Origin of magnetic anomalies in the large Ebersbrunn diatreme, W Saxony, Germany

Abstract

The Ebersbrunn diatreme is a deeply eroded (>1 km) diatreme structure in western Saxony, Germany. At current erosion levels, this ultramafic to carbonatitic diatreme is about 2 × 1.5 km in map view, which makes it a large one. Based on shallow drill cores, the diatreme contains coarse unbedded volcaniclastic rocks with up to 80 % country rock fragments. The diatreme is characterised by positive and negative magnetic anomalies, which are controlled mostly by the changing proportions of magnetic minerals in the rocks. The magnetic minerals are themselves contained in the juvenile fraction. Rock magnetic studies on three drill cores, one from the magnetic low and two from magnetic highs, including bulk susceptibility and its anisotropy, temperature dependent susceptibility, various remanence measurements (natural remanent magnetisation, anhysteretic remanence and isothermal remanent magnetisation) and alternating field demagnetisation have been performed. Additionally scanning electron microscope imaging and energy-dispersive X-ray spectroscopy were performed to better characterise the magnetic minerals. Magnetisation is caused by titanomagnetite with slightly varying Ti contents within all cores. Samples from the positive magnetic anomalies have a relatively high degree of anisotropy, but no preferred orientation of one of the principal axes can be seen. The magnetic highs are caused by non-bedded volcaniclastic rocks comparatively rich in juvenile particles. The negative magnetic anomaly is associated with a compaction-like magnetic fabric but without macroscopically visible bedding. Hence, the magnetic low is caused by rocks with a lower content of juvenile material. To the authors’ knowledge, this is the first time rock magnetic methods have been applied to diatreme rocks.