Isothermal Remanent Magnetization Acquisition Research Articles

Quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation

A new method of analysing isothermal remanent magnetisation (IRM) acquisition curves based on cumulative log Gaussian analysis [Robertson and France, Phys. Earth Planet. Inter. 82 (1994) 223–234] is proposed. It is based on the curve fitting of the IRM acquisition curve versus the logarithm of the applied field with: (i) the acquisition curve on a linear scale, (ii) the acquisition curve expressed as a gradient, and (iii) the acquisition curve on a probability scale. Even when a sample is not saturated, its magnetic properties can be defined, although with less certainty. The number of magnetic components required for an optimal fit to a measured IRM acquisition curve is evaluated statistically. The method discriminates on the basis of different mineral coercivity. Therefore, additional rock-magnetic tests are still required to separate minerals with similar coercivities.

Rock-magnetism and ore microscopy of the magnetite-apatite ore deposit from Cerro de Mercado, Mexico

Rock-magnetic and microscopic studies of the iron ores and associated igneous rocks in the Cerro de Mercado, Mexico, were carried out to determine the magnetic mineralogy and origin of natural remanent magnetization (NRM), related to the thermo-chemical processes due to hydrothermalism. Chemical remanent magnetization (CRM) seems to be present in most of investigated ore and wall rock samples, replacing completely or partially an original thermoremanent magnetization (TRM). Magnetite (or Ti-poor titanomagnetite) and hematite are commonly found in the ores. Although hematite may carry a stable CRM, no secondary components are detected above 580°, which probably attests that oxidation occurred soon enough after the extrusion and cooling of the ore-bearing magma. NRM polarities for most of the studied units are reverse. There is some scatter in the cleaned remanence directions of the ores, which may result from physical movement of the ores during faulting or mining, or from perturbation of the ambient field during remanence acquisition by inhomogeneous internal fields within these strongly magnetic ore deposits. The microscopy study under reflected light shows that the magnetic carriers are mainly titanomagnetite, with significant amounts of ilmenite-hematite minerals, and goethite-limonite resulting from alteration processes. Magmatic titanomagnetites, which are found in igneous rocks, show trellis, sandwich, and composite textures, which are compatible with high temperature (deuteric) oxy-exsolution processes. Hydrothermal alteration in ore deposits is mainly indicated by martitization in oxide minerals. Grain sizes range from a few microns to >100 μm, and possible magnetic state from single to multidomain, in agreement with hysteresis measurements. Thermal spectra, continuous susceptibility measurements, and IRM (isothermal remanent magnetization) acquisition suggest a predominance of spinels as magnetic carriers, most probably titanomagnetites with low-Ti content. For quantitative modeling of the aeromagnetic anomalies, we used data on bulk susceptibility and natural remanent intensity for quantifying the relative contributions of induced and remanent magnetization components and allow a better control of the geometry of source bodies. The position and geometry of this magnetic source are shown as an ENE-striking tabular body, steeply inclined (75°) to the south.

Palaeomagnetic and rock magnetic properties of sediment cores from Chalkidiki, Greece

A detailed palaeomagnetic investigation was performed on Eocene and Oligocene core samples from 4 deep boreholes (1–3.1 km) from the Chalkidiki peninsula. Both low field magnetic susceptibility and intensity of the natural remanent magnetization (NRM) indicate rather weakly magnetised materials. 134 samples have been subjected to either stepwise thermal or alternating field demagnetization. Both procedures indicate that the NRM is carried by magnetite. Isothermal remanent magnetization (IRM) acquisition curves suggest the dominance of magnetite. Finally, 12 thin sections were studied in order to more precisely characterise the magnetic mineralogy of the samples. This investigation reveals the presence of magnetite and pyrite. In general, the observed inclinations of characteristic remanence in these rocks are much lower than expected inclinations but converge with those obtained from formations on land.

