Direct observation of the thermal demagnetization of magnetic vortex structures in non-ideal magnetite recorders

The thermal demagnetization of pseudo-single-domain (PSD) magnetite (Fe3O4) particles, which govern the magnetic signal in many igneous rocks, is examined using off-axis electron holography. Visualization of a vortex structure held by an individual Fe3O4 particle (~ 250 nm in diameter) during in situ heating is achieved through the construction and examination of magnetic-induction maps. Step-wise demagnetization of the remanence-induced Fe3O4 particle upon heating to above the Curie temperature, performed in a similar fashion to bulk thermal demagnetization measurements, revealed its vortex state remains stable under heating close to its unblocking temperature, and is recovered upon cooling with the same or reversed vorticity. Hence, the PSD Fe3O4 particle exhibits thermomagnetic behavior comparable to a single-domain carrier, and thus vortex-states are considered reliable magnetic recorders for paleomagnetic investigations

Palaeomagnetic studies of Eyjafjardardalur, northern Iceland: does the geocentric axial dipole exist during 3-7 Ma?

The geocentric axial dipole (GAD) hypothesis states that when we average the geomagnetic field over sufficient geological time intervals, the time-averaged field (TAF) behaves like a dipole aligned along the Earth’s spin axis and positioned at the Earth’s centre. This hypothesis is crucial in palaeomagnetic research, e.g., it is a key to tectonic reconstructions. However, there is some evidence for the persistence of long-term hemispheric asymmetry on time scales of 10^5-10^6 yrs, particularly at high-latitudes. As most palaeomagnetic research is conducted under the GAD hypothesis, this hypothesis needs to be rigorously tested. This thesis investigates the symmetry of the palaeomagnetic field and tests the GAD hypothesis during ~2.6-8.5 Ma using full-vector palaeomagnetic data - including palaeodirection and palaeointensity - from dated lava piles in northern Iceland. Demagnetisation measurements including alternating field (AF) and thermal were made to determine palaeomagnetic directions. A mean declination and inclination of 354.1° and 71.4° were found in this study, with a 95% confidence limit of only 2.4°. The mean virtual geomagnetic pole (VGP) is located at 80.6°N and 184.6°E, which is tilted ~10° from the geographic north; this VGP position does not agree with the GAD hypothesis. It is argued that this shallow inclination is a result of the non- dipole fields which contribute to the GAD field. This was confirmed by the palaeosecular variation (PSV) model compilation of the global dataset during 0-8.5 Ma. The PSV model suggests that 4% of axial quadrupole and 1% of axial octupole contribute to the TAF during 0-8.5 Ma. Palaeointensity data reveals a weak mean intensity of 26.9 ± 1.8 μT, which is lower than the intensity of the expected GAD field (55.9 μT) at 65°N for the current day field. This weak intensity equates to a virtual dipole moment (VDM) of 37.4 ± 2.5 ZAm^2. After combining this study data with 1-3 Ma data also from Iceland to improve temporal resolution, a mean VADM of 44.0 ± 2.2 ZAm^2 was obtained. The direct comparison of the VADM between high-northern and high-southern latitudes (37.3 ± 3.8 ZAm2) was made. This study reveals that there is hemispheric asymmetry of the field between Northern and Southern Hemisphere.Open Acces