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| Titel |
Magnetic anisotropy of chloritoid |
| VerfasserIn |
Tom Haerinck, Timothy Debacker, Manuel Sintubin |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250076719
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| Zusammenfassung |
| The anisotropy of magnetic susceptibility (AMS) is commonly used as a petrofabric tool. Whereas qualitative relationships between AMS and the petrofabric are well established, quantitative correlations are often ambiguous. For a quantitative interpretation of the paramagnetic component of a rock’s AMS, the mineral source(s) of the paramagnetic fabric and their intrinsic contribution(s) should be understood. This requires knowledge about the intrinsic AMS of the rock-forming, paramagnetic minerals.
For this study, the magnetocrystalline anisotropy of monoclinic chloritoid, a relatively common mineral in aluminium-rich, metapelitic rocks, has been determined for the first time by measuring the high-field anisotropy of magnetic susceptibility (HF-AMS) on a collection of single crystals, collected from different tectonometamorphic settings worldwide. Magnetic remanence experiments, i.e. (a) alternating-field (AF) demagnetization of a 1 T isothermal remanent magnetization (IRM) and a 200 mT anhysteretic remanent magnetization (ARM) and (b) low-temperature cycling of a room temperature saturation isothermal remanent magnetization (RT-SIRM), show that all specimens contain ferromagnetic (s.l.) impurities, being mainly magnetite. The determined HF-AMS ellipsoids have a highly oblate shape with the minimum susceptibility direction subparallel to the crystallographic c-axis of chloritoid. In the basal plane of chloritoid, however, the HF-AMS can be considered isotropic. The paramagnetic Curie temperature (θ), which has been determined parallel and perpendicular to the basal plane, indicates a weak antiferromagnetic interaction in the direction of the crystallographic c-axis and a ferromagnetic interaction within the basal plane. The degree of anisotropy is found to be 1.48, which is significantly higher than the anisotropy of most paramagnetic silicates, i.e. the Fe-bearing phyllosilicates and the mafic silicates pyroxene, orthopyroxene, amphibole and olive, and even well above the frequently used upper limit (i.e. 1.35) for the paramagnetic contribution to AMS in siliciclastic rocks.
As a consequence, chloritoid-bearing metapelites with a pronounced mineral alignment can have a higher degree of anisotropy than expected, without a significant contribution of strongly anisotropic, ferromagnetic (s.l.) minerals. The newly discovered magnetocrystalline anisotropy of chloritoid thus calls for a revised approach of magnetic fabric interpretations in chloritoid-bearing rocks. |
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