Models of Maghematization: Observational Evidence in Support of a Magnetic Unstable Zone

Ge, Kunpeng; Williams, Wyn; Nagy, Lesleis; Tauxe, Lisa

doi:10.1029/2020gc009504

Cited by 8 publications

(13 citation statements)

References 39 publications

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“…(2019) conducted studies on the magnetic domain state transition of greigite and iron, and Ge et al. (2021) supported the observational existence of magnetically unstable (MU in this study) particles by combining micromagnetic modeling and low‐temperature oxidation experiments. The present study provides a more detailed micromagnetic investigation of the MU zone and the domain state behavior in applied fields to determine whether magnetic information can be reliably extracted from such particles.…”

Section: Introductionsupporting

confidence: 57%

“…The dramatic decrease of magnetic blocking temperature and relaxation time has also been preliminarily captured in searching for the boundary of SD-PSD in the earlier studies (Enkin and Williams, 1994;Winklhofer et al, 1997;Muxworthy et al, 2003). Subsequently, Valdez-Grijalva et al (2018) and Nagy et al (2019b) conducted studies on the magnetic domain state transition of greigite and iron, and Ge et al (2021) supported the observational existence of magnetically unstable (MU in this study) particles by combining micromagnetic modeling and low-temperature oxidation experiments. The present study provides a more detailed micromagnetic investigation of the MU zone and the domain state behavior in applied fields to determine whether magnetic information can be reliably extracted from such particles.…”

Section: Introductionmentioning

confidence: 55%

“…Ge et al. (2021) reported similar stability calculations for magnetite grains with a shape factor of 1.3 and found a slightly broader room temperature unstable zone of ∼100–150 nm, so it is possible that the unstable zone will grow further with elongation. Therefore, the unstable zone is likely not a fixed region but will also shift with changes both in grain elongation and mineralogy (Valdez‐Grijalva et al., 2018).…”

Section: Discussionmentioning

confidence: 83%

“…(2021) whereby the blocking temperature spectra of non‐SD samples appears to change even over a few years, a result unexpected from classic Néel theory. As the particle size distribution in natural samples could be estimated by laser particle size analyzer (LPSA) or FIB‐SEM methods, then more appropriate samples/specimens could be selected out with high coercivities or via low‐temperature oxidation for paleo‐direction and paleointensity studies (Ge et al., 2021). Alternatively, to actively avoid the MU zones, it may be better to use samples that contain larger SV grains for paleo‐direction and paleointensity studies since larger SV grains are much more SD‐like in their stability and remanence (Nagy et al., 2017) or seek the finest grain sizes by targeting quenched materials like submarine basaltic glass (e.g., Pick & Tauxe, 1993).…”

Section: Discussionmentioning

confidence: 99%

“…Compared with the SD grain size range of ∼30-70 nm, our results suggest that the MU zone contributes to ∼10% of B c and M rs /M s spectrum for samples containing magnetite particles between 25 and 300 nm, and thus the influence of the MU zone on paleo-direction and especially paleointensity observations in fine-grained material should not be ignored and may explain the "fragile curvature" effect documented by Tauxe et al (2021) whereby the blocking temperature spectra of non-SD samples appears to change even over a few years, a result unexpected from classic Néel theory. As the particle size distribution in natural samples could be estimated by laser particle size analyzer (LPSA) or FIB-SEM methods, then more appropriate samples/specimens could be selected out with high coercivities or via low-temperature oxidation for paleo-direction and paleointensity studies (Ge et al, 2021). Alternatively, to actively avoid the MU zones, it may be better to use samples that contain larger SV grains for paleo-direction and paleointensity studies since larger SV grains are much more SD-like in their stability and remanence (Nagy et al, 2017) or seek the finest grain sizes by targeting quenched materials like submarine basaltic glass (e.g., Pick & Tauxe, 1993).…”

Section: The Evolution Process Of the Magnetically Unstable Zonementioning

confidence: 99%

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Micromagnetic Modeling of a Magnetically Unstable Zone and Its Geological Significances

Wang

Williams

et al. 2022

JGR Solid Earth

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Recent micromagnetic simulations have found that particles in the transitional zone between the single domain (SD) and single vortex (SV) zone are prone to thermal and external magnetic field instabilities that could adversely affect the accuracy of interpretations of paleomagnetic recordings. In this study, we attempt to evaluate the internal magnetization characteristics of these magnetically unstable (MU) particles and the influence on paleomagnetic observations by simulating the magnetic behavior of 68–104 nm truncated octahedral magnetite particles using the MERRILL modeling software. We found that: (a) The size region of the “MU zone” for grains of truncated octahedron shape is different from cubic octahedrons and spheres, indicating that the zone may be controlled by the geometry and shape of particles; (b) The MU zone has a range of 79–97 nm region, which is dominated by a hard‐axis aligned single vortex (HSV); and (c) MU particles are unstable as a function of temperature and have low coercive fields. Finally, the numerical fitting of hysteresis parameters for experimental data suggests that the influence of such MU particles in samples should not be ignored, especially for samples with fine‐grained magnetic minerals as the primary magnetic recording carriers. This research has extended our understanding of the behavior of the “MU zone” and its significance on paleomagnetic records.

