An official website of the United States government
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
The Effect of Differential Weathering on The Magnetic Properties of Paleosols: A Case Study of Magnetic Enhancement vs. Magnetic Depletion in the Pleistocene Blackwater Draw Formation, Texas
The type-section of the Blackwater Draw Formation (BDF) consists of a series of five paleosol horizons developed on eolian deposits and an overlying surficial soil. Previous work has shown that magnetic properties (e.g., χ, ARM, and IRM) as a function of depth in this type-section, display both magnetically enhanced and magnetically depleted signals for different paleosols. To better understand the magnetic mineralogy responsible for these varying responses, various rock-magnetic experiments, scanning electron microscopy, and Mössbauer spectroscopy were conducted on representative samples from the six soil units which constitute the BDF type-section. Our results show that sub-micron hematite [with a minor contribution from single-domain sized hematite (Hc = ∼500 mT) dominates all the soils in terms of weight percent concentration. Whereas, low coercivity (Hc = ∼35 mT or less) magnetite/maghemitized-magnetite grains, largely in the PSD state (Mr/Ms=∼0.14 +/– 0.03588, Hcr/Hc=∼2.68 +/– 0.298789), dominate the magnetic signal. Magnetically depleted soils show a relatively higher proportion of goethite, while magnetically enhanced soils show an increased contribution from SP/SSD magnetite/maghemite phases.By combining our data-set with geochemically-derived climofunctions, we have correlated the magnetically preserved, depleted, and enhanced sections of the type-section to three distinct environmental phases (I-III). The basal sediments of Phase I displays relatively homogenous (neither enhanced nor depleted) magnetic properties due to relatively arid conditions and minimal alteration of southerly derive eolian sands. Conversely, Phase II-III represents a change in weathering intensities and provenance, resulting in a mix of southerly derived sands and northerly derived silts. Phase II, experienced greater precipitation levels, resulting in the dissolution of Fe-oxide phases and thus magnetic depletion. The uppermost Phase III experienced intermediate precipitation intensities resulting in magnetic enhancement.Using previously published age models we tentatively interpret these changing environmental conditions to be influenced by the Middle-Pleistocene Transition (1.2-0.7 Ma), where the Earth’s climatic cycles shifted from a ∼41 kyr to ∼100 kyr cycles. However, ambiguities persist due to uncertainties in the currently published age model. Due to the complexity of the magnetic signal, we recommend future studies utilize a holistic approach, incorporating rock-magnetic, geochemical, and microscopy observations for more accurate reconstruction of regional paleoenvironments.
more »
« less
- Award ID(s):
- 1642268
- PAR ID:
- 10318867
- Date Published:
- Journal Name:
- Frontiers in Earth Science
- Volume:
- 9
- ISSN:
- 2296-6463
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract In well‐buffered modern soils, higher annual rainfall is associated with enhanced soil ferrimagnetic mineral content, especially of ultrafine particles that result in distinctive rock magnetic properties. Hence, paleosol magnetism has been widely used as a paleoprecipitation proxy. Identifying the dominant mechanism(s) of magnetic enhancement in a given sample is critical for reliable inference of paleoprecipitation. Here, we use high‐resolution magnetic field and electron microscopy to identify the grain‐scale setting and formation pathway of magnetic enhancement in two modern soils developed in higher (∼580 mm/y) and lower (∼190 mm/y) precipitation settings from the Qilianshan Range, China. We found that both soils contain 1–30 μm aeolian Fe‐oxide grains with indistinguishable rock magnetic properties, while the higher‐precipitation soil contains an additional population of ultrafine (<150 nm) magnetically distinct magnetite grains. We show that the in situ precipitation of these ultrafine particles, likely during wet‐dry cycling, is the only significant magnetic enhancement mechanism in this soil. These results demonstrate the potential of quantum diamond microscope magnetic microscopy to extract magnetic information from distinct, even intimately mixed, grain populations. This information can be used to evaluate the contribution of distinct enhancement mechanisms to the total magnetization.more » « less
-
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 per cent, whereas the coercivity (Hc) of the average modelled 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 multidomain (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 palaeomagnetic 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 modelled small-particle fraction only explains 6 per cent of the best fit to the measured VSM hysteresis loop, it contributes 28 per cent of the remanent magnetization. The modelled direction of maximal Mrs/Ms in plagioclase is subparallel 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, 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.more » « less
-
Abstract Large impact craters on Earth are associated with prominent magnetic anomalies, residing in magnetite of the shocked target rocks and impactites. Shock experiments on magnetite suggest that up to 90% of magnetic susceptibility is lost at pressures >5 GPa, but can be partially restored by post‐shock thermal annealing. The magnetic property changes are caused by shock induced grain size reduction and fragmentation, as well as domain wall‐pinning at crystal lattice defects. A recent study of granitoids from the peak‐ring of the Chicxulub crater found that annealing may occur naturally, but can also be overprinted by high‐temperature hematite‐to‐magnetite transformation in non‐oxidizing environments. In this study, we isolate the effect of defect annealing and hematite‐to‐magnetite transformation using the evolution of hysteresis, isothermal remanent magnetization components and first order reversal curve (FORC) diagrams at different high‐temperature steps. We used a laboratory‐shocked magnetite‐quartz ore, a non‐shocked naturally oxidized granite, and a naturally shocked and oxidized granite. Our findings suggest that annealing of shock‐induced lattice defects partially restores some pre‐shock magnetic behavior and causes an apparent average bulk‐sample domain state increase. Hematite‐to‐magnetite transformation creates new fine‐grained magnetite that strongly overprints the original signal, and decreases the average bulk‐sample domain state. Where annealing and hematite‐to‐magnetite transformation both occur, the new magnetite masks the annealing‐induced property restoration and apparent domain state modification in the shocked magnetite. As magnetite oxidation is a ubiquitous process in surface rocks, these findings are fundamental to understand hematite‐to‐magnetite transformation as a potential overprint mechanism, and could have broad implications for paleomagnetic interpretations.more » « less
-
SUMMARY The magnetic properties of iron-bearing minerals at above-ambient temperatures control their magnetic expression at depth in the Earth and other planets, as well as the permanent memory they retain as thermoremanence or thermochemical remanence when brought to the surface and cooled. This paper reports magnetic hysteresis parameters measured at temperatures up to the Curie point TC for natural pyrrhotite and hematite and for suites of sized magnetites, both natural and synthesized. Domain structure changes can be inferred from the ratio of saturation remanence Mrs to saturation magnetization Ms. In almost all magnetites and pyrrhotites studied, Mrs decreases more rapidly with increasing measurement temperature T than Ms, indicating thermal unblocking or vortex development in single-domain grains and addition or remobilization of domain walls at high T in multidomain grains. During cooling of a rock, iron minerals might then denucleate domains or vortices. Coercive force Hc, a measure of stability against changing magnetic fields, also decreases with increasing measurement T, usually at a rate similar to that of Mrs, but often retains a finite value near the Curie point.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3389/feart.2021.601401
