HR: 0800h
AN: GP11C-0837    [Abstracts]
TI: Spot Reading of the Absolute Paleointensity of the Geomagnetic Field During the Kiaman Superchron: The Exeter Lavas Case
AU: * Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: SOEST-HIGP, Paleomagnetics and Petrofabrics Laboratory, 1680 East West Rd, Honolulu, HI 96822 United States
AU: Fuller, M D
EM: mfuller@soest.hawaii.edu
AF: SOEST-HIGP, Paleomagnetics and Petrofabrics Laboratory, 1680 East West Rd, Honolulu, HI 96822 United States
AB: It is well known that Superchrons provide an opportunity to study the geomagnetic field in an extreme state, i.e. when reversals are at their lowest frequency or totally absent. Such Superchrons represent features that may provide constraints for theoretical calculations and numerical models of the geodynamo. Thus, one way to contribute to the understanding of the generation of the paleofield particularly during Superchrons is to determine the absolute paleointensity of rocks formed during those periods of time. We have sampled a member of the Exeter lavas (ca 290.8+/-0.8 Ma, Ar/Ar date) from the large, abandoned quarry in the northeastern slope of the southernmost hill of Killerton Park, 10 km northeast of Exeter where we collected three samples from three different stratigraphic levels. We stepwise demagnetized the samples collected from the three different levels by both alternating field (5mT to 100mT) and thermal (from 28$^{o}$C to 575-650$^{o}$ C) methods, and the mean directions obtained by principal component analysis (D=198$^{o}$ and I=-25$^{o}$). All samples yielded a strong and stable ChRM trending towards the origin based on no less than seven to nine steps, with thermal and AF results agreeing to a very high degree. Low field susceptibility versus temperature (k-T) analyses were conducted for individual samples and the majority of them show reversible curves. Curie point determinations revealed a temperature close to or equal to 580$^{o}$ C, indicative of almost pure magnetite for most of the samples. Magnetic grain sizes analysis indicated SD-PSD sizes. We used the modified Thellier-Coe double heating method to determine paleointensities. pTRM checks were performed systematically one temperature step down the last pTRM acquisition in order to document magnetomineralogical changes during heating. The temperature was incremented by steps of 50$^{o}$ C between room temperature and 500$^{o}$C and every 25-30$^{o}$ C. The paleointensity determinations were obtained from the slope of the Arai diagrams. Special care was taken to interpret the Arai diagrams within the same range of temperatures lower than 300$^{o}$C unless a clear and unique slope would be present. We were able to obtain paleointensity determinations for the three set of samples. The three independent estimates of paleofield range between 23.5 to 30 uT ( i.e. VADM 5.5 to 9.03 X10$^{22}$ Am$^{2}$). The range of these values includes the present dipole moment (i.e. 8X10$^{22}$ Am$^{2}$), indicating that the paleofield during the studied period of time was strong, stable and is consistent with available time-averaged intensity and directional data.
DE: 1521 Paleointensity
DE: 1530 Rapid time variations
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
DE: 1560 Time variations--secular and long term
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting