HR: 13:55h
AN: GP23B-02    [Abstracts]
TI: Fundamental AMS for Single Crystals of Carbonate Minerals
AU: * Schmidt, V
EM: schmidt@mag.ig.erdw.ethz.ch
AF: ETH-Hoenggerberg, Institute of Geophysics, Schafmattstrasse 30 , Zurich, 8093 Switzerland
AU: Hirt, A M
EM: hirt@mag.ig.erdw.ethz.ch
AF: ETH-Hoenggerberg, Institute of Geophysics, Schafmattstrasse 30 , Zurich, 8093 Switzerland
AB: The anisotropy of magnetic susceptibility (AMS) can serve as a good qualitative indicator of strain in deformed carbonate rocks with diamagnetic susceptibility. An open question is how well the AMS serves as a quantitative indicator of strain in rocks whose minerals undergo twinning and recrystallization with deformation. We are investigating this quantitative relationship by numerical models which requires knowledge of the AMS of carbonate minerals. There is, however, a lack of reliable values for the intrinsic AMS of carbonate crystals. Although the AMS for calcite is well defined, measurements from other carbonate minerals show a large range of anisotropies. This is due partially to the very low paramagnetic or diamagnetic susceptibility of these minerals but also partly due to the fact that the chemical composition of the crystals in these earlier studies was not well-constrained. We have investigated the room temperature AMS of important carbonate minerals; i.e., calcite, dolomite, siderite, aragonite, rhodochrosite, azurite, magnesite, cerussite. Single crystals of best quality were investigated, and the chemical composition of the crystals was analyzed by Laser Ablation Inductively Coupled Plasma - Mass Spectrometry. High-field (torsion magnetometry) and low-field (KLY-4S) anisotropies were measured on all crystal samples. Acquisition of isothermal remanent magnetization was used to detect possible ferromagnetic inclusions in the samples. The high-field torsion magnetometry allows for the ferrimagnetic and paramagnetic contributions to be separated. AMS values obtained by both low-field and high-field methods agree very well, which suggests that ferrimagnetic contamination is negligible. The high-field torsion magnetometer was more accurate in determining the AMS for small and weakly magnetic samples. The bulk susceptibility of calcite increases linearly with Fe and Mn content. Pure calcite has a perfectly oblate AMS with the minimum susceptibility subparallel to the crystallographic c-axis. With increasing content of ferrous ions the susceptibility changes to positive values with the maximum susceptibility axis subparallel to the c-axis. Pure dolomite shows a perfectly oblate AMS. For slightly impure dolomite the AMS shape changes to perfectly prolate.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting