Anisotropy of Out-of-Phase Magnetic Susceptibility and Its Potential for Rock Fabric Studies: A Review

Abstract

In anisotropic materials such as minerals and rocks, the AC magnetic susceptibility is also anisotropic, and consists of two components, one in-phase with the applied field (ipMS) and the other out-of-phase (opMS). Correspondingly, anisotropies of these components, in-phase magnetic anisotropy (ipAMS) and out-of-phase anisotropy (opAMS), can be defined. In non-conductive dia- and paramagnetic materials, and in pure multi-domain magnetite, the opMS is effectively zero and only ipAMS can be measured. In some ferromagnetic minerals, such as pyrrhotite, hematite, titanomagnetite, or small magnetically viscous grains of magnetite, the opMS is clearly non-zero, and not only ipAMS but also opAMS can be determined. The opAMS can then be used as a tool for the direct determination of the magnetic sub-fabrics of the minerals with non-zero opMS. The precision in measurement of opMS decreases non-linearly with decreasing opMS/ipMS ratio, which may result in imprecise determination of the opAMS if the ratio is very low. It is highly recommended to inspect the results of the statistical tests of each specimen and to exclude the specimens with statistically insignificant opAMS from further processing. In rocks with a mono-mineral magnetic fraction represented by the mineral with non-zero opMS, the principal directions of the opAMS and ipAMS are virtually coaxial, while the degree of opAMS is higher than that of ipAMS. In some cases, the opAMS provides similar results to those provided by anisotropies of low-field-dependent susceptibility and frequency-dependent susceptibility. The advantage of the opAMS is in its simultaneous measurement with the ipAMS during one measuring process, whereas the other two methods require measurement in several fields or operating frequencies.