Field‐impressed anisotropies of magnetic susceptibility and remanence in minerals

Abstract

The application of alternating (AF) or direct (DF) magnetic fields to samples containing dispersed particles of magnetite, titanomagnetite, or maghemite has been found to alter significantly the measured low‐field susceptibility anisotropy even though the particle orientation is unchanged. The acquisition of isothermal remanent magnetization (IRM) is also strongly dependent on the previous AF treatment given to the samples. A method of quantifying the changes brought about by application of the fields is described. The samples studied were intrinsically isotropic or only weakly anisotropic, and the computed impressed susceptibility or IRM ellipsoids produced by the applied fields were ellipsoids of revolution with the unique axis aligned with the applied field axis. The size and shape of the field‐impressed ellipsoids were strongly dependent upon particle size. These effects might thus form the basis of a rapid, nondestructive means of estimating particle size in rocks. The results also have important implications for magnetic fabric analysis and the static AF demagnetization of rocks. In particular, to ensure that the intrinsic magnetic fabric is measured without any field‐impressed component of anisotropy, samples should not be subjected to static AF demagnetization prior to the determination of either low field susceptibility anisotropy or IRM anisotropy. Samples should also not carry a significant remanence (such as an IRM acquired in a field greater than about 5 mT) prior to low field susceptibility anisotropy analysis.

Can remanence anisotropy detect paleomagnetic inclination shallowing due to compaction? A case study using Cretaceous deep‐sea limestones

“…Many specimens had a K too weak for anisotropy to be reliably determined; we rejected specimens with significantly more than 1% rms error (defined as the root-mean-square of the differences between rep_•.at measurements of the same matrix element divided by K). Results for the remaining 20 specimens are given in Table 4, Susceptibility anisotropy was measured after ARM anisotropy and hence may be affected by field-impressed susceptibility anisotropy [Potter and Stephenson, 1990]. Any such effect was evidently not great enough to change the basic shape of the susceptibility ellipsoid, which is strongly foliated in the bedding plane like the ARM ellipsoid.…”

Induced Magnetization of Magnetite-titanomagnetite in Alternating Fields Ranging from 400 A/m to 80,000 A/m; Low-field Susceptibility (100 – 400 A/m) and Beyond

“…Observations of k or AMS are truly unique and universal only where RBM are absent or of low abundance and dispersed. Furthermore, exposure to high fields changes ''k 0 '' and AMS noticeably for rocks rich in certain multidomainal accessory RBM (B1% by volume) (POTTER and STEPHENSON, 1990), especially magnetite and pyrrhotite. For reproducible determination of low-field susceptibility in specimens rich in multidomain, low coercivity RBM, it may be advisable to demagnetize carefully first, by a nondestructive but isotropic technique.…”

“…For reproducible determination of low-field susceptibility in specimens rich in multidomain, low coercivity RBM, it may be advisable to demagnetize carefully first, by a nondestructive but isotropic technique. Low-temperature demagnetization is preferable since thermal demagnetization may cause mineralogical change and static three-axis AF demagnetization may reset ''k 0 '' and AMS (POTTER and STEPHENSON, 1990). Tumbling AF demagnetization can never expose every axis through the specimen to every demagnetizing field value and thus risks imposing some anisotropy.…”

“…1a). To measure k 0 with sufficient precision and reproducibility for anisotropy studies (AMS) any exposure to high fields must be avoided (e.g., POTTER and STEPHENSON, 1990). First the specimen should be demagnetized in a nondestructive manner; low-temperature demagnetization being the most cautious technique, although neither universally nor completely successful.…”

Structures and Magnetic Fabrics from the Lower Sheeted Dike Complex of Hole 504B Reoriented Using Stable Magnetic Remanence

“…For the majority of the samples (39) the AMS measurements were conducted before demagnetization, but some (25) were shipboard samples that were demagnetized by AF cleaning on the drill ship before AMS fabric was determined. The sequence of measurement is important because the application of a static alternating field can impress an anisotropy (Potter and Stephenson, 1990), an effect we have been able to reproduce in some of these samples. For this reason we have excluded the shipboard samples from our discussion of the AMS fabrics.…”