Magnetic anisotropy and paleomagnetism

The natural remanent magnetization of certain Welsh slates lies in the cleavage plane, which was found to be a plane of high susceptibility defined by the alignment of anisotropic crystals of pyrrhotite. A correlation between the magnitude of susceptibility anisotropy and the bulk susceptibility of the slates has been observed. This is interpreted as a partial masking of the susceptibility anisotropy of the dispersion of magnetic particles by an isotropic contribution to the susceptibility of the rock due to the paramagnetic matrix. As a consequence of this masking, or dilution, rocks with low concentrations of hematite or pyrrhotite may be expected to exhibit susceptibility anisotropy which is small compared with that of the magnetic particles. Small susceptibility anisotropy controlled by the bedding plane has been observed in collections of sedimentary rocks including red sandstones. Laboratory experiments demonstrated that thermoremanent magnetization, isothermal remanent magnetization, and pressure-promoted remanent magnetization may be acquired anisotropically by rocks. Only certain red sandstones, of the rocks commonly used in paleomagnetism, appear to show a comparable divergence of the natural remanence from the geomagnetic field in which remanence was acquired.