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Section: Introductionsupporting

confidence: 57%

Section: Introductionmentioning

confidence: 55%

Section: Discussionmentioning

confidence: 83%

Section: Discussionmentioning

confidence: 99%

Section: The Evolution Process Of the Magnetically Unstable Zonementioning

confidence: 99%

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Micromagnetic Modeling of a Magnetically Unstable Zone and Its Geological Significances

Wang

Williams

et al. 2022

JGR Solid Earth

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The Origin of Magnetofossil Coercivity Components: Constraints From Coupled Experimental Observations and Micromagnetic Calculations

Xue,

Chang,

Pei

et al. 2024

JGR Solid Earth

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Biogenic magnetite crystals produced by magnetotactic bacteria (MTB) and associated magnetofossils in sediments are characterized by variable morphologies, grain sizes, and chain structures. Magnetofossils are widely used in paleomagnetic and paleoenvironmental studies, but the complex magnetofossil shapes and particle arrangements significantly affect magnetic properties, hampering their magnetic detection and proxy interpretation. Here we perform coupled experimental and micromagnetic modeling analyses of typical magnetofossil‐rich sediments, where the effects of magnetofossil crystal forms and microstructures on magnetic properties can be quantitatively separated. Since the in situ magnetofossil chain structures in sediments remain poorly known, we compare results from magnetic measurements and micromagnetic simulations based on realistic magnetofossil shapes and grain size distributions. Our results suggest that bullet‐shaped magnetofossils certainly contribute to the biogenic hard (BH) coercivity component with a minor contribution from elongated prismatic particles, and collapsed equidimensional grains to the biogenic soft (BS) component. Micromagnetic simulations with different collapse models of bullet‐shaped magnetofossils produce variable FORC (first‐order reversal curve) central‐ridge contributions with similar coercivity distributions. Sensitivity test suggests that samples containing different forms of magnetofossils can produce the BH coercivity component if the proportion of the bullet‐shaped particles is more than ∼2%. Magnetofossil assemblages with a higher proportion of bullet‐shaped particles have higher coercivities, squareness ratios, and larger BH contents. Our data shed new light on understanding the origin of magnetofossil coercivity components and the in situ magnetofossil microstructures in sediments, which is widely useful for interpreting magnetofossil proxy signals in geological records.

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Vortex Magnetic Domain State Behavior in the Day Plot

Williams,

Moreno,

Muxworthy

et al. 2024

Geochem Geophys Geosyst

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The ability of rocks to hold a reliable record of the ancient geomagnetic field depends on the structure and stability of magnetic domain‐states contained within constituent particles. In paleomagnetic studies, the Day plot is an easily constructed graph of magnetic hysteresis parameters that is frequently used to estimate the likely magnetic recording stability of samples. Often samples plot in the region of the Day plot attributed to so‐called pseudo‐single‐domain particles with little understanding of the implications for domain‐states or recording fidelity. Here we use micromagnetic models to explore the hysteresis parameters of magnetite particles with idealized prolate and oblate truncated‐octahedral geometries containing single domain (SD), single‐vortex and occasionally multi‐vortex states. We show that these domain states exhibit a well‐defined trend in the Day plot that extends from the SD region well into the multi‐domain region, all of which are likely to be stable remanence carriers. We suggest that although the interpretation of the Day plot and its variants might be subject to ambiguities, if the magnetic mineralogy is known, it can still provide some useful insights about paleomagnetic specimens' dominant domain state, average particle sizes and, consequently, their paleomagnetic stability.

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Models of Maghematization: Observational Evidence in Support of a Magnetic Unstable Zone

Abstract: Paleomagnetic studies contribute fundamentally to our understanding of plate tectonics, reconstruction of a global climate framework, the physics of dynamic processes deep within the Earth as well as providing constraints on theories of evolution of galaxies (

Cited by 8 publications

References 39 publications

Micromagnetic Modeling of a Magnetically Unstable Zone and Its Geological Significances

Micromagnetic Modeling of a Magnetically Unstable Zone and Its Geological Significances

The Origin of Magnetofossil Coercivity Components: Constraints From Coupled Experimental Observations and Micromagnetic Calculations

Vortex Magnetic Domain State Behavior in the Day Plot

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