AMS in basalts: Is there a need for prior demagnetization?

Abstract

One of the most prominent applications of anisotropy of magnetic susceptibility (AMS) measurements is the evaluation of flow fabrics in basalts. Basalts often contain Fe-Ti oxides with variations in grain/domain size fractions due to their variable cooling history. The origin of AMS in such rock types is of complex nature. Crystal shape, magnetocrystalline anisotropy and magnetic interactions are considered as the most influential parameters. It is still under debate if a remanent magnetization, commonly strong in such basalts, can influence the lowfield AMS measurements and therefore bias the magnetic fabric interpretation. This study shows a change of scalar AMS parameters and orientation of principal magnetic fabric axes in Deccan basalts during demagnetization experiments using static three-axial and tumbling devices (peak fields of 100 mT). The AMS was measured before and after the respective demagnetizing experiment. Scalar AMS parameters and orientation of the principal axes change during the experiments: Mean susceptibility rises due to demagnetization, the shape factor T and the corrected magnetic anisotropy P' show systematic change. In multidomain (MD) and pseudo-single-domain (PSD) grains, axial demagnetization will align the domain (bloch) walls according to the applied field. Especially specimens showing a normal magnetic fabric react very sensitively to changes in the domain structure. kmax will get aligned parallel to the last applied field and the other principal magnetic axes will follow the orthogonal geometry of the field. Specimens with an inverse magnetic fabric can also be affected by the demagnetization. However, inverse specimens with an apparent high fraction of SD particles show no redistribution of their principal axes. Demagnetization with a tumbling device will not impress a direction on the AMS but will remove the magnetic remanence in MD/PSD grains. This change of the domain structure can lead to aberrations of the principal magnetic axes from their initial position. These observations point to a significant influence of a remanence vector on the initial AMS. It is therefore advisable to demagnetize specimens showing a strong magnetic remanence by using a tumbling demagnetizer before any AMS experiments to get better information on the 'true' preferred distribution of ferrimagnetic grains. Axial demagnetization should be avoided, as an applied static magnetic field can impress on the AMS. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.