Isolating components of magnetic anisotropy for more reliable anisotropy corrections in paleomagnetism

Paleomagnetic studies use a rock’s magnetic history to make e.g. plate tectonic reconstructions, and one of the main assumptions made is that the rock’s magnetization indicates the direction of the field at the time of magnetization. It is known that rock deformation or compaction can lead to deflection of the magnetization, which, if not accounted for, will cause major problems for paleogeographic reconstructions. The aim of this project will be to provide an improved correction technique based on the isolated magnetic anisotropy of the mineral that carries the remanent magnetization. These results will benefit both the paleomagnetism and archeomagnetism communities, and possibly exploration scientists who model magnetic anomalies.

Swiss National Science Foundation project 167608


Influence of magnetic fabric on strong magnetic anomalies

The Earth’s magnetic field measured at the surface is a superposition of the field generated in the Earth’s core, external field and contributions from magnetic minerals in the crust. Differences in magnetization of rocks in the Earth’s crust usually cause spatial variations in the observed magnetic field, termed magnetic anomalies. As the shape and amplitude of these anomalies depends on the geometry of the source body, they can be used to model the shallow sub-surface. However, anomaly shape and amplitude also depend on the direction of magnetization, which is not necessarily parallel to the inducing field in the presence of remanence or anisotropy. To quantify the effect of magnetic anisotropy on an anomaly, we investigated magnetic fabrics, directions of remanent magnetization and magnetic anomalies associated with the Bjerkreim Sokndal layered intrusion, Rogaland, Southern Norway. Due to modal layering and solid state deformation, these rocks possess a strong magnetic fabric. In addition to characterizing the fabric, we investigated how anisotropy affects the direction of remanent magnetization, and the anomalies measured over the intrusion. These results can be applied to interpretation of anomalies in other areas with anisotropic rocks, such as ore deposits or fault zones.

Swiss National Science Foundation project 155517. Additional funding: IRM Visiting Fellowship, NTNU and The Research Council of Norway (to Suzanne McEnroe)


Magnetic anisotropy of common rock-forming minerals

The aim of this project was to characterize magnetic anisotropy in single crystals of the common rock-forming minerals olivine, pyroxene, amphibole, mica, and feldspar. During the first part of the study, existing measurement procedures were improved to allow for a more precise determination of magnetic anisotropy in samples with weak susceptibility. Then, magnetic anisotropy was described for each mineral group, with a special focus on how it relates to the crystal structure and chemical composition. Finally, the newly determined single crystal properties were used to model magnetic fabrics in rocks whose mineralogy and preferred orientation of minerals is known. The results from this project allow researchers to quantitatively interpret magnetic fabrics, and will help to understand complex magnetic fabrics.

Swiss National Science Foundation projects 129806 and 143438 (to Ann Hirt)