Anisotropy of magnetic susceptibility in salt rocks from the German Zechstein Basin, Sondershausen mine

Abstract

SUMMARY The anisotropy of magnetic susceptibility (AMS) of rocks reflects the alignment of certain minerals, and therefore it can be used to investigate the deformation history of rocks. However, for salt rocks, very few studies on the AMS of salt rocks and the influence of accessory minerals exist. In this study, we analysed the potential to use the AMS of salt rocks with low impurity content for fabric characterization. Samples of rock salt, sylvinite and carnallitite from a salt mine in Sondershausen (Germany) from the Late Permian (Zechstein 2, Stassfurt series) are investigated. The results of low-field AMS (LF-AMS) measurements show a very weak but significant magnetic anisotropy for sylvinite, carnallitite, and rock salt with an elevated content of accessory minerals. The AMS results are consistent in individual layers of the same rock type. In order to identify the magnetic minerals, which cause the magnetic anisotropy, the high-field AMS (HF-AMS) was measured using a torque magnetometer in order to separate ferrimagnetic and paramagnetic contributions to the AMS. A significant paramagnetic subfabrics exists, which reflects the alignment of phyllosilicates. The magnitude of the LF-AMS is considerably greater than that of the paramagnetic subfabric. This indicates the existence of a ferrimagnetic subfabric due to magnetite, which can have a different orientation than the paramagnetic subfabric. Differences in the orientation of the AMS in samples from two sites suggest a relationship of deformation history and AMS. At a site with dipping layers, the AMS orientation is independent of the bedding and shows large differences between individual lithological layers. In a tight fold, the AMS of all rock types has similar shape and orientation. We conclude that AMS in salt rocks can give meaningful information on the mineral fabric, which could be used in the analysis of the deformation history.