HR: 17:05h
AN: GP44A-04 INVITED     [Abstracts]
TI: Long-term trends in paleointensity examined using Thellier analyses of single silicate crystals
AU: * Tarduno, J A
EM: john@earth.rochester.edu
AF: University of Rochester, Department of Earth & Environmental Sciences 227 Hutchison Hall, Rochester, NY 14627, United States
AB: Available data suggest a remarkable feature of the Permian to Recent time-averaged geomagnetic field: its strength apparently varied by only a factor of ~3. Thus, the task of investigating more subtle trends becomes challenging, and has given rise to continued constructive debate. For example, several data sets (specifically Thellier analyses of single silicate crystals and submarine basaltic glass, and natural remanent magnetization intensities of submarine basalt) suggest an inverse relationship between reversal frequency and field strength. This relationship has not, to date, been seen in Thellier results from whole rocks, perhaps due to geologic and laboratory-induced alteration. In addition, it is likely that the presently available paleointensity database from whole rocks is underrepresented in samples that have seen high-temperature oxidation (and that might otherwise yield more accurate values due to favorable grain size/domain state characteristics). Long-term paleointensity trends are largely based on comparisons of values from the Cretaceous Normal Polarity Superchron and preceding and succeeding mixed polarity intervals. Here we discuss such comparisons using new paleointensity data from single silicate crystals. For Archean times, prior paleointensity estimates based on whole rocks suggest much weaker fields. On the basis of data from single silicate crystals, we argue that some prior results from whole rock samples underestimate field strength due to the ubiquitous thermochemical alteration seen in Archean terrains. Field strengths at 3.2 Ga appear to be comparable to those of the more recent field.
DE: 1507 Core processes (1213, 8115)
DE: 1521 Paleointensity
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly