An official website of the United States government
Abstract The minerals carrying the magnetic remanence in geological samples are commonly a solid solution series of iron‐titanium spinels known as titanomagnetites. Despite the range of possible compositions within this series, micromagnetic studies that characterize the magnetic domain structures present in these minerals have typically focused on magnetite. No studies systematically comparing the domain‐states present in titanomagnetites have been undertaken since the discovery of the single vortex (SV) structure and the advent of modern micromagnetism. The magnetic properties of the titanomagnetite series are known to vary strongly with composition, which may influence the domain states present in these minerals, and therefore the magnetic stability of the samples bearing them. We present results from micromagnetic simulations of titanomagnetite ellipsoids of varying shape and composition to find the size ranges of the single domain (SD) and SV structures. These size ranges overlap, allowing for regions where the SD and SV structures are both available. These regions are of interest as they may lead to magnetic instability and “partial thermal remanent magnetization (pTRM) tails” in paleointensity experiments. We find that although this SD + SV zone occupies a narrow range of sizes for equidimensional magnetite, it is widest for intermediate (TM30‐40) titanomagnetite compositions, and increases for both oblate and prolate particles, with some compositions and sizes having an SD + SV zone up to 100s of nm wide. Our results help to explain the prevalence of pTRM tail‐like behavior in paleointensity experiments. They also highlight regions of particles with unusual domain states to target for further investigation into the definitive mechanism behind paleointensity failure.
more »
« less
Absolute Paleointensity Study of Miocene Tiva Canyon Tuff, Yucca Mountain, Nevada: Role of Fine‐Particle Grain‐Size Variations
Abstract Fine‐grained, Ti‐poor titanomagnetite in the ~12.7 Ma Tiva Canyon (TC) Tuff systematically increases in grain size from superparamagnetic (SP) at the flow base to single domain (SD) at a few meters height. This allows us to examine the role of grain‐size variation on paleointensity, within the transition from SP to stable SD. We present magnetic properties from two previously unreported sections of the TC Tuff, as well as Thellier‐type paleointensity estimates from the lowermost ~7.0 m of the flow. Magnetic hysteresis, frequency‐dependent susceptibility, and thermomagnetic data show that sample grain‐size distribution is dominated by SP in the lower ~3.6 m, transitioning upwards to mostly stable SD. Paleointensity results are closely tied to stratigraphic height and to magnetic properties linked to domain state. SD samples have consistent absolute paleointensity values of 28.5 ± 1.94 μT (VADM of 51.3 ZAm2) and behaved ideally during paleointensity experiments. The samples including a significant SP fraction have consistently higher paleointensities and less ideal behavior but would likely pass many traditional quality‐control tests. We interpret the SD remanence to be a primary thermal remanent magnetization but discuss the possibility of a partial thermal‐chemical remanent magnetization if microcrystal growth continued at T < Tcand/or the section is affected by post‐emplacement vapor‐phase alteration. The link between paleointensity and domain state is stronger than correlations with water content or other evidence of alteration and suggests that the presence of a significant SP population may adversely impact paleointensity results, even in the presence of a stable SD fraction.
more »
« less
- Award ID(s):
- 1642268
- PAR ID:
- 10456078
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 20
- Issue:
- 12
- ISSN:
- 1525-2027
- Page Range / eLocation ID:
- p. 5818-5830
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The late Ediacaran to early Cambrian witnessed significant Earth system changes, including animal life diversification and an enigmatic paleomagnetic record. This study focuses on the Nama Group, a key geological unit for understanding the Ediacaran‐Cambrian transition. Previous paleomagnetic studies in the Nama Group identified complex remagnetization patterns but lacked a detailed examination of remanence carriers. To address this, we conducted a series of rock magnetic experiments on unweathered borehole core samples to better constrain the remagnetization mechanisms. Thermal demagnetization identified two magnetic components.C1, a recent viscous remanent magnetization, used for borehole core orientation, andC2, a stable remagnetization component carried by single‐domain (SD) pyrrhotite and magnetite. Magnetic mineralogy and paleomagnetic data suggest that the remanence acquisition mechanism ofC2is best explained by thermoviscous remanent magnetization (TVRM) and thermal remanent magnetization (TRM), rather than chemical remanent magnetization (CRM). The presence of low unblocking temperatures, coupled with thermochronological evidence of prolonged heating during tectonic collisions and subsequent cooling, supports this interpretation. The remagnetization event is linked to the final consolidation of West Gondwanaland during the late stages of megacontinent assembly (∼490–480 Ma), coinciding with regional uplift and a stable geomagnetic field during the Moyero reverse superchron. These findings challenge the CRM hypothesis, as the quasi‐synchronous remagnetization across cratonic blocks and the predominance of single reverse polarity are better explained by thermal processes. This study highlights the critical role of thermoviscous relaxation in large‐scale remagnetization and provides new insights into the tectonic evolution of West Gondwanaland.more » « less
-
Abstract Pressure remanent magnetization (PRM) is acquired when a rock is compressed in the presence of a magnetic field. This process can take place in many different environments from impact and ejection processes in space, to burial and subsequent uplifting of terrestrial rocks. In this study, we systematically study the acquisition of PRM at different pressures and temperatures, using synthetic magnetite in four different grain sizes ranging from nearly single‐domain to purely multidomain. The magnitude of the PRM acquired in a 300 μTfield is, within error, independent of the domain state of the sample. We propose that the acquisition of a PRM is mainly driven by the magnetostriction of the magnetic material. We further show that compared to a thermal remanent magnetization, the acquisition of PRM in large multidomain grains can be quite efficient, and may represent a significant component of magnetization in low‐temperature–high‐pressure environments.more » « less
-
Abstract Absolute paleointensity (API) of the geomagnetic field can be estimated from volcanic rocks by comparing the natural remanent magnetization (NRM) to a laboratory‐induced thermoremanent magnetization (Lab‐TRM). Plots of NRM unblocking versus Lab‐TRM blocking from API experiments often exhibit nonideal curvature, which can result in biased estimates. Previous work showed that curvature can increase with age; however, selection criteria designed to eliminate such behavior yielded accurate estimates for two‐year‐aged specimens (70.3 ± 3.8 μT;N = 96 specimens out of 120 experiments). API can also be estimated in coercivity space. Here, we use the Tsunakawa‐Shaw (TS) method applied to 20 specimens aged in the laboratory field of 70.0 μT for 4 years, after acquisition of zero‐age (fresh) Lab‐TRM in the same field. Selection criteria for the TS experiment also yielded accurate results (68.5 ± 4.5 μT;N = 17 specimens). In thermal API experiments, curvature is related to internal structure with more single domain‐like behavior having the least curvature. Here we show that the fraction of anhysteretic remanent magnetization demagnetized by low‐temperature treatment was larger for samples with larger thermal curvatures suggesting a magnetocrystalline anisotropy source. We also tested experimental remedies that have been proposed to improve the accuracy of paleointensity estimates. In particular, we test the efficacy of the multi‐specimen approach and a strategy pretreating specimens with low field alternating field demagnetization prior to the paleointensity experiment. Neither yielded accurate results.more » « less
-
Abstract Unmixing of remanent magnetization curves, either isothermal remanent magnetization (IRM) or backfield IRM, is widely used in rock magnetic and environmental magnetic studies to discriminate between magnetic coercivity components of different origins. However, the wide range of physical properties of natural magnetic particles gives rise to an ambiguous interpretation of these components. To reduce this ambiguity and provide a straightforward interpretation of coercivity components in terms of domain state, interactions, and constituent magnetic phases, we combined backfield IRM unmixing with unmixing of nonlinear Preisach maps for two typical mid‐latitude northern hemisphere loess‐paleosol sequences. Both backfield IRM and nonlinear Preisach maps unmixing are based on the same non‐parametric algorithm, and provide similar endmembers (EMs) in the two sections studied. The first EM (EM1) has a low median coercivity (∼21 mT) and is a non‐interacting single domain (SD) magnetite/maghemite of pedogenic origin. The second EM (EM2) has a moderate median coercivity (∼60 mT) and is a mixture of pseudo‐single domain/multidomain, SD magnetite/maghemite and non‐interacting SD hematite, all of eolian origin. The same EM1 found in both sections suggests that this component's grain size and coercivity are independent of pedogenesis intensity. The same EM2 indicates that a similar magnetic population is being transported and deposited, irrespective of the dust source area and loess granulometry. The approach outlined here provides strong evidence that non‐parametric backfield IRM unmixing isolates physically realistic EMs. Unmixing nonlinear Preisach maps elucidates these EMs in terms of domain states and their constituent magnetic phases.more » « less
