Proterozoic Magnetic Field Intensities Obtained Using the Microwave Palaeointensity Technique.

Geomagnetic field intensities have been obtained using the microwave palaeointensity technique from a number of Precambrian dyke swarms. This method uses high-frequency microwaves resonant with the magnetic carrier to demagnetise and remagnetise the sample. As the microwaves only interact the magnetic grains (which make up only a small component of the rock) bulk heating of the specimen is reduced, minimising sample alteration and increasing the chance of experimental success. The conventional Thellier palaeointensity method has also been applied as an independent standard. Chilled margin samples from a selection of Proterozoic dolerite dykes ranging in age 2.5 - 1.2Ga have been studied to ascertain the magnetic field strength at the time of formation. Rock magnetic analysis has been carried out to evaluate magnetic mineralogy and grain size. Curie point analysis shows single distinct Curie temperatures at 580° C for the majority of samples, indicating pure magnetite. Hysteresis and backfield IRM measurements indicate grains within the pseudo-single domain (PSD) grain size range. Lowrie-Fuller tests conducted to determine the grain size of remanence suggest that for samples that pass acceptance criteria, remanence is carried by single domain (SD) grains. High quality field estimates were obtained, passing alteration and multi-domain checks, with palaeointensities generally being lower than present. The corresponding virtual dipole moment (VDM) for the geomagnetic field across the Proterozoic ranges from 2 - 5 x10²² Am², showing the geodynamo to be stronger in intensity now (8 x10²² Am²) than it was earlier in geologic history.