Carriers Of Remanent Magnetization Research Articles

Occurrence and Distribution Patterns of Magnetic Particles Within Stalagmite Growth Laminae

AbstractThe concentrations of magnetic particles in stalagmites depend greatly on the regional hydrologic condition, and thus can act as an important proxy for paleoclimate reconstruction. The central axis region of a stalagmite is usually selected for such magnetism study. However, little is known about the distribution and influence factors of exogenetic magnetic particles within the growth laminae. In this study, the occurrence and variations of magnetic particles along the growth laminae of three shape stalagmites were investigated by a set of magnetic measurements, including isothermal remanent magnetization (IRM) acquisition curves and low‐temperature magnetic analysis. The results show the main magnetic remanence carrier is detrital magnetite/maghemite, coexisting with small amounts of hematite and goethite. The distribution of soft magnetic particles along the growth laminae showed different patterns among the three shape stalagmites (dome, cone, and candle). For the dome shape stalagmite, the soft magnetic particle concentrations in all studied growth layers showed an increasing trend from center to edge. Similar patterns were also appeared in the bottom growth layers of the cone and candle shape stalagmites. This edge enrichment of magnetic particles is caused by drip water washing effect. However, such enrichment pattern became weaker or even inverse in the top growth layers of the cone and candle shape stalagmites, indicating that the shapes of stalagmites could also affect the surface distribution of soft magnetic minerals. These uneven distributions of magnetic particles imply that the central axis region of a stalagmite may not always be the optimal specimen for environmental magnetism study.

Paleomagnetism of Gedemsa magmatic segment, Main Ethiopian Rift: Implication for clockwise rotation of the segment in the Early Pleistocene

One-hundred-seventy oriented paleomagnetic core samples from 23 sites were collected from volcanic lava flows belonging to the Nazareth Group (1.7–1.9 Ma) and the Wonji group (≤0.3 Ma) along the Gedemsa magmatic segment (MS) in the, Main Ethiopian Rift (MER). Samples were subjected to progressive thermal (Th) and Alternating Field (AF) demagnetization for directional analyses, and representative samples were used for rock-magnetic experiments. The rock-magnetic study indicated that titano-magnetite, titano-hematite and magnetite are the primary carrier of remanent magnetization. Two components of Natural Remanent Magnetization (NRM) were revealed: the first, low stability component is removed by AF of 20-30mT or heating up to 200 °C. The second, high stability component isolated above these steps is directed toward the origin and considered Characteristic Remanent Magnetization (ChRM). All specimens we studied show normal geomagnetic polarity. The overall mean direction for the Wonji group is Decs = 0.60, Incs = 5.10 (α95 = 6.50, K = 35.8, N = 14) while for the Nazareth group is Decs = 18.90, Incs = 13.40 (α95 = 10.30, K = 35.6, N = 7). When these mean directions from the two groups are compared with their respective reference geomagnetic dipole fields (Besse and Courtillot, 1993, 2002), the former shows no significant block rotation and a clockwise rotation R = 170 ± 3.40 were detected in the latter. These results indicate the Gedemsa magmatic segment is characterized by complex deformation due to the opening of DebreZeit-Butajira (DzB) and Yerer – Tullu Wellel Volcano-tectonic Lineament (YTVL segments. Our study indicates that detailed paleomagnetic observations in the Rift are important to identify quantitative deformation history of the region.

Improvements to the Shaw-Type Absolute Palaeointensity Method

Palaeointensity information enables us to define the strength of Earth’s magnetic field over geological time, providing a window into Earth’s deep interior. The difficulties in acquiring reliable measurements are substantial, particularly from older rocks. Two of the most significant causes of experimental failure are laboratory induced alteration of the magnetic remanence carriers and effects relating to multidomain magnetic carriers. One method that has been claimed to overcome both of these problems is the Shaw method. Here we detail and evaluate the method, comparing various selection criteria in a controlled experiment performed on a large, non-ideal dataset of mainly Precambrian rocks. Monte Carlo analyses are used to determine an optimal set of selection criteria; the end result is a new, improved experimental protocol that lends itself very well to the automated Rapid 2G magnetometer system enabling experiments to be carried out expeditiously and with greater accuracy.

Mayan limekilns as geomagnetic field recorders

We report new results from frontier research that relates the methodology and knowledge of geophysics (geomagnetism) with archeology and the data will represent a valuable contribution to both disciplines. A detailed magnetic survey was performed on limekilns from the northern Yucatan peninsula. We used Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) Spectroscopy to determine the temperature of calcination in order to select limekiln samples that were heated above the Curie temperature for common magnetic minerals. Using rock-magnetism, we identified the main carrier of remanent magnetization to be multi-domain or large pseudo-single domain low titanium titanomagnetite. Alternating magnetic field demagnetizations delivered reliable archaeomagnetic directions. Our summary of 12 limekiln ages shows a large span from 707 to 1900 yr AD. Such a wide time range signifies that Maya used limekilns in the Yucatan peninsula area in the Late Classic period (600 AD), and that some of these limekilns were reused in the Colonial period. From a geophysical point of view, information can be used about the behavior of the Earth's magnetic field in the recent historical past in Mesoamerica. In this context, the hundreds of Mayan limekilns which are distributed in large areas, open great opportunities to contribute to both archeology and geomagnetic field fluctuations during the past two millennia.

Hysteresis of Natural Magnetite Ensembles: Micromagnetics of Silicate‐Hosted Magnetite Inclusions Based on Focused‐Ion‐Beam Nanotomography

AbstractThree‐dimensional geometries of silicate‐hosted magnetic inclusions from the Harcus intrusion, South Australia have been determined using focused‐ion‐beam nanotomography (FIB‐nt). By developing an effective workflow, the geometries were reconstructed for magnetic particles in a plagioclase (162) and a pyroxene (282), respectively. For each inclusion, micromagnetic modeling using Micromagnetic Earth Related Rapid Interpreted Language Laboratory provided averaged hysteresis loops and backfield remanence curves of 20 equidistributed field directions together with average Ms, Mrs, Hc, and Hcr. The micromagnetic structures within each silicate are single‐domain (SD), single‐vortex (SV), multivortex (MV) and multidomain states. They have been analyzed using domain‐state diagnostic plots, such as the Day plot and the Néel plot. SD particles can be subdivided into groups with dominant uniaxial anisotropy (Mrs/Ms ∼0.5 and 10 < Hc < 100 mT) and mixed uniaxial/multiaxial anisotropy (Mrs/Ms ∼0.7 and 10 < Hc < 30 mT). Most SV particles lie on a trend with 0 < Mrs/Ms < 0.1 and 0 < Hc < 10 mT, while others display a broad range of intermediate Mrs/Ms and Hc values. SV and MV states do not plot on systematic grain‐size trends. Instead, the multicomponent mixture of domain states within each silicate spans the entire range of natural variability seen in bulk samples. This questions the interpretation of bulk average hysteresis parameters in terms of grain size alone. FIB‐nt combined with large‐scale micromagnetic simulations provides a more complete characterization of silicate‐hosted carriers of stable magnetic remanence. This approach will improve the understanding of single‐crystal paleomagnetism and enable primary paleomagnetic data to be extracted from ancient rocks.

Magnetoelastic properties and behaviour of 4C pyrrhotite, Fe7S8, through the Besnus transition

Pyrrhotite, Fe7S8, is a commonly occurring carrier of magnetic remanence and has a low temperature transition, the Besnus transition, involving a change in spin state. Variations of the thermodynamic, magnetic and elastic properties through this transition at ∼33 K in a natural sample of 4C pyrrhotite have been tested against a group theoretical model for coupling between order parameters relating to Fe/vacancy ordering (irrep U1(1/2,0,1/4)) and magnetic ordering (irreps m and m). Magnetoelastic coupling is weak but the pre-existing microstructure of ferroelastic and magnetic domains, that develop as a consequence of Fe/vacancy and ferrimagnetic ordering during slow cooling in nature (P63/mmc → C2′/c′), causes subtle changes in the low temperature transition (C2′/c′ → P). The Besnus transition involves a rotation of magnetic moments out of the a–c plane of the monoclinic structure, but it appears that the transition temperature might vary locally according to whether it is taking place within the pre-existing domain walls or in the domains that they separate. Evidence of metamagnetic transitions suggests that the magnetic field–temperature phase diagram will display some interesting diversity. Low temperature magnetic transitions in minerals of importance to the palaeomagnetism community have been used to identify the presence of magnetite and haematite in rocks and the Besnus transition is diagnostic of the existence of pyrrhotite, Fe7S8.

Constraining the Intracontinental Tectonics of the SW Central Asian Orogenic Belt by the Early Permian Paleomagnetic Pole for the Turfan‐Hami Block

AbstractIn order to better understand the kinematics and geodynamics of the late Paleozoic to early Mesozoic transition from accretionary to intracontinental tectonics in the southwest Central Asian Orogenic Belt, we performed a paleomagnetic study on Lower Permian strata in the Turpan‐Hami Basin, NW China. Magnetite and hematite are shown to be the principal magnetic remanence carriers. A typical sedimentary fabric that has not been modified by postdepositional deformation is recognized via magnetic fabric study. Stable characteristic remanent magnetizations, all of reverse magnetic polarity, are revealed by step demagnetizations. Remanence age is determined to be of Early Permian affinity, based on a fold test, remanence behavior of minerals, and geochronologic data. With inclination shallowing corrections, the first Early Permian paleomagnetic pole for the Turpan‐Hami Block is acquired as λ = 75.7°N, φ = 276.3°E, A95 = 5.7°, and N = 10 sites. Comparisons of this new pole with published ones from neighboring cratonic blocks provide kinematic constraints on the tectonic transition and subsequent intracontinental tectonics of this region. The Turpan‐Hami and Yili blocks have kept their relative positions since the Early Permian. Significant latitudinal movement of 14.7° ± 7.2° occurred between the Turpan‐Hami and South Junggar blocks and could be mainly accommodated by late Paleozoic crustal shortening of at least ~530 km across an intra‐arc basin in the Bogda belt. The Turpan‐Hami Block has experienced large‐magnitude relative rotations of −41.1° ± 7.1° and 21.2° ± 9.4° with respect to Tarim and South Junggar since the Early Permian, respectively, corresponding to dominant intracontinental tectonics characterized by large‐magnitude strike‐slip displacements along megashear zones.

Paleomagnetic study of basaltic rocks from Baengnyeong Island, Korea: efficiency of the Tsunakawa\u2013Shaw paleointensity determination on non-SD-bearing materials and implication for the early Pliocene geomagnetic field intensity

Finding the statistical intensity signatures of the Earth’s magnetic field over geologic time has helped understanding of the evolution of the Earth’s interior and its interactions with other integral parts of Earth systems. However, this has been often hampered by a paucity of absolute paleointensity (API) data, which are difficult to obtain primarily because of non-ideal magnetic behaviors of natural materials. Here, we present new API determination data with paleodirectional and rock magnetic analyses from basaltic rocks probably aged ~ 4‒5 Ma in Baengnyeong Island, Korea. Paleodirectional analysis obtained an overall mean direction of D = 347.3° and I = 38.3° (α95 = 4.9°, k = 113.4) corresponding to a virtual geomagnetic pole at 342.1° E and 70.2° N. Comprehensive rock magnetic analyses identified Ti-poor titanomagnetite with, in part, multi-domain (MD) particles as a main carrier of remanent magnetization. The Tsunakawa–Shaw (TS) method yielded 12 qualified API estimates with a high success rate, efficiently removing possible MD influences, and resulted in a mean value of 13.1 μT with good precision (1.7 μT, standard deviation). The Thellier method of the IZZI protocol with pTRM checks, coupled with the use of a bootstrap approach instead of the “conventional best-fitting” in API determination, gave 6.6‒19.7 μT as a 95% confidence interval of its mean API estimate, which supports the reliability of our TS-derived API mean estimate; but it is not considered in the final mean value because of the relatively large uncertainty. The virtual dipole moment corresponding to the TS-derived API mean, 2.9 (± 0.4) × 1022 Am2, is somewhat lower than the expectations of the past few Myr averages. Combined with a global API database, our new data implies a larger dispersion in the dipole moment during the early Pliocene than previously inferred. This also suggests that the issue of whether the early Pliocene average dipole strength was moderately high (> 5 × 1022 Am2) or consistent (4‒5 × 1022 Am2) should be discussed further.

