Determining ancient magnetic field strengths from the Earth and Solar System

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Palaeomagnetic recordings in ancient rocks and meteorites hold the key to answering some of the most fundamental questions in Earth and Planetary Sciences including the evolution of the Core and geodynamo, plate tectonics and palaeogeography, and the formation of the Solar System. Recently, our fundamental understanding of how rocks record the geomagnetic field has been challenged. Hitherto palaeomagnetists regarded ideal recorders as those particles that are both uniformly magnetized and thermally stable, and referred to these as single-domain (SD) grains. However, it has long been recognised that the SD range size range for most grain shapes (expressed as the diameter of their equivalent volume sphere) is extremely narrow, only existing in particles between approximately 30 to 80 nm in size. Most palaeomagnetic samples are dominated by larger grains that contain non-uniform magnetic domain structures referred to a pseudo-single-domain (PSD), which reflects the ambiguity with which their magnetic properties are known. The origin of PSD grains' magnetization and apparently high stability has remained largely a mystery; palaeomagnetic protocols designed for SD behaviour often have very high failure rates (as much as 90 %). These high failure rates are then usually attributed to the presence of PSD grains. The advent of numerical micromagnetic modelling and nanometric magnetic imaging, has given us the ability to understand these complex systems. A remarkable recent discovery, from a collaboration between the PI, CoI and named PDRA, who showed that these highly magnetised PSD grains usually exist in a single magnetic vortex domain state, which has both a high magnetic signal and a much higher recording stability than even the 'ideal' uniformly magnetised SD particles. With these recent developments in numerical modeling, we are now in position to construct a full model of the many millions of magnetic particles contained within a real palaeomagnetic sample. This will be achieved through the development of a micromagnetic database that contains the full domain structure and magnetic characteristics as a function of grain size, shape, temperature and external field strength, for particles in the most palaeomagnetic significant size range of 30 - 1000 nm in magnetite. This database will form a unique resource from which we can mine the data to: 1) Provide a comprehensive and fundamental new understanding of PSD domain state characteristics. This is essential in order to provide a means of linking laboratory rock-magnetic observations to the thermal stability and magnetic blocking temperatures that control a samples ability to retain an accurate recording of the geomagnetic field over many millions of years. 2) Reconstruct the recording fidelity of palaeomagnetic samples. For any distribution of grain sizes we will be able to predict the ability of a sample to acquire a natural remanent magnetisation through a simulated cooling from above its Curie temperature. The effect of subsequent partial remagnetization in laboratory magnetic fields and cooling rates can then be determined to determine the impact of the experimental process on our ability to extract the correct value of the ancient geomagnetic field. 3) Establish a non-heating method of obtaining reliable palaeointensity values. A major drawback in current methods of paleointensity determinations is that the magnetic minerals often undergo chemical alteration during laboratory re-heating. By establishing the relationship between domain state thermal and magnetic field stability (blocking temperatures and coercivities) it will be possible to provide a theoretical basis for replacing laboratory heating by room-temperature isothermal or anhysteretic magnetization measurements.