Absolute paleointensities on cretaceous kimberlites in Botswana
Final Report Abstract
Long periods of stability of the Earth’s Magnetic Field (superchrons) provide a remarkable opportunity to test the various theories and models regarding convection in the Earth’s liquid outer core and the generation of the geomagnetic field. However, despite significant progress during the last two decades, we are still far from answering some of the key questions related to the geodynamo and processes in the outer liquid core to which long term changes in intensity of the geomagnetic field are related . Was the intensity of the Earth Magnetic Field, respectively the Virtual Dipole Momement of the Earth, lower during the Mesozoic than during the Cenozoic or was it higher, especially during the Cretaceous Normal Superchron where the field was stable for roughly 38ma. Answering the later will provide more insights into geodynamo processes and in particular help to understand how the vigor of outer core convection is related to reversals of the Earth Magnetic Field. Taking the Virtual Dipole Moment of the Earth as a measure for vigor of outer core convection, studies of absolute paleointensities can provide important clues for answering this question. Unfortunately, however, the paleointensity database is still rather small and every new high quality data point is improving the data set. Kimberlites of Cretaceous age, which are quite numerous in southern Africa, may provide an opportunity to obtain a high amount of intensity data for the Cretaceous Normal Superchron. The target of this study was, thus, to sample a variety of kimberlite pipes in Botswana and to check their suitability for paleointensity determination. Previous studies, which have focused on their paleodirectional characteristics, yielded high quality data and the resulting paleomagnetic poles form an important part of Africa’s apparent polar wander path for the mid to late Cretaceous. In the course of the project 84 samples from the Orapa and two neighboring kimberlite mines in Botswana were studied in detail. The sampling material originated from a collection of kimberlite rocks originally sampled and rockmagnetically studied in detail by Dr. Conall MacNiocaill, Oxford. Only samples were used for paleoinstensity studies which didn’t show any evidence for low-temperature oxidation. Two thirds of the paleointensity determinations were successful and yield interpretable results. The mean intensity for these rocks which have been dated radiometrically to be ~93m.a. in age is about 35μT which is smaller but still comparable to today’s values. Although these results are based on a small number of samples and must be considered preliminary they show that kimberlites, which have been neglected by the paleointensity community, have a high potential for the determination of paleointensities.
