Hysteresis parameters and magnetic anisotropy of silicate-hosted magnetite exsolutions

Abstract

Summary Anisotropy of remanent magnetization and magnetic susceptibility are highly sensitive and important indicators of geological processes which are largely controlled by mineralogical parameters of the ferrimagnetic fraction in rocks. To provide new physical insight into the complex interaction between magnetization structure, shape, and crystallographic relations, we here analyse “slice-and-view” focused-ion-beam (FIB) nano-tomography data with micromagnetic modelling and single crystal hysteresis measurements. The data sets consist of 68 magnetite inclusions in orthopyroxene (Mg60) and 234 magnetite inclusions in plagioclase (An63) were obtained on mineral separates from the Rustenburg Layered Suite of the Bushveld Intrusive Complex, South Africa. Electron backscatter diffraction was used to determine the orientation of the magnetite inclusions relative to the crystallographic directions of their silicate hosts. Hysteresis loops were calculated using the finite-element micromagnetics code MERRILL for each particle in 20 equidistributed field directions and compared with corresponding hysteresis loops measured using a vibrating sample magnetometer (VSM) on silicate mineral separates from the same samples. In plagioclase the ratio of remanent magnetization to saturation magnetization (Mrs/Ms) for both model and measurement agree within $1.0\%$, whereas the coercivity (Hc) of the average modeled curve is 20 mT lower than the measured value of 60 mT indicating the presence of additional sources of high coercivity in the bulk sample. The VSM hysteresis measurements of the orthopyroxene were dominated by multi-domain (MD) magnetite, whereas the FIB location was chosen to avoid MD particles and thus contains only particles with diameters <500 nm that are considered to be the most important carriers of paleomagnetic remanence. To correct for this sampling bias, measured MD hysteresis loops from synthetic and natural magnetites were combined with the average hysteresis loop from the MERRILL models of the FIB region. The result shows that while the modeled small-particle fraction only explains $6\%$ of the best fit to the measured VSM hysteresis loop, it contributes $28\%$ of the remanent magnetization. The modelled direction of maximal Mrs/Ms in plagioclase is sub-parallel to [001]plag, whereas Hc does not show a strong orientation dependence. The easy axis of magnetic remanence is in the direction of the magnetite population normal to (150)plag and the maximum calculated susceptibility (χ*) is parallel to [010]plag. For orthopyroxene, the maximum Mrs/Ms, maximum χ* and the easy axis of remanence is strongly correlated to the elongation axes of magnetite in the [001]opx direction. The maximum Hc is oriented along [100]opx and parallel to the minimum χ*, which reflects larger vortex nucleation fields when the applied field direction approaches the short axis. The maximum Hc is therefore orthogonal to the maximum Mrs/Ms, 16 controlled by axis-aligned metastable single-domain states at zero field. The results emphasize that the nature of anisotropy in natural magnetite does not just depend on the particle orientations, but on the presence of different stable and metastable domain states, and the mechanism of magnetic switching between them. Magnetic modelling of natural magnetic particles is therefore a vital method to extract and process anisotropic hysteresis parameters directly from the primary remanence carriers.

Mapping Magnetic Signals of Individual Magnetite Grains to Their Internal Magnetic Configurations Using Micromagnetic Models

“…A limitation of the microCT technique is that it cannot be used to directly measure crystallographic orientations beyond estimating it from the particle shape, for example, if it is an octahedral magnetite particle. In this context, Electron backscattered diffraction data may help to directly or indirectly determine the spacial orientation of the magnetocrystalline anisotropy axes (Nikolaisen et al., 2022 ) which will help to constrain the micromagnetic model.…”

MagNet: Automated Magnetic Mineral Grain Morphometry Using Convolutional Neural Network

“…MagNet is designed to analyze 2D images, which requires certain assumptions to be made regarding the 3D shapes of particles based on their 2D projected shapes and categorization before micromagnetic analysis (Figure 5) can be performed. However, much progress has been made recently in the application of focused ion beam nanotomography (FIB-nt) to generate detailed 3D information for micromagnetic modeling from sequences of 2D images obtained using a "slice-and-view" approach (Einsle et al, 2016;Lascu et al, 2018;Nikolaisen et al, 2020Nikolaisen et al, , 2022. The methods developed here may be applied automatically to each of the 2D images obtained, accelerating and improving the accuracy of the segmentation process.…”

“…(a) Biogenic magnetite nanoparticles synthesized by magnetotactic bacteria (MTB; Bazylinski & Frankel, 2004) and their fossil remains in the geological records (i.e., magnetofossils; Kopp & Kirschvink, 2008) have diverse crystal shapes and chain structures indicative of key paleoenvironmental signals (e.g., Amor et al., 2022; Berndt et al., 2020; Chang et al., 2019, 2018; Yamazaki & Kawahata, 1998). (b) Silicate‐hosted magnetic nanoparticle inclusions carrying important paleomagnetic signals (e.g., Tarduno et al., 2006, 2010) have variable morphologies and complex microstructures (Chang et al., 2016; Nikolaisen et al., 2020, 2022). (c) Tetrataenite in meteorites, which can record the magnetic activity of planetesimals in the early solar system (Bryson et al., 2014, 2015), has a variety of particle shapes and its particle size correlates with the metallographic cooling rate of meteorites (Einsle et al., 2018; Goldstein et al., 2009; Yang et al., 2010).…”

Mapping magnetic signals of individual magnetite grains to their internal magnetic configurations using micromagnetic models

“…A limitation of the microCT technique is that it cannot be used to directly measure crystallographic orientations beyond estimating it from the particle shape, for example, if it is an octahedral magnetite particle. In this context, Electron backscattered diffraction data may help to directly or indirectly determine the spacial orientation of the magnetocrystalline anisotropy axes (Nikolaisen et al, 2022) which will help to constrain the micromagnetic model.…”