Anisotropy of (partial) isothermal remanent magnetization: DC-field-dependence and additivity

Abstract

SUMMARY Anisotropy of isothermal remanent magnetization (AIRM) is useful for describing the fabrics of high-coercivity grains, or alternatively, the fabrics of all remanence-carrying grains in rocks with weak remanence. Comparisons between AIRM and other measures of magnetic fabric allow for description of mineral-specific or grain-size-dependent fabrics, and their relation to one another. Additionally, when the natural remanence of a rock is carried by high-coercivity minerals, it is essential to isolate the anisotropy of this grain fraction to correct paleodirectional and paleointensity data. AIRMs have been measured using a wide range of applied fields, from a few mT to several T. It has been shown that the degree and shape of AIRM can vary with the strength of the applied field, for example, due to the contribution of separate grain subpopulations or due to field-dependent properties. To improve our understanding of these processes, we systematically investigate the variation of AIRM and the anisotropy of partial isothermal remanence (ApIRM) with applied field for a variety of rocks with different magnetic mineralogies. We also test the additivity of A(p)IRMs and provide a definition of their error limits. While A(p)IRM principal directions can be similar for a range of applied field strengths on the same specimen, the degree and shape of anisotropy often show systematic changes with the field over which the (p)IRM was applied. Also, the data uncertainty varies with field window; typically, larger windows lead to better-defined principal directions. Therefore, the choice of an appropriate field window is crucial for successful anisotropy corrections in paleomagnetic studies. Due to relatively large deviations between AIRMs calculated by tensor addition and directly measured AIRMs, we recommend that the desired A(p)IRM be measured directly for anisotropy corrections.

Anisotropy of Full and Partial Anhysteretic Remanence Across Different Rock Types: 2—Coercivity Dependence of Remanence Anisotropy

“…Only by measuring ApARMs for a set of coercivity windows can we begin to identify the principal components of composite anisotropy. By virtue of (1) and (3), the AARM as measured over the entire coercivity range in a sample may not be representative of the remanence anisotropy exhibited by the minerals that carry (the largest part of) the NRM. In some cases (e.g., the Mauch Chunk red beds), the main NRM carriers may reside in a coercivity range which is higher than what may be reached with AARM (Biedermann, Bilardello, Jackson, & Feinberg, ). However, even when the NRM is carried by magnetite, and the ARM coercivity spectrum matches that of the NRM, the full‐spectrum AARM may be a composite of subfabrics with different orientations and/or degrees of anisotropy, and not ideal for correcting the NRM for inclination shallowing or anisotropic NRM acquisition (Biedermann, Bilardello, Jackson, Tauxe, et al, ).…”