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

Abstract

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.

Anisotropy of magnetic susceptibility impressed during rock magnetic procedures (AF, IRM) and information on the domain state of the magnetic carriers

“…1) AF and IRM acquisition induce significant changes in bulk susceptibility as already discussed in previous papers (Biedermann et al, 2017;Jordanova et al, 2007). Volcanic rocks with titanomagnetite are especially prone to these changes.…”

“…As discussed previously, the application of a static AF demagnetization induces a change in bulk magnetic susceptibility (Biedermann et al, 2017;Jordanova et al, 2007). Although no further change in mean susceptibility is observed in successive applications of an AF above 30 mT, a change is observed in AMS: an AMS lineation created by an AF field along X, re-aligns with the last direction along which the AF is successively applied (Y or Z) (Fig.…”

“…Field-impressed Anisotropy of Magnetic Susceptibility (AMS) in samples exposed to large direct fields (DF) or alternating fields (AF) has long been recognized in samples with single domain (SD) and multidomain (MD) magnetite particles (Bhathal and Stacey, 1969;Stephenson, 1990a, 1990b;Violat and Daly, 1971). More recent studies have mainly discussed the effect of AF on AMS (Biedermann et al, 2017;Henry et al, 2007;Jordanova et al, 2007;Liu et al, 2005;Schöbel et al, 2013). Biederman et al (2017) studied the effect of static AF demagnetization upon 4 sets of samples from different rock types.…”

A Practical Approach for Identification of Magnetic Fabric Carriers in Rocks

“…These fields usually fit the coercivity of stable single‐domain (SSD) and pseudosingle‐domain (PSD) ferromagnetic minerals. While in many cases, the magnetic remanence is carried mostly by these minerals, ferromagnetic grains such as MD magnetite or hematite are usually not detected by the AARM measurements (Biedermann et al., 2017; Bilardello & Jackson, 2014; Jackson, 1991; Jackson & Tauxe, 1991; McCabe et al., 1985). Furthermore, the anisotropies of the different ferromagnetic grain populations may combine or subtract from each other (Biedermann et al., 2020), which may affect the total AARM as well as the RT‐AMS (Lanci & Zanella, 2016).…”