A comparison of anisotropy of magnetic remanence methods – a user’s guide for application to palaeomagnetism and magnetic fabric studies
David K. Potter, 2004. "A comparison of anisotropy of magnetic remanence methods – a user’s guide for application to palaeomagnetism and magnetic fabric studies", Magnetic Fabric: Methods and Applications, F. Martín-Hernández, C. M. Lüneburg, C. Aubourg, M. Jackson
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Anisotropy of magnetic remanence (AMR) is increasingly being applied to palaeomagnetic and structural fabric studies. AMR techniques measure the anisotropy of the remanence carrying particles, and thus are directly relevant to palaeomagnetic studies concerned with computing the direction and intensity of the Earth’s ancient magnetic field from the natural remanent magnetization (NRM) recorded in anisotropic rocks. This paper provides a comparison of several AMR methods, including some of the less well-known techniques, and highlights the relative merits of each. Results from a strongly anisotropic rock and a pottery sherd are presented. The anisotropies of anhysteretic remanent magnetization (AARM) and isothermal remanent magnetization (AIRM) are currently the most commonly applied types of AMR, since they have provided reasonably good analogues of the anisotropy of thermoremanent magnetization (ATRM) acquired in the Earth’s field. They have also helped to correct for inclination shallowing of detrial remanent magnetization (DRM) in sediments. IRM anisotropy is the most rapid AMR technique, and is particularly useful for very low concentrations of remanence carrying particles. The gyroremanences, gyroremanent magnetization (GRM) and rotational remanent magnetization (RRM), are preferentially acquired by stable single-domain (SD) particles, and are thus directly relevant to the particles of major interest in palaeomagnetism. GRM anisotropy is the most sensitive AMR method. It is essentially the remanence equivalent of the anisotropy of magnetic susceptibility (AMS) delineator, since a single application of an alternating field will only produce a GRM in a sample containing an anisotropic distribution of particles. Static ARM methods need to take account of components of GRM that are simultaneously acquired.
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Fabric is a ubiquitous and significant feature of geological materials. The processes involved in the formation and deformation of rocks and sediments leave their mark on the orientations of the constituent mineral grains. Petrofabrics thus provide essential keys to understanding the history of geological materials. Magnetic anisotropy is directly related to petrofabric, and has become one of the most rapid, sensitive and widely used tools for its characterization. The relationship between magnetic fabric and petrofabric is complex and depends on various factors including the composition, concentration and grain size of mineral grains. Ongoing research in geological applications is paralleled by studies of the fundamental mineral magnetic phenomena involved.
The papers in this book represent the current state of investigations in magnetic anisotropy studies as a discipline that integrates geological interpretations, mineral fabric development, technical advances and rock-magnetic properties.