Concentration Of Magnetic Grains Research Articles

A broad band magnetic susceptibility test study — The magnetic spectroscopy of a Neolithic ditch

The magnetic characteristics of soil, sediments and archaeological deposits are environmentally sensitive, and can therefore be used to understand formation processes at archaeological sites. The magnetic susceptibility (MS) of a material indicates the concentration of magnetic inclusions of a sample. This depends on the concentration of magnetic grains but also on the composition of the magnetic mineralogy as the size of those grains. The grain size relates to the size-dependent magnetic domain state, which vary from thermally unstable ultrafine superparamagnetic (SP) grains to stable single-domain (SD) and pseudo-single-domain (PSD) grains to large multi-domain (MD) grains. Standard dual frequency measurements (χFD), magnetic susceptibility measurements at two different frequencies, are applied to semi-quantitatively evaluate a sample's SP inclusion. Other methods are used to quantitatively evaluate the magnetic grain size distribution (GSD) of a sample. Recently, magnetic susceptibility meters are available permitting broad-band magnetic susceptibility (BBMS) measurements to be made over a large spectrum of frequencies. This allows quantifying a narrow GSD of SP grains based on their frequency-dependency. Although novel in archaeology, such measurements have been applied in a handful of environmental studies in recent years concerning loess deposits in China and Bulgaria as well as ceramics in a study from the Czech Republic. To assess the applicability of BBMS in archaeology, soil samples were taken from a ditch at the middle-Neolithic Circular Ditched Enclosure, Hornsburg I, Lower Austria. The samples were measured with the University of Toronto Electromagnetic Induction Spectrometer (UTEMIS II). Measurements at different frequencies, ranging between 140 Hz to 63 kHz, allowed the calculation of the samples' GSD's, the grain sizes' mean, standard deviation and range as well as the frequency dependence. These can be used to understand a sites or features soil formation history, as archaeological deposits often exhibit enhanced magnetic properties that are distinct from surrounding non-anthropogenic soils, sediments and deposits. Therefore understanding how these contrasts relate to the changes in magnetic concentration, composition and grain size can provide indispensable information towards the formation processes and post-depositional changes at an archaeological site. The results of the study show promise for BBMS studies being a useful tool for differentiating different soils, sediments and archaeological deposits and thus reflecting the different archaeological phases and soil formation processes within the examined ditch. This provides for a better understanding of the depositional and post-depositional processes of the ditch deposits allowing for further archaeological assessment and offering additional applicability for a wide range of archaeological fields.

Contribution des paramètres magnétiques à l’identification des niveaux stratigraphiques et de la pédogenèse (Grotte del Angel, Espagne)

[fr] La grotte d’Angel, datée du Pléistocène moyen et de la base du Pléistocène supérieur, a livré un important matériel lithique et faunistique, et présente un intérêt exceptionnel dans la Péninsule Ibérique. Vu son importance, et dans le cadre d’un programme de recherche européen (N°IDPH/ER/05), une étude pluridisciplinaire a été réalisée sur le remplissage sédimentologique et les vestiges archéologiques. Nous présentons dans cet article les résultats de l’étude des propriétés magnétiques du remplissage de la grotte. Cette approche a révélé la présence de deux unités stratigraphiques. L’unité magnetostratigraphique inférieure se caractérise par une forte concentration en grains magnétiques, parmi lesquels un fort pourcentage en grains superparamagnétiques (SP). Les minéraux magnétiques sont d’origine secondaire, de petite taille, de nature douce et dominés par la magnétite et la maghémite. Ces derniers ont subi une évolution géochimique post-dépositionnelle importante (forte pédogenèse). Au contraire, dans l’unité supérieure, les valeurs des différents paramètres magnétiques sont faibles et les minéraux magnétiques sont d’origine détritique et se caractérisent par une force coercitive élevée, conforme à la présence d’hématite.

