Influence of static alternating field demagnetization on anisotropy of magnetic susceptibility: Experiments and implications

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 18; no. 9; pp. 3292 - 3308
• Main Authors: Biedermann, Andrea R., Jackson, Mike, Bilardello, Dario, Feinberg, Joshua M., Brown, Maxwell C., McEnroe, Suzanne A.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.09.2017
• Subjects: AF cleaning; alternating field demagnetization; Anisotropy; anisotropy of magnetic susceptibility (AMS); field‐impressed anisotropy; field‐induced anisotropy; Igneous rocks; Low temperature; low‐temperature demagnetization; magnetic fabric; Magnetic susceptibility; Magnetism; Minerals; Orientation; Remanent magnetization; Shape
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1002/2017GC007073

Anisotropy of magnetic susceptibility (AMS) indicates the preferred orientation of a rock's constituent minerals. However, other factors can influence the AMS, e.g., domain wall pinning or domain alignment in ferromagnetic minerals. Therefore, it is controversial whether samples should be alternating field (AF) demagnetized prior to AMS characterization. This may remove the influence of natural remanent magnetization (NRM) or domain wall pinning on AMS; however, it may also result in field‐induced anisotropy. This study investigates the influence of stepwise AF and low‐temperature demagnetization on mean susceptibility, principal susceptibility directions, AMS degree and shape for sedimentary, metamorphic, and igneous rocks. Alternating fields up to 200 mT were applied along the sample x, y, and z axes, rotating the order for each step, to characterize the relationship between AMS principal directions and the last AF orientation. The changes in anisotropy, defined by the mean deviatoric susceptibility of the difference tensors, are between <2% and 270% of the AMS in NRM‐state. Variations in AMS parameters range from small changes in shape to complete reorientation of principal susceptibility axes, with the maximum susceptibility becoming parallel to the last AF direction. This is most prevalent in samples with low degrees of anisotropy in the NRM‐state. No clear correlations were found between field‐induced anisotropy and hysteresis properties. Therefore, we propose that future studies check any samples whose AMS is carried by ferromagnetic minerals and low anisotropy degrees for AF‐induced artifacts. These results highlight the need for understanding the AMS sources and carriers prior to any structural interpretation. Key Points Field‐induced anisotropy between <2% and 270% of initial AMS, and can influence AMS shape, degree, or principal directions Increase of susceptibility parallel to last applied AF orientation may lead to switching of principal axes Problematic if AMS is carried by ferromagnetic minerals, and degree of anisotropy is low