Demagnetization Of Natural Remanent Magnetization Research Articles

On the early development of Huastecs (Gulf of Mexico) revealed by the Earth's magnetic field recorded in domestic hearths

AbstractA detailed magnetic mineralogy and archaeomagnetic study was carried out on recently discovered domestic hearths and burned floors at the Chak Pet archaeological settlement (Tamaulipas, Mexico). The study aimed to obtain reliable absolute chronological constraints on the early development of Huastecs during the Formative period. Oriented hand samples corresponded to four domestic hearths and one burned floor. Continuous thermomagnetic curves revealed mostly irreversible behavior, while titanomagnetites, titanomaghemites, and goethites are assumed to carry the remanent magnetization. In total, 87 specimens were subjected to stepwise demagnetization of natural remanent magnetization using an alternating field procedure. Characteristic remanent magnetization directions were obtained for 29 samples of two hearths and one burned floor. No single, technically acceptable paleointensity determination was obtained. The new archaeomagnetic age intervals for Chak Pet allow locating the origin of this settlement at the Gulf of Mexico within the Middle Formative (900–600 BCE) continuing until the Late Formative period (350–100 BCE). New archaeomagnetic ages are in accordance with the diagnostic pottery analysis. Dated archaeological elements are associated with both ceramic types and different sets of burials, providing a reliable tool to calibrate their chronological and stratigraphic positions.

Magnetic characterization of the Daule chondrite (Ecuador's first meteorite fall): The case of elusive tetrataenite?

We investigated the magnetic properties of Ecuador's first reported meteorite fall (March 23, 2008), the Daule ordinary chondrite (L5, S4, W0) using thermomagnetic analyses at high and cryogenic temperatures, analyses of magnetic hysteresis and first-order reversal curves, and thermal and alternating field (AF) demagnetization of natural remanent magnetization (NRM). The mineralogical and chemical composition of Daule was examined using scanning electron microscopy with energy-dispersive x-ray spectroscopy. Most methods indicate that the magnetic properties of Daule are dominated by multidomain FeNi alloys (kamacite) with Ni content varying between ∼4% and ∼17%. However, backfield demagnetization (BFD) analyses revealed the presence of high-coercivity tetrataenite that survived shock metamorphism. The differential survival of tetrataenite at the millimeter scale indicates heterogeneity of the impact-related temperature and pressure fields within the Daule meteorite. BFD curves may serve as an efficient tool for identifying minor amounts of tetrataenite that otherwise cannot be discerned from the signal from magnetically-soft FeNi mineral phases by methods based on induced magnetization. Thermal demagnetization experiments unveiled the presence of a well-defined characteristic component of NRM, which remains resistant to AF demagnetization. We interpret this component as a pre-impact thermochemical remanence carried by tetrataenite and acquired during the thermal metamorphism of the parent body. At cryogenic temperatures, the magnetic properties of Daule are dominated by low-Mg magnesiochromite with the Curie temperature at 60–70 K.

Behavior of Greigite‐Bearing Marine Sediments During AF and Thermal Demagnetization and Its Significance

AbstractGyro‐remanent magnetization (GRM) is a frequently occurring yet unwanted remanence contamination for certain samples during alternating field (AF) demagnetization of the natural remanent magnetization. The origin and detailed properties of GRM have not yet been fully understood. In this study, systematic rock magnetic analyses were conducted on marine greigite‐bearing samples of Hole U1433A drilled by the IODP Expedition 349 from the South China Sea. Results show that GRM is mostly acquired above ~55 mT AF demagnetization and can be effectively removed by heating to ~400°C during thermal demagnetization but a secondary tail could remain until ~585°C. In addition, no apparent GRM was observed during the AF demagnetization for the 400°C thermally treated samples. These results strongly suggest that GRM is dominantly carried by single domain (SD) greigite but with minor contributions from SD magnetite. Thus, thermal treatment alone or the hybrid demagnetization (i.e., thermal demagnetization at ~400°C first then systematical AF demagnetization) can efficiently avoid the GRM acquisition and be beneficial for relative paleointensity estimation for greigite‐bearing samples. Besides, GRM carried by greigite has a low thermal stability. Our results also show AF demagnetization spectra of anhysteretic remanent magnetization (ARM) could be strongly distorted by GRM effects due to both have a preference of SD particles. Thus, the median destructive field of ARM is improper to be used as a coercivity proxy for greigite‐bearing samples. Instead, the biplot analysis of AF demagnetization of natural remanent magnetization and ARM can be used to evaluate the relative content of greigite.

Magnetic properties of the remagnetized Middle-Ordovician limestones of the Ponón Trehué Formation (San Rafael Block, central-western Argentina): Insights into the Permian widespread Sanrafaelic overprint

The widespread Sanrafaelic remagnetization reset most of the early Cambrian to mid-Ordovician carbonate platform of the Argentine Precordillera and the calcareous units of the San Rafael Block. We conducted a detailed rock-magnetic study on the Middle-Ordovician limestones of the Ponón Trehué Formation at both limbs of a tight anticline exposed in the San Rafael Block (Mendoza province, central-western Argentina) that are carriers of a syntectonic magnetization of Permian age. We found that the magnetic overprint in the Ponón Trehué Formation is carried by both pyrrhotite and magnetite, with goethite and subordinate haematite likely related to weathering. Hysteresis parameters, frequency dependence of magnetic susceptibility, Cisowski and modified Lowrie-Fuller tests suggest the presence of ultrafine particles of chemical origin. Demagnetization of natural remanent magnetization and of three-axis isothermal remanence confirm pyrrhotite and magnetite as important contributors to the remanence. Both minerals carry the same magnetic syntectonic component suggesting a coeval or nearly coeval remanence acquisition and therefore mineral formation. This and the results of the magnetic fabric analyses indicate an authigenic origin of the magnetic minerals during folding associated with the Sanrafaelic tectonic phase (ca. 280 Ma). Although the chemically active (oxidizing?) fluids expelled from the orogen as it developed in the early Permian is a viable explanation for the Sanrafaelic remagnetization, the role of the nearly coeval magmatism in Precordillera and the San Rafael Block remains to be properly evaluated.

Eight good reasons why the uppermost mantle could be magnetic

Wasilewski et al. (1979) concluded that no magnetic remanence existed in the uppermost mantle and that even if present, such sources would be at temperatures too high to contribute to long wavelength magnetic anomalies (LWMA). However, new collections of unaltered mantle xenoliths indicate that the uppermost mantle could contain ferromagnetic minerals. 1. The analysis of some LWMA over cratons and forearcs suggest magnetic sources in the uppermost mantle. 2. The most common ferromagnetic phase in the uppermost mantle is stoichiometric magnetite. Assuming a 30. km-thick crust, and crustal and mantle geotherms of 15. °C/km and 5. °C/km, respectively, the 600. °C Curie temperature implies a 30. km-thick layer of mantle. 3. The uppermost mantle is cooler than 600. °C in Archean and Proterozoic shields (>. 350. °C), subduction zones (>. 300. °C) and old oceanic basins (>. 250. °C). 4. Recently investigated sets of unaltered mantle xenoliths contain pure magnetite inclusions in olivine and pyroxene formed in equilibrium with the host silicate. 5. The ascent of mantle xenoliths occurs in less than a day. Diffusion rates in olivine suggest that the growth of magnetite possible within this time frame cannot account for the size and distribution of magnetite particles in our samples. 6. Demagnetization of natural remanent magnetization (NRM) of unaltered mantle xenoliths unambiguously indicates only a single component acquired upon cooling at the Earth's surface. This is most easily explained as a thermoremanent magnetization acquired by pre-existing ferromagnetic minerals as xenoliths cool rapidly at the Earth's surface from magmatic temperatures, acquired during ascent. 7. Modern experimental data suggest that the wustite-magnetite oxygen buffer and the fayalite-magnetite-quartz oxygen buffer extend several tens of km within the uppermost mantle. 8. The magnetic properties of mantle xenoliths vary consistently across tectonic settings. In conclusion, the model of a uniformly non-magnetic mantle should be revisited. © 2014 Elsevier B.V.

Origin of apparent magnetic excursions in deep‐sea sediments from Mendeleev‐Alpha Ridge, Arctic Ocean

Arctic deep‐sea sediments often record intervals of negative inclination of natural remanence that are tens of centimeters thick, implying magnetic excursions with durations of tens of thousand years that far exceed excursion durations estimated elsewhere, and the lack of tight age control usually provides excessive freedom in the labeling of Arctic excursions. Fortuitous variations in sedimentation rate have been invoked to explain the “amplified” excursions. Alternating field demagnetization of natural remanent magnetization (NRM) of sediment cores 08JPC, 10JPC, 11JPC, and 13JPC recovered by the Healy Oden Trans‐Arctic Expedition in August 2005 (HOTRAX05) to the Mendeleev‐Alpha Ridge yields apparent magnetic “excursions” in sediments deposited in the Brunhes Chron. Thermal demagnetization of the NRM, however, implies multiple magnetization components with negative inclination components usually “unblocked” below ∼350°C. Analysis of isothermal remanent magnetization acquisition curves from magnetic extracts indicates two magnetic coercivity components superimposed on one another. Magnetic experiments conducted under high and low temperatures show features that are characteristic for (titano)magnetite and titanomaghemite. Presence of the two magnetic phases is further confirmed by elemental mapping on a scanning electron microscope equipped for X‐ray energy dispersive spectroscopy (EDS) and by high‐resolution X‐ray diffraction (XRD). It is unlikely that anomalously thick intervals of negative inclination in these Brunhes‐aged sediments are caused by unusual behavior of the magnetic field in the Arctic area. We therefore attribute low and negative NRM inclinations in these cores to partially self‐reversed chemical remanent magnetizations, apparently carried by titanomaghemite and acquired during the oxidation of detrital (titano)magnetite grains. The high Ti contents and high oxidation states indicated by EDS and XRD data provide the conditions required for partial self‐reversal by ionic reordering during diagenetic maghemitization, and this process appears to have affected all HOTRAX05 cores collected from the Mendeleev‐Alpha Ridge.

Rock-magnetic analyses as a tool to investigate archaeological fired sediments: a case study of Mirador cave (Sierra de Atapuerca, Spain)

SUMMARY Here we report a detailed mineral magnetic study of Neolithic burnt levels in the Mirador Cave (Sierra de Atapuerca, Burgos, Spain) to reconstruct the burning history and to investigate their suitability for archaeomagnetic purposes. As a consequence of the ancient burning, a characteristic facies sequence was developed along the Holocene stratigraphy. From top to bottom it includes: (i) 2–10 cm ash layers, (ii) ∼2 cm underlying rubefied layers and (iii) clay, mainly unburnt and of variable thickness. In some cases a thermally altered facies (2–6 cm) with a heterogeneous texture was identified (facies TF), usually between rubefactions and the unburnt levels. 126 oriented samples from 4 units (MIR12, 15, 18 and 21) and a 2 m section, all comprised between units MIR21 (6380 ± 40 14C BP) and MIR9 (5090 ± 40 14C BP) were analysed with rock magnetic methods. In addition, bulk sediment from each facies that comprise the Neolithic sequence was investigated. Measurements included: stepwise alternating field and thermal demagnetization of natural remanent magnetization (NRM), viscosity experiments, determination of the anisotropy of the magnetic susceptibility (AMS), the susceptibility frequency dependence at room temperature and determination of the temperature dependence of the susceptibility. Additional experiments consisted in the determination of the behaviour of anhysteretic and isothermal (IRM) remanences, magnetic hysteresis loops, first-order-reversal-curve diagrams, and thermal demagnetization of three-axial IRM. It appeared that the facies all show a fairly similar magnetic mineralogy and grain size dominated by low-titanium magnetite that is often partially maghaemitized. Main differences constitute the amount of superparamagnetic particles that is higher in unburnt strata concurring with a less well-defined NRM behaviour. The magnetic mineral concentration is notably higher in ashes. This homogeneity strongly suggests that similar sources and burial conditions prevailed during Neolithic times. Agreeing with archaeological observations and favoured by rapid burial conditions, very limited alterations have been deduced. AMS data revealed the absence of fluid flow in the ash lenses sampled. Demagnetization revealed a stable single NRM component in ashes, a single or two-component NRM in rubefactions and less stable multicomponent behaviour in clays. In ashes, magnetic minerals are likely secondary in origin formed by low-temperature oxidization soon after burning. Although this thermochemical nature of the NRM invalidates the use of these sediments for palaeointensity studies, archaeomagnetic (directional) data can be successfully obtained because the burning and oxidation are closely confined in time.

テリエ法実験における強磁性粒子の非理想挙動の検出と補正―新たな古地磁気強度推定方法について―

We present a new method of paleointensity determination based on comparing the thermal demagnetization of natural remanent magnetization (NRM) with that of an artificial total thermoremanent magnetization (TRM). Igneous rocks often contain pseudo-single domain (PSD), multidomain (MD), and/or single domain (SD) particles as magnetic remanence carriers under strong magnetic grain (domain) interactions. The magnetic grain interactions have particular disastrous effects on paleointensity experiments, which make determination of paleointensity unreliable. We have critically examined how magnetic grain interactions affect the Thellier experiment, and have developed a new technique for correcting grain-interaction effects in the experiment of paleointensity estimation. The essential point of our experimental method is that by comparing the thermal demagnetization of natural remanent magnetization (δ NRM_loss) with that of an artificial total TRM (δ TRM_loss) for estimating its paleointensity, rather than that by comparing the remaining of NRM during thermal demagnetization (NRM_remaining) with a progressive TRM_gain in the traditional Thellier-Coe method, which essentially requires the additivity of partial TRM and independence of pTRMs. Using our new method, a mild alternating field (AF) demagnetization pre-treatment is applied to destroy most of the low coercivity remanence, which makes the samples behave more suitebly for a paleointensity study. We also make an apparent paleointensity estimation with pTRM, which is acquired in the perpendicular direction of NRM in a narrow non-overlapping temperature interval and cooled slowly in air. In this way, the non-ideal behavior of samples is detected most sensitively by the discrepancy between NRM loss and pTRM gain. Finally, we employ an artificial total TRM test to elucidate the relation between TRM_loss and pTRM_gain, and to correct interference caused by the non-ideal behavior. We have applied our new method to several representative suites of historical lava flows of known geomagnetic field intensity, and successfully extracted reliable paleointensity with a precision higher than 95% from samples even containing PSD and MD grains.

Relationships between contact metamorphism and magnetite formation and destruction in a plutonʼs aureole, White-Inyo Range, eastern California

Magnetic-petrologic studies on metasedimentary rocks from the low-pressure–high-temperature contact-metamorphic aureole of the Joshua Flat–Beer Creek pluton at the southern rim of the Inyo batholith, California, reveal the effect of contact metamorphism on magnetic minerals. The regional metamorphic biotite zone and three different contact-metamorphic zones (andalusite- cordierite, transition, and cordierite–K- feldspar) in the aureole are identified by silicate petrology and their rock magnetic behavior. Phengite barometry yields pressure data in the range of 0.2–0.3 GPa, and Na-in-cordierite thermometry indicates a peak temperature of ∼730 °C for the highest-temperature contact-metamorphic zone. Compositional changes of iron-magnesium silicates and iron oxides correspond to changes in magnetic susceptibility, temper ature dependence of magnetic susceptibility, and AF (alternating field) demagnetization of NRM (natural remanent magnetization) and can be correlated with the metamorphic history of the aureole. Magnetic susceptibility ranges from 0.2 × 10 –3 to 0.5 × 10 –3 SI in the biotite zone and is highly variable in the andalusite-cordierite and transition zones, for which the values range from 0.35 × 10 –3 to 28 × 10 –3 SI. In prograde metamorphism, the assemblage ilmenite + rutile ± hematite transforms to magnetite and ilmenite-hematite solid solutions, producing an increase in magnetic susceptibility that can be correlated with the cordierite-in isograd. In the cordierite–K-feldspar zone and in samples having undergone retrograde metamorphism, prograde magnetite reacts to ilmenite and hematite. This breakdown is associated with decreased magnetic susceptibility (0.2–0.8 × 10 –3 SI) and blurs the magnetite isograd that formed during prograde metamorphism in the outer contact aureole. Retrograde metamorphism in the contact aureole is defined by chlorite composition [ T : 280 and 420 °C, log f (O 2 ):−35 and−28]. Contact-metamorphic aureoles, as long as they are not strongly altered, may contribute significantly to magnetic anomalies around magnetite- series plutons. This fact has to be considered when interpreting aeromagnetic data of granitoid rocks.

AF demagnetization characteristics of NRM, compared with anhysteretic and saturation isothermal remanence: an aid in the interpretation of NRM

AF demagnetization characteristics and isothermal remanent magnetization (IRM) acquisition of a variety of rocks are analysed in the form of Cisowski plots and plots of progressive demagnetization of natural remanent magnetization (NRM) and anhysteretic remanent magnetization (ARM) against IRMs. The igneous rocks studied carrying a primary NRM acquired during initial cooling have a ratio of NRM:IRMs of parts in 100, whereas altered igneous rocks have much smaller ratios of parts in 1000 or 10,000. Conversely, the detrital sedimentary rocks carrying a primary NRM have a ratio of parts in 1000, but altered sedimentary rocks carrying secondary magnetization have ratios of parts in 100. Although not as discriminatory as the ratios of NRM:IRMs, altered igneous and sedimentary rocks frequently have strongly concave up curves, reflecting the common admixture of a small amount of hard material in samples carrying secondary magnetization, The ratio of ARM to IRMs serves as a proxy for weak field magnetization and is an indicator of grain size.

A theoretical treatment of paleointensity determination experiments on rocks containing pseudo-single or multi domain magnetic particles

A phenomenological theory of thermoremanent magnetization (TRM) is applied to model the possible results of paleointensity determination experiments using the Thellier method. It is shown that intrinsic curvature of the Arai plot for samples containing pseudo-single or multi domain particles can be attributed exclusively to the presence of remanences which are unblocked below their blocking temperature. Thus the thermal demagnetization tail of partial TRM (pTRM) is not a direct measure for a sample’s tendency to yield a curved Arai plot. Here it is proposed to use the advancing decay of a pTRM( T 1, T 2) which during heating occurs already below the lower temperature T 1< T 2 as a more reliable pre-selection criterion. In addition, the effect of an average magnetostatic interaction on the Thellier experiment turns out to be negligible. An extended Thellier method which compares the thermal demagnetization of natural remanent magnetization with that of an artificial TRM according to the applied TRM theory should result in a perfectly linear modified Arai plot even for pseudo-single or multi domain samples if no alteration or viscous effects occur.

Paleointensity experiments using alternating field demagnetization

Experiments for absolute paleointensity involve a large number of heatings and thus increase the probability of mineralogical changes which considerably reduce the success rate. Magnetic granulometry also plays an important role. In the presence of multidomain grains thermal demagnetization of natural remanent magnetization (NRM) is not linearly related to acquisition of thermal remanence (TRM) in the laboratory, so that the NRM–TRM diagrams are characterized by two slopes, neither of them providing a correct field determination. After briefly reviewing most current techniques for paleointensity and various attempts to improve the results, the present paper investigates approaches relying on alternating field (a.f.) demagnetization. Measurements have been performed on at least eight samples from seven contemporary flows from Hawaii. Classical thermal experiments were also conducted with poor success. In contrast, 30% of the NRM–TRM curves obtained after a.f. demagnetization were perfectly linear over the entire spectrum of coercivities and thus appropriate to determine a paleointensity. This difference was caused by the possibility of measuring a much larger number of samples. The mean field is less than 10% higher than the present field and in better agreement for a subset of samples with NRM and TRM heated at 300°C prior to a.f. demagnetization. These values are acceptable within the uncertainties inherent in any study of absolute paleointensity. Thus a.f. techniques can be used with confidence provided that the whole spectrum of coercivities is involved in the field determination and exempt from any correction. This condition is necessary and sufficient to demonstrate the absence of alteration after heating. Because of the rapidity of such experiments, an approach combining a.f. and thermal experiments is certainly the best way to increase considerably the number of field determinations per lava flow and hence the overall quality of paleointensity studies.

Magnetic glasses in the Azuki and AT volcanic ashes in Japan

Magnetic properties of volcanic glass shards have been described on the widespread Azuki and AT ashes in Japan. Thermomagnetic analysis and isothermal remanent magnetization (IRM) acquisition experiments were performed on both bulk samples and separated glass samples of the Azuki volcanic ash. Together with progressive thermal demagnetization of natural remanent magnetization (NRM), the results demonstrate that titanomagnetite contained in volcanic glass shards is the sole contributor to the stable NRM of the Azuki ash. The results of a Lowrie-Fuller test suggest that the remanence carrier of the ash layer is single-domain grains of titanomagnetite, which can be seen as submicron particles using electron microscopy. On the basis of progressive IRM acquisition in samples with various contents of AT volcanic glass, it is also suggested that titanomagnetite in glass shards is a significant magnetic carrier of NRM for the AT ash. Since most of the glass shards are tens of microns in size, they are expected to acquire depositional remanent magnetization (DRM) without experience of succeeding post-depositional process under aqueous conditions. A redeposition experiment with the Azuki ash shows that stable DRM, whose intensity is comparable to that of the NRM of the water-laid Azuki ash, is acquired during several hours.

Viscosité magnétique potentielle, aimantation détritique et viscosité réelle d'un sédiment lacustre quaternaire

The magnetic viscosity of sediments, as indicated by thermal demagnetization of natural remanent magnetization (NRM), depends on two main parameters: (i) the specific magnetic viscosity of the material and (ii) the efficiency of the primary magnetization process. In an attempt to determine the relative importance of these two variables, we studied a Lower Pleistocene lacustrine sequence bearing a primary reversed detrital remanent magnetization (DRM).Using natural samples and small cores made of crushed sediment, our study is based on the thermodynamic theory of rock magnetism developed by L. Néel, who established an equivalence between time and temperature, that is, between viscous remanent (VRM) and thermoremanent (TRM) magnetization processes. The determination of the blocking temperature spectra from 20 to 152 °C allows us to calculate the maximum theoretical VRM acquired in situ at each horizon, while the detailed thermal study of the NRM permits an appraisal of the DRM quality. This leads us to define a geological viscosity index, which accounts, in a rather convincing manner, for the behaviour of the samples observed during the classical paleomagnetic study. For the section studied, it appears that the variations of this index are closely correlated with the efficiency of the DRM acquisition process.

Review of archaeointensity methods

Most of the traditional methods of determining the intensity of the ancient geomagnetic field from archaeological materials utilized thermal demagnetization of the natural remanent magnetization (NRM) and of the laboratory induced thermoremanent magnetization (TRM). When applied rigorously these methods are foolproof. They are, however, very time consuming and the number of samples with which they can be used is limited. Attempts to speed up these traditional methods have generally led to the use of subjective criteria in assessing the reliability of the results and archaeomagnetic research has recently been concentrated on extending the range of samples to which the method can be applied. Through the use of alternating field, rather than thermal, demagnetization of NRM and TRM it has become possible to apply corrections for alteration occurring during laboratory firing of the archaeological samples and develop objective criteria of reliability. Recent research has shown that it may be possible to determine archaeointensities the laboratory redeposition of lake sediments.

Short geomagnetic episodes in the Matuyama epoch

Two short geomagnetic episodes in the middle Matuyama epoch have been identified within thin (4.3-mm) sections of two deep-sea calcareous sediment cores taken in the western equatorial Pacific. Both cores are correlated by magnetostratigraphic and micropaleontological methods. Magnetic stability and paleomagnetic reliability are tested by alternating field demagnetization of natural remanent magnetization as well as by the ratios of intensities of anhysteretic remanent magnetization to those of saturation isothermal remanent magnetization. One episode is dated to be 1.06 m.y. BP. The other is identified to be about 1.94 m.y. BP, which is presumably in agreement with the Reunion event. Both episodes seem to be accompanied by conspicuous drops in field intensity. Possible correlation of field intensity with biological productivity in the ocean is also postulated from the present paleomagnetic results.

Thermoremanent magnetization in submicroscopic magnetite

Induction and demagnetization characteristics of thermoremanent magnetization (TRM) have been studied for four synthetic magnetites with mean particle sizes of 0.04–0.22 μm. The experiments are intended to simulate the acquisition and laboratory demagnetization of natural remanent magnetization carried by submicroscopic but non-single-domain titanomagnetite particles in igneous rocks. Although hysteresis properties indicate that the magnetites are above single-domain (SD) size, their weak-field TRM are SD-like. TRM intensity, for a 1-oe inducing field, ranges from 6 to 12 emu/cm3. Median alternating demagnetization fields are between 275 and 400 oe, while blocking temperatures are generally within 50°C of the Curie point. However, neither Neel's SD theory nor Stacey's pseudo-SD theory can account for the observed dependence of TRM on inducing field, since theoretically TRM should saturate in rather low fields. Neel's two-domain theory fits the strong-field data very well but is inadequate in the weak-field region. These failures indicate that theoretically assumed two- and four-domain structures are not appropriate in particles just above SD size. Blocking temperature data are consistent with well-developed multidomain (MD) structure (i.e., thermal activation volume associated with a domain wall notably less than particle volume) in 0.22-μm particles but a wavelike spin structure without recognizable domains in particles ≤0.1 μm in size. The dependence of weak-field TRM on grain size d also supports this interpretation: TRM varies approximately as d−1 between 0.1 μm and the pseudo-SD threshold at 15–20 μm, but below 0.1 μm there is no clear size dependence. Particles with wavelike spin structure have not been recognized previously but may well explain the MD-like hysteresis and SD-like TRM of many igneous rocks.

Magnetism of natural pyrrhotite, haematite and ilmenite

The paper deals with the magnetic properties of the natural minerals haematite, ilmenite and pyrrhotite. The natural remanent magnetization Jn, the volume susceptibility ℵ, the specific magnetization of saturation σso, the Curie temperature Θ, the A.C. demagnetization of natural remanent magnetization, the thermal demagnetization Jn, the temperature dependence of the specific magnetization of saturation, and several other parameters were determined for the individual sets of minerals. Since natural minerals can and do contain various heterogeneous and isomorphic admixtures, their magnetic properties may change depending on the content and type of these admixtures. Therefore, all the investigations of the magnetic properties described in this paper were carried out with extensive sets of minerals, representing various Czechoslovak and world localities. A number of interesting results were obtained from the executed experiments, e.g., the different stability of the natural remanent magnetization of haematite with respect to A.C. demagnetization, the presence and type of heterogeneous inclusions in ilmenite, the phase changes connected with the γ-transition in natural pyrrhotites, etc. The types of distributions of the investigated values of the magnetic properties were also determined.