Magnetostatic interactions in deep‐sea sediments inferred from first‐order reversal curve diagrams: Implications for relative paleointensity normalization

Understanding magnetostatic interactions in sediments is important for paleomagnetism and rock magnetism. Magnetostatic interactions are known to affect significantly the efficiency of anhysteretic remanent magnetization (ARM) acquisition, and hence the ratio of ARM susceptibility (χARM) to saturation isothermal remanent magnetization (SIRM), a magnetic grain‐size proxy widely used for environmental applications, can be controlled by magnetostatic interactions. Relative paleointensity estimations may also be influenced by magnetostatic interactions because ARM is often used as a normalizer to correct for the difference in magnetizability of sediments. In this study, characterization of magnetic grains in deep‐sea sediments from the North Pacific Ocean was conducted using first‐order reversal curve (FORC) diagrams and IRM acquisition curves. The FORC diagrams indicate that the magnetic grains consist mainly of a noninteracting single‐domain (SD) component and an interacting SD component, and additionally of a multidomain (MD) component. The relative abundances of these components were semiquantitatively estimated by curve fitting of cross sections along a line parallel to the axis of local interaction fields (Hu) and through a peak in the coercivity (Hc). Three components were fitted assuming a Gaussian distribution of interaction fields (Hu). The IRM acquisition curves can be described by two dominant components assuming a log‐Gaussian distribution. The mean coercivities of the two components are ∼40 and ∼100 mT, respectively. It is estimated that the former component is carried by biogenic magnetite, and roughly corresponds to the noninteracting SD component derived from the FORC diagrams, and that the latter component is carried by eolian maghemite, corresponding to the interacting SD component. The χARM/SIRM ratio decreases with increasing concentration of the interacting SD component. This implies that the χARM/SIRM ratio is significantly affected by magnetostatic interactions and that it does not necessarily reflect magnetic grain size. The effect of magnetostatic interactions on natural remanent magnetization acquisition in sediments is not well understood, but if it is similarly sensitive to the SIRM, normalization by ARM in relative paleointensity estimations would overcompensate for the concentration of magnetic grains and cause the significant coherence between the normalized intensity and the normalizer that has been reported in the literature.

Rock-magnetic and remanence properties of synthetic Fe-rich basalts: Implications for Mars crustal anomalies

We characterized the magnetic mineral assemblage and remanence properties of a set of synthetic samples patterned on the meteorite-derived basalt composition A ⁎, which contains 18.9% total FeO. Basalts were synthesized at conditions that track 4 oxygen fugacity ( fO 2) buffer curves, from 3.4 log units below the quartz–fayalite–magnetite (QFM) buffer to 5 log units above QFM, and 6 cooling rates from 10 5 to 3 °C/h. The resulting array of samples was characterized using magnetic hysteresis loops, temperature dependence of saturation magnetization and saturation remanence (10 to 1000 K), and the acquisition and demagnetization of anhysteretic remanent magnetization (ARM) and thermoremanent magnetization (TRM). The magnetic mineral assemblage characteristics are strongly dependent on fO 2. Samples synthesized at the iron–wüstite (IW) buffer have a very low concentration of remanence-carrying grains, which are likely near the superparamagnetic-stable-single-domain boundary. Samples synthesized at the QFM and nickel–nickel oxide (NNO) buffers contain a slightly higher concentration of remanence-carrying grains, which are stable-single-domain to fine pseudo-single-domain particles, respectively. Samples synthesized at the manganese oxide (MNO) buffer contain the highest concentration of magnetic grains, which are up to 100 μm in diameter. The dominant Fe–Ti oxide produced is an Mg- and Al-bearing titanomagnetite with 2.4–2.7 Fe cations per formula unit. The Curie temperatures of the QFM samples are consistent with their electron-microprobe derived compositions. Those of the NNO sample set are very slightly elevated with respect to their electron microprobe derived compositions. The Curie temperatures of the MNO samples are elevated up to 200 °C above what they should be for their composition. We attribute the Curie temperature elevation to high-temperature nonstoichiometry of the titanomagnetite. The IW sample set acquired very weak TRMs with intensities of 0.02 to 0.5 A/m. This intensity of remanence is a factor of 50–500 too low to generate the observed 1000 nT anomalies detected on Mars by the Mars Global Surveyor MAG–ER experiment. The QFM, NNO, and MNO samples acquired TRMs up to 40 A/m in a 10-μT applied field, and up to 200 A/m in a 50-μT field, with little or no dependence on cooling rate. Our results suggest that Fe-rich melts that crystallize extensive titanomagnetite can generate an intensely magnetized layer in the Martian crust, even if the remanence was acquired in a weak field. The QFM sample set can easily account for the observed 1000-nT Mars magnetic anomalies, even in a magnetized layer as thin as 15–30 km.