Magnetic mineralogy and properties of the Peña Colorada iron ore deposit, Guerrero Terrane: implications for magnetometric modeling

A wide range of physical and magnetic parameters have been used to characterize the iron ores and lithologies of the Peña Colorada Mine, southern Mexico, including saturation magnetization, Curie temperature, density, magnetic susceptibility, remanence intensity, Königsberger ratio, and hysteresis parameters. Rock magnetic properties are mainly controlled by variations in magnetite content, deuteric oxidation, and hydrothermal alteration. Magnetite grain sizes in iron ores range from 4 to >20 μm, which suggest the predominance of multi-domain (MD) states over single-domain (SD) and pseudo-single-domain (PSD) states. Curie temperatures are 580±5°C, characteristic of magnetite. Hysteresis parameters indicate that most samples present PSD to MD behavior. AF demagnetization and isothermal remanent magnetization (IRM) acquisition curves show that natural remanent magnetization (NRM) and laboratory remanences are carried by MD magnetite in iron ores and PSD–SD magnetite in the host rocks. The variation of NRM intensity and susceptibility suggests that the carriers of remanent and induced magnetization are the same in all cases, Ti-poor titanomagnetites. The Königsberger ratio falls in a narrow range between 0.1 and 4, indicating the presence of MD and PSD magnetites. There is a clear correlation among the magnetic parameters measured with high- and low-temperature oxidation states in both types of iron ore and host rock. A large dipolar magnetic anomaly is observed over the deposit. Data for susceptibility, intensity and direction of the NRM provide constraints for modeling of the magnetometric anomalies by quantifying the relative contributions of induced and remanent magnetization components and allowing better control of the geometry of source bodies.

Crossover plots: A useful method for plotting SIRM data in paleomagnetism

The effective grain‐size of magnetic minerals, and therefore domain type, controls the saturation isothermal remanent magnetization acquisition and subsequent demagnetization response and can be used to create “crossover” plots for paleomagnetic specimens, where %SIRM is plotted as a function of the field applied, using a logarithmic scale. This paper presents templates of such plots for magnetite, pyrrhotite, hematite and goethite, onto which sample data can be plotted for conventional paleomagnetic specimens to characterize their magnetic mineralogy and effective grain‐size.

Identifying goethite and hematite from rock magnetic measurements of soils and sediments

We describe a sequence of experiments involving the acquisition of isothermal remanent magnetization (IRM) in high (>2 T) fields, followed by orthogonal demagnetization and the cooling and heating of remanent magnetization (Mr) over the temperature range −196°C to +680°C. The main aim of these experiments is to improve the basis for identifying the imperfect antiferromagnetic minerals contributing to the magnetic properties of natural soil and sediment samples, which often contain complex mineral mixtures. The procedures developed are first applied to a series of ore and synthetic samples of hematite and goethite, each characterized by X‐ray diffraction (XRD) spectra, and second to controlled mixtures of the two minerals. The results confirm the mineral‐diagnostic nature of the experiments used. They are subsequently used to identify goethite and hematite contributions to the rock magnetic properties of a variety of soils and recent sediments. Goethite can be unambiguously identified, often accompanied by hematite, in the majority of the samples studied, despite indications from the synthetic mixtures that the ratio of goethite to hematite may have to exceed an order of magnitude before the goethite signatures are readily recognizable. The rock magnetic tests are a much more sensitive basis for recognizing the presence of goethite in natural samples than are XRD analyses. The results suggest that goethite's contribution to the rock magnetic properties of soil and sediment samples may often have been underestimated.

Study of magnetic contrasts in the Guafita oil field (south-western Venezuela)

We have carried out magnetic susceptibility (MS) studies of near-surface samples (drill cuttings) from 11 wells at the Guafita oil field and environs (southwestern Venezuela). In order to characterize magnetic mineralogies and lithological contrasts, MS results have been complemented by S-ratio logs, scanning electron microscopy (SEM), X-ray energy dispersion (EDX), isothermal remanent magnetization (IRM) acquisition curves, visual examination of mineral compositions using a stereomicroscope of 8X to 66X and X-ray diffraction studies of whole rock, clay and magnetic fractions. Two different kinds of MS anomalies have been recognized (i.e. A and B). Anomaly A was observed only in three producer wells and seems to be related to the presence of Fe-rich spherical aggregates and low coercivity magnetic phases, probably of authigenic origin, whereas B appears in most of the producer and non-producer wells. Although B is not associated to framboids of magnetic minerals, X-ray diffraction analyses reveal the presence of quartz, kaolinite, magnetite and hematite at those depth levels where it has been detected. We argue that anomaly A could be linked to a reducing environment induced by the underlying reservoir, whereas B probably reflects a lithological contrast. This hypothesis also seems to be supported by preliminary visual examination of mineral compositions. A contour map of anomaly B amplitude values appears to indicate the sediment input and transport directions in the field. On the other hand, an average-depth contour map of this same anomaly shows the existence of a major fault that has been previously identified by geological studies in the region.

The superparamagnetism of Yucca Mountain Tuff

Yucca Mountain Tuff contains small titanomagnetite grains with narrow size distributions in the superparamagnetic range [Schlinger et al., 1988]. Magnetic measurements on three samples (comprising hysteresis loops at low and ambient temperatures, acquisition and demagnetization of isothermal remanent magnetization, thermal demagnetization of the saturation remanence and of a low‐temperature thermoremanence, and frequency‐ and temperature‐dependent susceptibilities) allow evaluation of the magnetic properties in terms of Néel's [1949] single‐domain theory. Precise grain volume distributions have been obtained by applying the blocking volume concept to thermal demagnetization results. In contrast, an attempt to derive mean particle volumes by fitting a Langevin curve to the room temperature magnetization curves fails, probably because the precondition for the Langevin function, KV/kT≪1, is not met. It is only for the sample with the smallest grains and in weak fields (<20 mT) that a Langevin fit provides a reasonable volume estimate. There is good agreement between the experimental results and the calculated frequency and temperature dependence of susceptibility, thus verifying that Néel's theory is sufficient for the magnetic description of single‐domain assemblages spanning the superparamagnetic/stable single‐domain boundary. However, some deviations between modeled and measured susceptibilities exist, and the physical causes may include size‐dependent anisotropy, nonuniform magnetizations, and also an uncertain preexponential time “constant” τ0. While τ0 = 10−11 s gives the best fit for the sample with the largest grains, τ0 = 10−9 s is more reasonable for the others. Thus τ0 may indeed be size‐ and temperature‐dependent as predicted by Brown [1959]. The commonly cited parameter χfd (frequency dependence of susceptibility) reaches 30% at room temperature (RT) for one sample with a blocking temperatures just below RT, while χfd = 0 at RT for a superparamagnetic sample with smaller grains. These results thus exemplify that χfd is not limited to 15%, as a number of studies suggest, and that χfd = 0 must not be taken to imply the absence of superparamagnetic grains.

Characterization of tephras using magnetic properties: an example from SE Iceland

Abstract Mineral magnetic techniques have been applied to two different stratigraphic profiles excavated in the Oraefi Volcano area, SE Iceland. The profiles include sequences of soils and tephra layers of inferred age. Each profile includes the distinctive pumice tephra layer associated with the eruption of Oraefi in 1362, which is a stratigraphic marker for the area. Detailed magnetic measurements included: magnetic susceptibility, anhysteretic remanent magnetization, isothermal remanent magnetization (IRM) acquisition, back-field IRM’s and strong field thermomagnetic behaviour (Curie temperature). The results indicate that the most useful magnetic properties for distinguishing between soils and tephras are the back-field IRM’s and the Curie temperatures. Principal component analysis has been applied using all the measured magnetic properties. In this way it is possible to discriminate between different types of material in the sections, and even more importantly to correlate some tephras between the two sections. The potential of this technique to correlate tephras is promising, but further evaluation is required.

Authigenesis of magnetite in organic-rich sediment next to a dike: implications for thermoviscous and chemical remagnetizations

Rock magnetic results from organic-rich sediment around a Tertiary dike in Scotland indicate changes in magnetic mineralogy and magnetite content as well as grain size in the contact zone. Sediment specimens within 85 cm of a 90 cm thick dike contain a Tertiary magnetization that is equivalent in direction to the thermal remanent magnetization in the dike whereas the sediment at greater distances does not carry a stable remanence. Based on acquisition of isothermal remanent magnetization (IRM), thermal decay of triaxial IRMs, and S-ratio determinations, magnetite is the dominant magnetic phase in the sediment except near the dike (<8 cm), where hematite is abundant. Maximum laboratory unblocking temperatures (Tb max) for the magnetization are higher than predicted by a thermal cooling model at distances greater than 40 cm but the Tb max values are not corrected for gradual cooling of the dike and are, therefore, overestimates. Trends in rock magnetic parameters such as IRM at 300 mT, and anhysteretic remanence (ARM) are consistent with an increasing contribution of single domain/pseudosingle domain magnetite towards the dike. An increase in saturation IRM (SIRM) along with a decrease in ARM/SIRM values < 25 cm from the dike also suggest an increase in multidomain magnetite. Low temperature experiments indicate the presence of abundant superparamagnetic magnetite. Changes in hysteresis parameters are consistent with changes in the relative amounts of the different magnetite size fractions towards the dike. The onset of changes in the magnetite grain size budget starting at 85 cm or approximately one dike-width suggests that chemical processes contributed to the remanence at temperatures as low as 220°C. Petrographic and geochemical studies suggest that the contact zone was not pervasively altered by hydrothermal fluids. Utilizing the dike as an analog for thermal processes that occur during burial, the results of this study suggest that moderately elevated burial temperatures are sufficient to cause magneto-chemical changes.

Some new aspects for the modelling of isothermal remanent magnetization acquisition curves by cumulative log Gaussian functions

Rock samples containing (ferri)magnetic minerals acquire an isothermal remanent magnetization (IRM) after exposure to a direct magnetic field. Measuring IRM(B) allows to produce IRM acquisition curves which were modelled by cumulative log Gaussian (CLG) functions (Robertson and France, 1994) to discriminate the different magnetic phases. An approach of how to find the best model has been added to this method. From the investigation of a magnetite sample it is inferred that with the method the remanence acquisition coercive force for each magnetic component in a sample can be deduced. The method allows to detect minute concentrations of magnetically hard material in a predominant magnetically soft sample and vice versa. It furthermore allows to extrapolate the IRM behaviour of samples that do not reach saturation IRM in the experiment. The study of a natural hematite renders it possible that even well‐defined hematite may be contaminated by (ferri)magnetic impurities biasing the remanence acquisition coercive force of the sample towards lower values.

Rock magnetic properties of the late Pleistocene Loess-Paleosol deposits in Haro River area, Attock basin, Pakistan: Is magnetic susceptibility a proxy measure of paleoclimate?

Variations of rock-magnetic properties in a late Pleistocene (approx. 18–130 ka) Loess-Paleosol sequence of Haro River area, Attock basin, Pakistan were studied. Isothermal remanent magnetization (IRM) acquisition and thermomagnetic analyses of the samples indicate that magnetite and maghemite are the major magnetic minerals in these samples. Saturation isothermal remanent magnetization (SIRM), anhysteretic remanent magnetization (ARM), and the ratio ARM/SIRM disclose relative enrichment of ultra-fine magnetite/maghemite grains in the paleosols as compared to loess deposits, which suggests that the magnetite/maghemite is of pedogenic origin. Despite this enrichment, the low-field magnetic susceptibility is not always enhanced in paleosols in this sequence. In the Chinese Loess it has been believed that the observed lows and highs of magnetic susceptibility values respectively correspond to loess and paleosol sequences which indicates paleoclimatic change during the formation of the sequences. However, our observations in Pakistan’s Loess-Paleosol sequence demonstrate that, although the ultrafine grained magnetite/maghemite may be produced by the process of pedogenesis, the low-field magnetic susceptibility record does not directly correlate with the alternation of loess and paleosol, and therefore the magnetic susceptibility can not simply be interpreted as a proxy for paleoclimate in the area.

Analysis of magnetic material in the human heart, spleen and liver.

Isothermal remanent magnetization (IRM) acquisition and alternating field (A.F.) demagnetization analyses were performed on human heart, spleen and liver samples resected from cadavers. The magnetic properties of the samples were measured both at 77K and at 273K. A.F. demagnetization was performed at 273K. Results from the analyses of the tissue indicate the presence of ferromagnetic, fine-grained, magnetically interacting particles which, due primarily to magnetic properties, are thought to be magnetite and/or maghemite. The presence of superparamagnetic particles can be inferred from the increase in saturation IRM values when measured at 77K compared with measurements at 273K and the decay of remanent magnetization upon warming from 77K. The concentration of magnetic material (assuming it is magnetite or maghemite) in the samples varies from 13.7 ng g-1 to 343 ng g-1, with the heart tissue generally having the highest concentration. The presence of magnetic material in these organs may have implications for the function of biogenic magnetite in the human body.

Palaeomagnetism of the Jurassic Asphaltkalk deposits, Holzen, northern Germany

SUMMARY Palaeomagnetic and rock magnetic studies on the Kimmeridgian-Portlandian Asphaltkalk deposits of the Hils Syncline near Holzen in northern Germany suggest a connection between bitumen impregnation and magnetization in the limestones. A stable remanent magnetization is only found in the Asphaltkalk deposits, not in non-bitumen-impregnated limestones of the same age. The stable remanence was acquired after folding in this region, as revealed by a palaeomagnetic fold test. The timing of remanence acquisition is Late Cretaceous-early Tertiary, as derived from a comparison of the mean pole of the magnetic component with the European apparent polar wander path (APWP). The decay pattern of the natural remanent magnetization (NRM) during alternating field (AF) and thermal demagnetizaton indicates that the magnetization is carried by magnetite. Isothermal remanent magnetization (IRM) acquisition and thermal decay measurements, as well as hysteresis parameters, are consistent with this interpretation. This study suggests that the creation of authigenic magnetite during the lower Tertiary in the Asphaltkalk deposits might coincide with the timing of hydrocarbon migration into the unit.

The application of high resolution IRM acquisition to the discrimination of remanence carriers in Chinese loess

High resolution isothermal remanent magnetisation (IRM) acquisition was performed on forty-one Chinese loess samples with the aim of investigating the effects of weathering on the remanence carrying mineralogy. Magnetic susceptibility was taken as a measure of the degree of weathering. This is a valid assumption based on previous studies which have demonstrated a close relationship between climate and magnetic susceptibility in the Chinese loess. Detailed analysis, based on fitting accumulative log-gaussian curves to IRM acquisition data, reveals up to four distinct coercivity components. The remanences of the two harder coercivity components are found to be almost independent of magnetic susceptibility. On the other hand, the remanences of the two softer coercivity components are found to be positively correlated with magnetic susceptibility. Most probably the two harder components are associated with detrital iron-oxides in the primary dustfall, whereas the softer components were created during pedogenesis.

Palaeomagnetism of some Cretaceous Nubian Sandstones, Northern Sinai, Egypt

Abstract A total of 23 sites have been sampled from the Nubian Sandstone of Gabal El-Minsherah and Gabal El-Halal, northern Sinai. Isothermal remanent magnetization acquisition showed that hematite is the predominant magnetic mineral carrying the remanence after the destruction of goethite at temperatures of 100–150°C. The in situ directions correspond to palaeomagnetic poles at 84°N 257°E (G. El Minsherah) and 78°N 288°E (G. El Halal). A fold-test shows that the magnetizations were acquired later than the Late Cretaceous (Laramide) tectonic disturbance. The presence of both normal and reversed polarities and in situ pole positions close to the expected Late Tertiary palaeomagnetic pole suggest that the magnetizations are of Miocene age. They are probably chemical magnetizations acquired during occasional fluid migrations through these permeable beds during arid Miocene climatic phases.

Relative paleointensity of the geomagnetic field during the Brunhes Chron

We conducted a paleomagnetic study of four sediment cores taken from different regions in the low latitudes of the Pacific Ocean in order to recover relative paleointensity variations during the Brunhes chron. Three cores have good age control from oxygen isotope stratigraphy. We extensively examined rock magnetic uniformity by the iso-thermal remanent magnetization (IRM) acquisition and the alternating-field demagnetization of the anhysteretic remanent magnetization (ARM) and the IRM. Only rock-magnetically homogeneous sediments were used. Biogenic magnetites were found in all but one core and their dissolution in early diagenesis was not significant. The four cores showed coherent variations of the remanent intensity normalized by the ARM, implying that these variations represent the behavior of the geomagnetic field. Long-term paleointensity variations during the Brunhes chron are characterized by an asymmetric pattern. Rapid increases in the intensity occurred just after the Brunhes-Matuyama transition, in the mid-Brunhes from about 500 to 400 ka, and during the last 100 ka, and gradual decreases between them. The first one supports the idea of rapid regeneration of the geomagnetic field after polarity reversals [1]. The other two may have been triggered by very short polarity reversals.

Rock magnetism and magnetic surveys in the iron ore deposit of El Encino, Mexico

In this study, we report results of petrophysical properties such as bulk (low field) magnetic susceptibility, remanent magnetization, Köenigsberger ratios, density, unblocking temperature spectra, isothermal remanent magnetization acquisition, and thermomagnetic analysis from surface and borehole samples of rock units and iron ore bodies in the iron mining district of El Encino, Jalisco, southwestern Mexico. Iron mineralization consists mainly of magnetite (from 65% to 90% volume) and hematite. It is associated with granodioritic intrusions and volcanic units in a mid-late Cretaceous volcano-sedimentary sequence. The magnetite in the iron ore has generally large grain size and a multidomain behavior. Stable remanent magnetization components associated with pseudo- or single domain behaviors are also observed. Iron ore bodies and associated intrusives show large dipolar magnetic anomalies. For the iron ore bodies examined, either the induced or the remanent component is responsible for the magnetic anomaly response. Data on bulk susceptibility, remanent intensity, and direction provide important constraints for modeling the magnetometric anomalies i.e., by quantifying the relative contributions of induced and remanent magnetization components; and thus permitting a better control of the geometry of source bodies.

Metastability of domain state in multidomain magnetite: Consequences for remanence acquisition

Experimental studies on isothermal remanent magnetization (IRM) acquisition of synthetic specimens containing crushed natural magnetite in seven grain sizes from 5–10 μm to 100–150 μm show that the initial demagnetization state before IRM acquisition strongly affects the samples' ability to acquire IRM. It is easier to acquire IRM from an initially thermally demagnetized state than from an alternating field (AF) demagnetized state, and low‐temperature demagnetization of the thermally demagnetized state increases resistance to IRM acquisition. These data are interpreted as confirmation of the existence of a number of possible domain states (metastable states) which have variable capacity for remanence acquisition. Our results also demonstrate that activation of domain structure is much more effective by AF than by direct (dc) fields. We suggest that the continually changing external field experienced during AF demagnetization allows the magnetic structure to reorganize incrementally towards the demagnetized state, requiring a considerably lower peak field (about 3 times lower) to achieve the same demagnetization effect. Thermoremanent magnetization (TRM) in larger grain sizes is partially self‐reversed. A difference in grain size dependence is observed for a number of magnetic parameters. All parameters vary with grain size below about 50 μm. Above 50 μm, parameters associated with coercivity, AF demagnetization, or low‐temperature treatment (Hcr; saturation IRM (SIRM) memory; TRM memory; median acquisition field (MAF) of IRM after AF; median destructive fields (MDF) of SIRM) are independent of grain size; parameters associated with TRM or thermal demagnetization (MDF ofTRM; MAF of IRM after thermal demagnetization; MAF of IRM after thermal demagnetization followed by low temperature demagnetization; self‐reversal as percentage of TRM) show grain size dependency throughout the grain size range. Our interpretation of the data invokes the existence of subtle material inhomogeneities in both natural and synthetic magnetites, giving rise to a limited range of Curie temperatures below 580°C (perhaps down to 560°C). We suggest that their control over the local energy minima states available to the grain is more significant in the TRM or thermally demagnetized state, while shielding by “soft” domain walls is dominant in the AF demagnetized state where walls are in the simplest domain configuration, leading to differing grain size dependencies of these different states.

Palaeomagnetic results from southern Albania and their significance for the geodynamic evolution of the Dinarides, Albanides and Hellenides

Mesozoic and Cenozoic rocks were studied in three tectonic units in southern Albania. The area belongs to the Ionian Zone, Outer Albanides. Rock magnetic investigations, comprising alternating field and thermal cleaning, stepwise acquisition of isothermal remanent magnetization and hysteresis experiments were carried out. They established magnetite and titanomagnetite as the main carriers of the natural remanent magnetization. In the high field behaviour, haematite and goethite can also be seen. Several fold tests on the site and on a regional scale indicate that the directions are characteristic. These directions prove a general clockwise rotation of about 40° since the Tortonian. Using all available palaeomagnetic data from Albania, individual movements of the different tectonic units within the Inner and Outer Albanides can be deduced. This is also valid for the Albanian Alps, which are understood to be the southernmost part of the Dinarides.