sinton, J
EM: sinton@hawaii.edu
AF: University of Hawai'i, 1680 East-West Road, Honolulu, HI 96822 United States
AB: Cooling rate effects on the intensity of thermoremanent magnetization (TRM) have been well documented in ceramics. In that case, lab cooling is generally more rapid than the initial cooling, leading to an overestimate of the paleofield by 5-10% in Thellier-type paleointensity experiments. For slowly-cooled plutons, this effect may be as great as 50%. The reverse scenario, however, has never been tested. We investigate the theory that rapidly-quenched submarine basaltic glass (SBG) may produce an underestimate of the paleofield when subjected to Thellier experiments cooled in air. If attempts to use SBG paleointensity as a dating tool are to be successful, this question must be addressed. We combine Thellier-Thellier paleointensity experiments with relaxation geospeedometry determinations of natural cooling rate. Experiments were performed on a set of young SBG from 6 sites at $17\deg$S on the East Pacific Rise, where the present-day field is 30.8 $\mu$T, and the field has been monotonically decreasing for the past $\sim$200 yr. Cooling rate determinations were made on chips taken from the Thellier specimens, as well as on previously unheated specimens. Paleointensity values range from 26.9 $\pm$ 1.4 $\mu$T (1$\sigma$) to 44.1 $\pm$ 4.9 $\mu$T. Age estimates based on these paleointensity values corroborate radiometric ages and geologic observations that place the samples in 3 age groups. The youngest group is very recent (indistinguishable from present day) and the oldest at least 100, and probably several hundred, years old. Absolute cooling rate determinations range from $\sim$10 to $\sim$$250\deg$C min$^{-1}$, as captured at the glass transition at $\sim$$650\deg$C. Such absolute determinations were not possible for all specimens. However, using an estimate of relative cooling rate variation, several sites suggest a weak correlation between cooling rate and paleointensity, the magnitude of which roughly agrees with theory. We estimate lab cooling rates to be on the order of 10-100$\deg$C min$^{-1}$ over the blocking temperature range ($\sim$200-400$\deg$C). Taking the differing temperature ranges into account (650$\deg$C vs. 200-400$\deg$C), the cooling rate data suggest that natural cooling rates may be only slightly faster than the lab rates. The implications are that while the cooling rate effect might produce some within-site scatter, it should not often result in a significant bias in paleointensity from SBG.
DE: 8400 VOLCANOLOGY
DE: 3035 Midocean ridge processes
DE: 1521 Paleointensity
DE: 1527 Paleomagnetism applied to geologic processes
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting