HR: 0800h
AN: T51B-0447    [Abstracts]
TI: Magnetic Properties of Magnetite and Titanomagnetite Under Pressures 0 to 6 GPa: Implications for Earthquake Prediction
AU: LeGoff, M
EM: legoff@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris cedex 05, IdF 75252 France
AU: * Gilder, S A
EM: gilder@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris cedex 05, IdF 75252 France
AB: Through the development of new diamond anvil cell technology, we can now measure hysteresis loops and full magnetic vectors of magnetic materials under pressures in excess of 30 GPa. These new techniques are advantageous for investigating the pressure dependence on the magnetic properties of magnetic minerals whose compositions and grain-size spectra are well characterized. Pure multidomain (MD) and single domain (SD) magnetite respond quite differently to stress, with MD grains being much more stress sensitive than SD grains. For the titanomagnetite series, we find that initial susceptibility and saturation moment decrease as a function of increasing pressure and Ti concentration. We define a constant called the median destructive stress, which is the pressure needed to remove one-half of the original (pre-stressed) initial susceptibility, and we show that this value is proportional to the inverse of the magnetostriction constant, consistent with established theory. Thus, by merely knowing the magnetostriction constants of a given magnetic mineral, one should be able to predict its sensitivity to an imposed stress. With this in mind, one can revisit the relevance of piezo-remanent magnetization and stress demagnetization to earthquake prediction, or the build up or relaxation of stress in the crust due to tectonic motion, which has proved elusive. Terrains containing Ti-rich titanomagnetite phases, such as oceanic crust (ophiolites), should be considered the best places to look for changes in the local magnetic field during an earthquake.
DE: 5109 Magnetic and electrical properties
DE: 3924 High-pressure behavior
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting