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The influence of Magnetic Anisotropy on the Direction of Natural Remanences in Pyrrhotite and Hematite Bearing Rocks

Ziel

Eminences carried by rocks provide information about position of the terrains at the time of formation, relative movements and history of deformation. But the direction of eminence can be altered among other reasons by the intrinsic magnetic anisotropy of the ferromagnetic particles. More precisely, this effect has been reported to be very important in ferromagnetic minerals with magneto crystalline anisotropy, such as pyrrhotite and hematite, because the magnetization is constrained to be within the crystallographic basal plane. However, the eminence carried by hematite-bearing rocks as red-beds is very important in the determination of the Apparent Polar Wander Path of many regions (e.g. North America, China...). Pyrrhotite is often carrying secondary eminence, but it has been increasing the interest of it in order to evaluate models of thermoremanent evolution of mountain ranges.
Pyrrhotite has also become very important in the analysis of recent sediments for the evaluation of pale climatic models. Magnetic anisotropy of hematite is poorly known. Previous models present an isotropic basal plane in which magnetization is able to rotate and a hard-axis in the perpendicular direction. The high coercively of hematite makes difficult to measure magnetic anisotropy of eminences because saturation is not reached. In low-field magnetic susceptibility measurement, it presents an additional difficulty since the susceptibility depends on the frequency of the applied DC field. Early models of magnetic anisotropy of pyrrhotite presented a crystalline anisotropy with isotropic basal plane. But soon, it was reported an influence of the domain state in the anisotropy of pyrrhotite and a non-isotropic basal plane.
The purely crystalline anisotropy has been discarded recently with the observations of a possible grain shape effect on the magnetic anisotropy.

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FP6-2002-MOBILITY-5
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PALEOMAGNETIC LABORATORY "FORT HOOFDDIJK", DEPARTMENT OF EARTH SCIENCE, UNIVERSITY OF UTRECHT
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