Paleomagnetic evidence for cold emplacement of eruption-fed density current deposits beneath an ancient summit glacier, Tongariro volcano, New Zealand

The thermal structures of eruption-fed, subglacial volcaniclastic deposits are poorly understood because their emplacement is hazardous to observe or obscured by the glacier. Determining deposit emplacement temperature, however, supports improved understanding of the flow dynamics and emplacement processes of volcaniclastic material beneath a glacier. Understanding the emplacement temperatures of ancient volcanic deposits is also important because they can be used, in combination with field studies, to infer the eruptive environment. Here, we use paleomagnetic techniques to quantify the emplacement temperatures of two ancient, proximal, eruption-fed density current deposits at Tongariro volcano, New Zealand. Stepwise thermal demagnetisation of lithic and recycled juvenile block-sized clasts reveal randomly orientated directions of magnetisation, suggesting that the clasts were rotated within the flow but not heated. Additional data from thermomagnetic, hysteresis, and isothermal remanent magnetisation tests indicate that the principal carrier of magnetic remanence is magnetite, and that the magnetisation directions are a primary remanence rather than post-depositional chemical remanent magnetisations. Following systematic removal of any viscous remanent magnetisation, the post-emplacement equilibrium temperatures for the deposits can be estimated at <150°C. The paleomagnetic data support field evidence for rapid cooling of clasts and waterlain deposition. The deposit-forming eruptions took place beneath a summit glacier where the freshly erupted tephra was efficiently cooled by mixing with meltwater. Lithic blocks and recycled juvenile bombs were entrained and remobilised within the cool currents that drained along meltwater channels beneath the ice. This is the first study in which paleomagnetic data have been used to determine the equilibrium temperatures of subglacial density current deposits. The data provide new insight into volcaniclastic flow dynamics beneath a glacier and current-meltwater interactions.

Paleomagnetic Secular Variations During the Past 40,000 Years From the Bay of Bengal

AbstractPaleomagnetic secular variations in South Asia are not well understood due to insufficient records. In this study, we produce a new paleomagnetic secular variation curve covering the past 40,000 years by stacking four marine sediment records from the middle portion of Bay of Bengal. Age models are based on radiocarbon dating and core correlation. Stepwise alternating field demagnetization was used to isolate the primary remanent component. Rock magnetic measurements indicate that the primary magnetic remanence carriers are single domain and pseudo‐single domain magnetites with no sign of diagenetic alteration. Our Bengal relative paleointensity stack is broadly consistent with local records and the PISO‐1500 relative paleointensity stack (Channell et al., 2009; https://doi.org/10.1016/j.epsl.2009.03.012). Two geomagnetic excursions presented by paleointensity minima are recognized, which correlate to the Mono Lake (29–32 ka) and Laschamp excursion (38–40 ka), respectively. We conclude that the Mono Lake and Laschamp excursion are two separate events.

Field Response of Magnetic Vortices in Dusty Olivine From the Semarkona Chondrite

AbstractRecent paleomagnetic studies have constrained the strength and longevity of the magnetic field generated by the solar nebula, which has broad implications for the early evolution of the solar system. Paleomagnetic evidence was recorded by nanoscale iron inclusions in olivine crystals in the Semarkona LL 3.0 chondrite. These dusty olivines have been shown to be credible carriers of ancient magnetic remanence. The small scale of the iron inclusions presents several challenges for defining their fundamental magnetic properties. Here we present the first correlative study of the response of these magnetic structures under applied laboratory fields. Results show that the majority of particles are in a single‐vortex state and exhibit stable magnetic behavior in applied fields up to 200 mT. Experimental observations using Lorentz microscopy and magnetic transmission X‐ray microscopy are shown to compare well with the results of finite‐element micromagnetic simulations derived from 3‐D models of the particles obtained using electron tomography. This correlative approach may be used to characterize the fundamental magnetic behavior of many terrestrial and extraterrestrial paleomagnetic carriers in the single‐vortex to multivortex size range, which represent the vast majority of stable magnetic carriers in rocks and meteorites.

The Vortex State in Geologic Materials: A Micromagnetic Perspective

AbstractA wide variety of Earth and planetary materials are very good recorders of paleomagnetic information. However, most magnetic grains in these materials are not in the stable single domain grain size range but are larger and in nonuniform vortex magnetization states. We provide a detailed account of vortex phenomena in geologic materials by simulating first‐order reversal curves (FORCs) via finite‐element micromagnetic modeling of magnetite nanoparticles with realistic morphologies. The particles have been reconstructed from focused ion beam nanotomography of magnetite‐bearing obsidian and accommodate single and multiple vortex structures. Single vortex (SV) grains have fingerprints with contributions to both the transient and transient‐free zones of FORC diagrams. A fundamental feature of the SV fingerprint is a central ridge, representing a distribution of negative saturation vortex annihilation fields. SV irreversible events at multiple field values along different FORC branches determine the asymmetry in the upper and lower lobes of generic bulk FORC diagrams of natural materials with grains predominantly in the vortex state. Multivortex (MV) FORC signatures are modeled here for the first time. MV grains contribute mostly to the transient‐free zone of a FORC diagram, averaging out to create a broad central peak. The intensity of the central peak is higher than that of the lobes, implying that MV particles are more abundant than SV particles in geologic materials with vortex state fingerprints. The abundance of MV particles, as well as their single domain‐like properties point to MV grains being the main natural remanent magnetization carriers in geologic materials.

Distribution of magnetic remanence carriers in the human brain

That the human brain contains magnetite is well established; however, its spatial distribution in the brain has remained unknown. We present room temperature, remanent magnetization measurements on 822 specimens from seven dissected whole human brains in order to systematically map concentrations of magnetic remanence carriers. Median saturation remanent magnetizations from the cerebellum were approximately twice as high as those from the cerebral cortex in all seven cases (statistically significantly distinct, p = 0.016). Brain stems were over two times higher in magnetization on average than the cerebral cortex. The ventral (lowermost) horizontal layer of the cerebral cortex was consistently more magnetic than the average cerebral cortex in each of the seven studied cases. Although exceptions existed, the reproducible magnetization patterns lead us to conclude that magnetite is preferentially partitioned in the human brain, specifically in the cerebellum and brain stem.

First Early Permian Paleomagnetic Pole for the Yili Block and its Implications for Late Paleozoic Postorogenic Kinematic Evolution of the SW Central Asian Orogenic Belt

AbstractWe conducted a paleomagnetic study on the Early Permian volcanic and sedimentary rocks, and the Neoproterozoic mafic dikes in the Yili Block, NW China. Magnetite and hematite were proven to be the principal magnetic remanence carriers. Demagnetizations revealed stable characteristic remanence magnetizations with a sole reversed magnetic polarity. The magnetic remanence of only the Early Permian strata turned out to be primary based on positive fold tests; meanwhile, the magnetic remanence age of the mafic dikes is ambiguous. Accordingly, the first Early Permian paleomagnetic pole for the Yili Block is calculated at λ = 81.5°N, φ = 256.5°E, N = 11, and A95 = 10.9°. Comparisons of this new pole with published ones from the Yili, Tarim, and South Junggar blocks provide new quantitative constraints on late Paleozoic kinematic evolution of the SW Central Asian Orogenic Belt: (1) Between the Yili and Tarim blocks, significant relative movement took place along major strike‐slip faults during the Late Carboniferous to Early Permian (580 ± 290 km) and the Early to Late Permian (585 ± 340 km), and the displacement rate increased from the Late Carboniferous to Early Permian (~19.3 ± 9.7 mm/yr) to the Early to Late Permian (~29.3 ± 17.0 mm/yr); (2) a significant relative rotation of 28.3° ± 18.3° in the Late Permian, and a lateral displacement of 630 ± 295 km after the Late Permian occurred between the Yili and South Junggar blocks. The significant strike‐slip movements played an important role in the formation of the Central Asian Orogenic Belt and should be considered with great attention in tectonic and paleogeographic reconstructions.

Paleomagnetic results from Lower Devonian sandstones of the Niqiuhe Formation in the Duobaoshan area and its constraints on paleoposition of the Xing&amp;rsquo;an block

Two common but contradictory views toward the timing of the final closure of the eastern Paleo-Asian Ocean (PAO) are either the Pre-Late Devonian or the Late Permian to Early Triassic. Previous paleomagnetic studies support a consolidation of the unified North China and Mongolia by the Late Permian. However, Pre-Late Permian paleogeography of the eastern Central Asian Orogenic Belt (CAOB) is not well understood due to the scarcity of reliable paleomagnetic records. The Xing’an block plays a key role in the evolution of the CAOB, however, no paleomagnetic results are available so far to provide constraints on its Late Paleozoic paleo-position. We performed a systematic paleomagnetic study on a purple siltstone section of the Lower Devonian Niqiuhe Formation near the Duobaoshan County, Heilongjiang Province. Representative fresh end samples were subjected to rock magnetic analysis to identify their dominant magnetic carriers and particle size distribution. Rock magnetic experiments include measurements of magnetic susceptibility variation dependence of temperature ( κ - T ) curves, hysteresis loops, acquisition of isothermal remanent magnetization (IRM) and back-field demagnetization of saturated IRM (SIRM) curves. The results indicate that the magnetic remanence carriers are dominant by hematite with minor contributions of magnetite. 112 cylindrical specimens were thermally demagnetized using an ASC TD-48S furnace and measured in a 2G-755-4K SQUID system in the Magnetotectonics Lab of Peking University. A total of 11 sites revealed identical bi-components character. The low temperature component resembles the present geomagnetic field in geographic coordinates, while the high temperature characteristic remanence (ChRM) component was reversed polarity both before and after tilt correction. Optimal concentration is achieved at 35.7% unfolding percentage, which suggests the magnetization was acquired during the late stage of a folding process. A previous structural study indicated that stress field of the research area switched from NW-SE to N-S orientation during the Late Permian, which is supported by the predominant NE-SW oriented Ordovician to Devonian strata as well as the angular unconformity between the Devonian and Upper Permian strata. The folding time of the Niqiuhe Formation is likely Middle Devonian to Middle Permian. Supported by evidence from a suite of granites near our sampling section (less than 3 km) dating at 309−299 Ma along with the universal reversed polarity of all 11 sites, we argue that the ChRM component was acquired during the Permo-Carboniferous Reversed Superchron (~318−262 Ma), when the study area was affected by a granitic intrusion thermal event (309−299 Ma). The ChRM component ( D / I =28.6°/−33.2°, k =18.6, α 95=10.9°) corresponds to a paleomagnetic pole of λ p/ φ p=17.3°S/97.1°E ( dp / dm =7.0°/12.4°). For a common reference site (43°N/114°E), our results suggest the Xing’an block was situated at 27.7°±12.4°N during the Late Carboniferous. Coeval results obtained from rocks of different lithology suggest 28.5°±9.7°N and 19.9°±14.8°N for the Uliastai continental margin and the Central Mongolia-Euruna block. After applying a blanket flattening factor f =0.6 for results obtained from clastic sedimentary rocks, the paleolatitudinal difference between the Xing’an block, Uliastai continental margin, and the Central Mongolia-Erguna block is less than 3.2°±12.8°. Late Carboniferous paleomagnetic poles from those three blocks are distributed along a small circle. Significant post-Permian local rotations are likely attributed to extensive Mesozoic strike-slip faults among the eastern CAOB. Those three blocks were probably consolidated into a unified block, situated at ~28°−30°N during the Late Carboniferous and dominant by the cold water Borel realm fauna. By contrast, the North China and Songliao-Xilinhot blocks was situated at an equatorial region (4.3°S−12°N) and dominant by the warm water Tethyan realm fauna, while Siberia was located at ~45°−50°N. The Paleo-Asian Ocean therefore remained open and the main basin was the Hegenshan ocean rather than the Solonker ocean.

Magneto- and biostratigraphy across the Jurassic-Cretaceous boundary in the Kurovice section, Western Carpathians, Czech Republic

An integrated magneto- and biostratigraphic study of the Jurassic-Cretaceous (J-K) boundary section at Kurovice, Czech Republic was carried out. The acquired paleomagnetic data showed dual polarity characteristic remanent magnetization (ChRM) with counter-clockwise rotation direction. Rock magnetic data indicated magnetite as the principal carrier of remanent magnetization as well as presence of e.g. hematite and goethite. Magnetozones from M21n up to M17r were identified and dated as early Tithonian (dinoflagellate Semiradiata and nannofossil NJT15b zones) to late early Berriasian (Calpionella elliptica and nannofossil NK-1 (sub)zones).

Nonlinear Preisach maps: Detecting and characterizing separate remanent magnetic fractions in complex natural samples

AbstractNatural remanent magnetization carriers in rocks can contain mixtures of magnetic minerals that interact in complex ways and are challenging to characterize by current measurement techniques. Here a nonlinear mapping scheme is described that efficiently enhances sensitivity and the resolution power of remanent Preisach maps. Using this scheme a large dynamic range of magnetic moments and coercivities can be reliably resolved. The method is applied to synthetic and natural standard samples containing magnetite and hematite, as well as to natural samples from remanent magnetic anomalies where complex microstructures are observed. It is shown that certain offset high‐coercivity patterns in remanent Preisach maps may serve as fingerprints for exsolution structures of ilmenite in hematite or hematite in ilmenite and that in some magnetite‐bearing remanent anomalies the magnetite coercivity is increased beyond its intrinsic coercivity range. Experimental results and theoretical considerations indicate a minimal coercivity of about 10 mT for single‐domain (SD) magnetite, such that observation of lower coercivities implies pseudo‐SD (PSD) or multidomain grain sizes. A diagnostic hematite pattern with a peak downward offset of 17 ± 2% of the intrinsic coercivity is found that is stable over a large range of intrinsic coercivities and may be related to shielding of internal defect or lamellar moments by a spin canting response to the internal field.

Linking speleothem and soil magnetism in the Pau d'Alho cave (central South America)

AbstractMineral magnetism of Pau d'Alho cave sediments, soils outside the cave, and in the stalagmite #6 (ALHO6) in Midwest Brazil is presented. This high growth‐rate speleothem (~168 mm/ka) encompasses the past 1355 years. Oxygen and carbon isotope data from the same stalagmite allow for a direct comparison of the magnetic signal with changes in paleoprecipitation and soil dynamics at the surface. Magnetic experiments include isothermal remanent magnetization, anhysteretic remanent magnetization, hysteresis loops, first‐order reversal curves, and low‐temperature superconducting quantum interference device magnetometry. The main magnetic remanence carriers in ALHO6 are magnetite and goethite, with a nearly constant relative proportion. Remanent coercivities of magnetite in all our samples are within 14–17 mT for an average grain‐size of ~1–2 µm, in the range of pedogenic magnetite, thus suggesting the detrital grains deposited in the stalagmite were produced in the soil above the cave. Magnetic remanence variations follow δ13C and δ18O data, suggesting a climatic control on the input of magnetic minerals into the Pau d'Alho cave system. The concentration of magnetic minerals in the stalagmite is governed by soil erosion above the cave, which by its turn is controlled by soil erosion and vegetation cover. Dry periods are associated with less stable soils and result in higher mineral fluxes carried into karst systems. Conversely, wetter periods are associated with soils topped by denser vegetation that retains micrometer‐scale pedogenic minerals and thus reduces detrital fluxes into the cave.

Variable defect structures cause the magnetic low-temperature transition in natural monoclinic pyrrhotite

Non-stoichiometric monoclinic 4C pyrrhotite (Fe7S8) is a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials. Because of its low-temperature magnetic transition around 30 K also known as Besnus transition, which is considered to be an intrinsic property, this mineral phase is easily detectable in natural samples. Although the physical properties of pyrrhotite have intensively been studied, the mechanism behind the pronounced change in magnetization at the low-temperature transition is still debated. Here we report magnetization experiments on a pyrrhotite crystal (Fe6.6S8) that consists of a 4C and an incommensurate 5C* superstructure that are different in their defect structure. The occurrence of two superstructures is magnetically confirmed by symmetric inflection points in hysteresis measurements above the transition at about 30 K. The disappearance of the inflection points and the associated change of the hysteresis parameters indicate that the two superstructures become strongly coupled to form a unitary magnetic anisotropy system at the transition. From this it follows that the Besnus transition in monoclinic pyrrhotite is an extrinsic magnetic phenomenon with respect to the 4C superstructure and therefore the physics behind it is in fact different from that of the well-known Verwey transition.

Magneto-electronic coupling in modulated defect-structures of natural Fe1−xS

We provide compelling experimental evidence that the low-temperature transition in natural non-stoichiometric Fe7S8, a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials, is a phenomenon caused by magnetic coupling between epitaxially intergrown superstructures. The two superstructures differ in their defect distribution, and consequently in their magnetic anisotropy. At T &amp;lt; 30 K, the magnetic moments of the superstructures become strongly coupled, resulting in a 12-fold anisotropy symmetry, which is reflected in the anisotropic magneto-resistance.