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.
Bibliographical noteFunding Information:
We are grateful to a number of people who helped collect the samples used in this study. M.J.J. collected the Thomson slate samples with Wei-Wei Sun, with guidance from Peter Hudleston. A.R.B. collected the Bjerkreim Sokndal samples under Swiss National Science Foundation (SNSF) project 155517, together with Alexander Michels, Norwegian University of Science and Technology (NTNU), on fieldwork supported by Research Council of Norway grant 222666 to S.A.M. Fogo basalts were collected under a National Science Foundation (NSF) grant to Brad Singer, and Fogo rock magnetic data was obtained under NSF grant EAR- 0911683. This study was funded by the SNSF, project 167608 to A.R.B. Measurements were performed at the Institute for Rock Magnetism (IRM) at the University of Minnesota. The IRM is a US National Multiuser Facility supported through the NSF, Earth Sciences Division, Instrumentation and Facilities program, and by funding from the University of Minnesota. This is IRM publication #1702. The data table can be obtained from the MagIC database, doi:10.7288/V4/MagIC/ 16232, or the supporting information. We thank editor Thorsten Becker, and reviewers Eric Ferré, Peter Selkin, and an anonymous referee for their constructive comments on the manuscript.
© 2017. American Geophysical Union. All Rights Reserved.
- AF cleaning
- alternating field demagnetization
- anisotropy of magnetic susceptibility (AMS)
- field-impressed anisotropy
- field-induced anisotropy
- low-temperature demagnetization
- magnetic fabric