Self-Reversal of Remanent Magnetisation of Basalts - Origin, Mechanisms and Consequences
One of the main goals of current palaeomagnetic research is the attempt to acquire high-resolution data on palaeodirections and -intensities in order to obtain detailed information about the Earth's magnetic field in the geological past. The material best suited for such studies are basaltic rocks. For these high-quality directional investigations and especially for palaeointensity determinations, a profound knowledge about the stability, magnetomineralogical character and the domain state of the carriers of remanence is imperative. The emphasis of the present work was placed on the investigation of basalts exhibiting partial or complete self-reversal of natural remanent magnetisation (NRM). This phenomenon is not an exotic rarity but a widespread characteristic of many basaltic rocks. However it remains usually unnoticed by routine palaeomagnetic measurements as it requires special techniques for its detection. In this work samples from Olby and Laschamp (France) and Vogelsberg (Germany) showing the phenomenon were studied with rock magnetic, microscopic and microanalytical techniques in order to identify the carriers of self-reversed remanent magnetisation. Further aims were to determine the exact mechanism of self-reversal acting in basalts and to evaluate the consequences for the reliability of palaeomagnetic data. On the basis of the experimental work a numerical model was developed which shows that, from the physical point of view, the observed magnetomineralogy is capable of causing self-reversal. The present work provides the following new insights: The phenomenon is caused by two magnetic phases with different blocking temperatures which are magnetically coupled. The lower blocking temperature corresponds to the primary titanomagnetite (mother phase) crystallising from the basaltic magma. The remanence with higher blocking temperature is carried by titanomaghemite (daughter phase) evolving from the primary titanomagnetite by partial low-temperature oxidation. The daughter phase forms narrow bands along cracks in the otherwise unaffected mother phase particles. This yields a close side-by-side assemblage of titanomagnetite and titanomaghemite with markedly different magnetic properties in one and the same grain. By applying the various microscopic techniques on identical grains, it was possible to directly correlate magnetomineralogy with magnetic domain structure. Numerical calculations of remanence acquisition demonstrate that two-phase particles with the experimentally observed geometry and magnetic properties are able to acquire a partially or completely self-reversed remanent magnetisation. The calculations also prove that the two magnetic phases present in the studied samples are coupled by magnetostatic interaction. The experimental results indicate that the low-temperature oxidation process responsible for the formation of the second magnetic phase takes place at temperatures at or above the blocking temperature of this daughter phase during primary cooling. This titanomaghemite phase is thus carrying a stable remanence in direction of the ambient magnetic field. Although the original titanomagnetite as the mother phase is in a strict sense the primary magnetic mineral, it does not carry the primary magnetic remanence but is at least in part magnetostatically coupled to the titanomaghemite. Therefore, its remanence is - at least in part - antiparallel to the external field. MFM domain observations present evidence that the mother phase is in the magnetic multidomain range. Hence, its magnetic remanence is not stable and is replaced by a viscous overprint acquired at ambient temperatures. In contrast, the daughter phase has a higher coercivity due to oxidation induced stresses and an increased domain width. Regarding the samples from Olby, these magnetomineralogical investigations directly lead to new arguments in favour of the existence of the Laschamp geomagnetic event: As the high blocking temperature daughter phase carries a stable remanence in direction of the external magnetic field, the local geomagnetic field direction was indeed reversed at the time of emplacement. Due to their complex magnetomineralogy and remanence acquisition, samples exhibiting partial or complete self-reversal are not suitable for palaeointensity determinations. In order to identify and exclude such samples in the course of such experiments, a modification of the existing Thellier-Thellier method is proposed. Additionally, this new procedure is also able to detect remanence carried by multidomain (MD) particles. The method substantially improves the reliability and quality of palaeointensity estimates as multidomain behaviour is among the most common reasons for erroneous results in Thellier-type palaeointensity determinations.