Separation of paramagnetic and ferrimagnetic anisotropies: A review

Abstract

[1] The magnetic anisotropy of rocks results from the contributions of diamagnetic, paramagnetic, and ferromagnetic (in the broad sense) minerals. This bulk anisotropy of magnetic susceptibility, which can be rapidly measured with modern instruments, generally provides a better understanding of the rock deformation history. Different minerals in a rock can form at different times and also respond to deformation in different manners. Therefore it is useful to separate their respective contributions to the whole rock magnetic fabric. Various techniques available to achieve this separation are presented and compared in this article. The variations of magnetic susceptibility with temperature can be used to selectively characterize the contribution of paramagnetic mineral phases following the Curie-Weiss law. The measurement of magnetic remanence-related anisotropy provides an efficient way to characterize the contribution of ferrimagnetic and antiferromagnetic species. Finally, measurement of the magnetic properties at high fields, above the saturation magnetization of ferromagnetic minerals, effectively separates the diamagnetic-paramagnetic magnetic anisotropy. The recent development of these techniques allows the separation of paramagnetic and ferrimagnetic anisotropies to be performed routinely on most specimens and shows promising potential for future magnetic anisotropy studies.

Magnetic properties of fault pseudotachylytes in granites

“…The high-field magnetic susceptibility (K hf ) was determined above the saturation field (H s ) following the approach of Belley et al [2009]. The independent measurement of K lf and K hf allows the calculation of the percentage of paramagnetic susceptibility [Martín-Hernández and Ferré, 2007]. Errors on the hysteresis parameters can be as high as 10% for weakly magnetic specimens such as those shown in Figures 9c and 9d. …”

Significance of AMS in multilayer systems in fold‐and‐thrust belts. A case study from the Eocene turbidites in the southern Pyrenees (Spain)

“…In some works (e.g. Martín et al, 2007) the paramagnetic behaviour has been linked to ferromagnetic inclusions along the internal structure of paramagnetic minerals. The parallelism between both orientation patterns in LTAMS and AGRM suggest that ferromagnetic acicular inclusions included in paramagnetic minerals can be responsible for these fabrics.…”

“…fabrics can be isolated by means of artificial remanent magnetization techniques, as anisotropy of anhysteretic (AARM), isothermal (AIRM) or gyroremanent magnetization (AGRM, see Potter, 2004or Martín et al, 2007 for a detailed description of techniques and methods). AARM permits the measurement of the anisotropy of the ferromagnetic particles.…”

Unraveling magnetic fabrics

“…These studies describe how magnetic subfabrics can be isolated using experimental methods or models. High-field measurements of AMS (HF-AMS) allow the separation of the saturated ferrimagnetic torque from the diamagnetic and paramagnetic torque signal that is related linearly to the square of the applied field (Owens and Bamford 1976;Hrouda and Jelínek 1990;Tarling and Hrouda 1993;Martín-Hernández and Hirt 2001;Martín-Hernandez and Férre 2007). The torque due to hematite is related linearly to the applied field, and this property was used by Martín-Hernández and Hirt (2004) to isolate the hematite subfabric in deformed Permian red beds from the Glarus Nappe.…”

“…There have been several excellent reviews, which describe different methods that are currently being used to measure AMS in both low and high fields (e.g., Borradaile and Henry 1997;Borradaile and Jackson 2004;Hrouda et al 2009b;Martín-Hernandez and Férre 2007;Tarling and Hrouda 1993) and the anisotropy of remanent magnetization (Hirt 2007;Jackson 1991;Lowrie 1989;Potter 2004). These studies describe how magnetic subfabrics can be isolated using experimental methods or models.…”