Magnetic mineralogy of the Laacher See deposits: Indicators of fractionation processes and emplacement conditions

The eruption history and geochemistry of the Laacher See tephra 12.9 ka ago are well-documented. This study is the first to investigate the magnetic properties of the approximately 55 m thick tephra deposits south of the vent. The goal is to establish a relationship between magnetic properties, magma fractionation processes, and emplacement conditions. Magnetic susceptibility, its field- and temperature-dependence, and natural remanent magnetization were measured, and the petrography of the opaque phases was studied. Magnetic susceptibility increases significantly with stratigraphic height, starting from very low values in the lowest deposits. This trend is also seen in remanent and total magnetization, due to an increase in the number and size of titanomagnetite grains in ash and pumice lapilli. The increase in crystal content is attributed to the highly zoned magma reservoir of the Laacher See volcano, resulting from fractional crystallization. Post-eruption, the tephra deposits represent the inverse magma column, with less fractionated crystals at the top. Field- dependence measurements of magnetic susceptibility reveal three zones with slight field dependence (up to 10 %) and a zone without field dependence in between. These four zones are also observed in temperature-dependent magnetic susceptibility. Heating curves typically show Curie temperatures between 480 °C and 530 °C, while cooling curves show significantly lower Curie temperatures (up to 200 °C), likely due to cation ordering effects. Total magnetization values provide new insights into the magnetic anomaly field in the Laacher See region, suggesting previous models overestimated magnetization by up to two orders of magnitude at depth.