HR: 11:35h
AN: GP22A-06    [Abstracts]
TI: Toward an optimal geomagnetic field intensity determination technique: Testing the IZZI protocol
AU: * Yu, Y
EM: yjyu@ucsd.edu
AU: Tauxe, L
EM: ltauxe@ucsd.edu
AB: Paleointensity determinations based on double heating techniques (Aitken, Coe, and Thellier) are generally considered to be functionally interchangeable, producing equally reliable paleointensity estimates. To investigate this premise, we have developed a simple mathematical model. We find that both the zero-field first and in-field first methods have a strong angular dependence on the laboratory field (parallel, orthogonal, and anti-parallel) while the two in-field steps method is independent of the direction of the laboratory-produced field. Contrary to common practice, each method yields quite different outcomes if the condition of reciprocity of blocking and unblocking temperatures is not met, even with marginal (10%) tails of partial thermoremanence. By far the best approach, however, is to alternatethe infield-zerofield (IZ) steps with zerofield-infield (ZI) steps. We tested the IZZI method associated with the partial thermoremanent magnetization (pTRM) tail check. As predicted, the IZZI method shows a strong angular dependence, resulting from the undemagnetized portions of pTRM tails. We also tested two fundamental assumptions embedded in Thellier experiments, the initial state dependence and the effect of multi-cycle heat treatment. We observed that the magnitude of pTRMs with initial state of thermal demagnetization was larger than that of pTRMs in Thellier analysis. A multi-cycle Thellier analysis on coarse-grained magnetites progressively produces more intense pTRMs and progressively erases more pTRM tails. Both pre-history and multi-cycle dependence will likely to enhance the non-linear features of the Arai plot for coarse-grained magnetites.
DE: 1512 Environmental magnetism
DE: 1521 Paleointensity
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting