Tiny Space Magnets: X-ray Microscopy and Nanopaleomagnetism of Meteoritic Metal
Meteorites provide information about the early history of our solar system and the formation and evolution of planetesimals. One of the few direct observations of internal geophysical processes within planetary bodies is the presence or absence of a dynamo-driven magnetic field. These observations provide essential constraints on the degree of differentiation, core solidification timescales and the driving forces for convection.
This thesis focusses on the paleomagnetic information recorded by iron and stony-iron meteorites, providing us with a unique view-point for the generation and variability of core dynamo activity.
Iron and stony-iron meteorites are primarily comprised of FeNi metal. The Widmanstätten pattern; an intergrowth of taenite and kamacite lamellae. Between these lamellae, a range of microstructures develop, dictated by the ‘M-shaped’ Ni diffusion profile. Among these microstructures is the cloudy zone, a region of tetrataenite islands in a Fe-rich matrix, formed by spinodal decomposition. The tetrataenite islands are extremely reliable paleomagnetic recorders. The direction of magnetisation and composition of FeNi microstructures was imaged using synchrotron X-rays. Magnetic contrast is generated using X-ray magnetic circular dichroism.
The dimensions of tetrataenite islands within the cloudy zone directly correlate with cooling rates. Cooling rates vary from ~0.5–10,000°C/Myr and correspond to island sizes of ~500–10nm, respectively. The slowest cooled group of iron meteorites reveal multidomain magnetic behaviour within the cloudy zone, whereas in faster-cooled meteorites islands are vortex state. This demonstrates that cooling rate influences the magnetic properties of the cloudy zone. The subtly different cooling rates between different pallasite meteorites means that each meteorite provides a ‘snapshot’ of the parent body magnetic field at a different point during its thermal evolution. Paleointensity results provide the first observations of a quiescent period in dynamo activity preceding core solidification. This also helps to constrain the paleomagnetic signals associated with core nucleation, which, in turn, constrains the mechanism of solidification.
Paleomagnetic studies of meteoritic metal were complemented with measurements of magnetic inclusions in olivines. Alternating-field and thermal demagnetisation experiments were carried out using both 2G SQUID and WSGI small-bore SQUID magnetometers. Results suggest that pallasite silicates are unreliable paleomagnetic recorders, and a planetary-strength paleointensity cannot be recovered. Paleomagnetic fidelity was also investigated for a dusty olivine grain from the Semarkona chondrite. Lorentz microscopy, transmission X-ray microscopy, nanotomography and micromagnetic simulations were used to rigorously test the behaviour of Fe-nanoparticles.
The final study in this thesis focusses on the IAB iron meteorites. These meteorites have an unusual and complex history. Paleomagnetic results are accompanied by a detailed microstructural study using X-PEEM and electron backscatter diffraction to constrain the formation of two microstructures: pearlitic and spheroidised plessite. Paleomagnetic results suggest the IAB parent body did not have an active core dynamo.
Meteorites represent the oldest material in our solar system, and their complex histories and susceptibility to alteration make them some of the most challenging samples to extract reliable paleointensity estimates from. Advanced electron microscopy and synchrotron techniques are now making it possible to extract reliable paleomagnetic information, with profound implications for the formation and evolution of the solar system.