Direct visualization of chemical and thermo-remanent magnetization of pseudo-single-domain magnetite grains and the implications for reliable paleomagentic signal acquisition

Abstract:

In order to reliably interpret paleomagnetic measurements, the mechanisms of chemical remanent magnetization (CRM) and thermoremanent magnetization (TRM) must be fully understood. Currently, most models of CRM and TRM processes only exist for the smallest, uniformly magnetized grains, termed single domain (SD). However, the magnetic signal from rocks is often dominated by slightly larger grains containing non-uniform magnetization states, termed pseudo-SD (PSD) grains.

Magnetite (Fe₃O₄) is the most magnetic naturally occurring mineral on Earth, carrying the dominant magnetic signature in rocks and providing a critical tool in paleomagnetism. The oxidation of Fe₃O₄ to other iron oxides, such as maghemite (γ-Fe₂O₃) and hematite (α-Fe₂O₃), is of particular interest as it influences the preservation of remanence of the Earth's magnetic field by Fe₃O₄. Further, TRM in Fe₃O₄ grains is acquired in the direction of the ambient geomagnetic field as they cool below their Curie temperature (T_C) of ~ 580 ˚C. The latest transmission electron microscopy (TEM) techniques like electron holography and environmental TEM (ETEM) allows for the imaging of magnetization in nano-scale minerals during in situ heating under vacuum and controlled atmospheres.

In the present study, synthetic Fe₃O₄ particles in the PSD size range (< 200 nm) were heated in situ in an ETEM under an O₂ atmosphere. Close examination of Fe₃O₄ particles after in situ heating revealed surface degradation, whilst electron energy-loss spectroscopy confirmed their oxidation. The effect of CRM was visualized using electron holography, in the form of reconstructed magnetic induction maps, where the oxidized grains exhibited a loss of overall remanence and change in remanent direction.

The thermomagnetic behavior of Fe₃O₄ particles in the PSD size range is also investigated using off-axis electron holography. Magnetic induction maps, which are recorded during in situ heating up to above the T_C, reveal the PSD nature of several Fe₃O₄ grains by visualizing their vortex domain states. The vortex states in small Fe₃O₄ grains (Fig. 1a & b) are shown to rotate or collapse into a single-domain state close to its unblocking temperature (Fig. 1c), rather than remaining thermally stable as seen in the vortex states of larger Fe₃O₄ grains.