Effect of the concentration of magnetic grains on the linear-optical-absorption coefficient of ferrofluid-doped lyotropic mesophases: deviation from the Beer-Lambert law.

In this paper is reported a systematic experimental study of the linear-optical-absorption coefficient of ferrofluid-doped isotropic lyotropic mixtures as a function of the magnetic-grains concentration. The linear optical absorption of ferrolyomesophases increases in a nonlinear manner with the concentration of magnetic grains, deviating from the usual Beer-Lambert law. This behavior is associated to the presence of correlated micelles in the mixture which favors the formation of small-scale aggregates of magnetic grains (dimers), which have a higher absorption coefficient with respect to that of isolated grains. We propose that the indirect heating of the micelles via the ferrofluid grains (hyperthermia) could account for this nonlinear increase of the linear-optical-absorption coefficient as a function of the grains concentration.

Stability of the uniform alignment of lyotropic nematic liquid crystals doped with magnetic grains

The stability of the uniform alignment of lyotropic nematic liquid crystals doped with magnetic grains is theoretically analysed. The sample is assumed to be prepared in a uniformly magnetized state by means of an external magnetic field, which is switched off subsequently. We show that there is a critical concentration of magnetic grains above which the distorting effect of the magnetic interaction overcomes the stabilizing effect of the steric interactions among grains and micelles.

The Magneto-Susceptibility Event and Cyclostratigraphy (MSEC) Method Used in Geological Correlation of Devonian Rocks from Anti-Atlas Morocco

Magneto-Susceptibility Event and Cyclostratigraphy (MSEC) is used here as a means of establishing sensitive chronostratigraphic markers and chronohorizons useful for high-resolution correlation. MSEC is a composite of the magnetic susceptibility (MS) record of marine strata and the coeval biostratigraphic record and, similar to chronozones, zones based on MSEC data have boundaries that are isochronous. MS, a measure of the concentration of magnetic grains in sediments, proxies for the ratio of lithogenic to biogenic components. Controls on the detrital input of lithogenic material include eustasy and climate, however induced, and sea floor/basin subsidence. The development of an initial MSEC composite reference curve for the Middle-Upper Devonian of the Anti-Atlas region of southern Morocco is presented and discussed in the context of global sea level transgressive and regressive cycles. MSEC trends, composed of increasing MS magnitudes, correlate well with episodes of regression, whereas trends of decreasing MS magnitudes correlate with episodes of transgression.

Elastic properties of magnetic suspensions in lyotropic nematic liquid crystals

A pseudo-molecular model is used to evaluate the elastic constants of a lyotropic nematic liquid crystal in which magnetic particles are suspended. The effective twist and bend elastic constants are shown to vary nonmonotonically with the concentration of magnetic grains. As the concentration is increased above a critical value, the twist constants K 22 and K 33 decrease below that of the undoped liquid crystal. This corresponds to the concentration above which the liquid crystal-magnetic grains composite has an effectively “collective” behaviour. The origin of the nonmonotonic behaviour is due to the destabilizing contribution of the magnetic grain-magnetic grain interaction to the total orientational energy.

Controls on magnetic grain-size variations and concentration in the Argentine Basin, South Atlantic Ocean

Measurement of two magnetic parameters, magnetic susceptibility (ξ) and anhysteretic remanent moment (ARM), was performed on 192 core-top samples from the Argentine Basin of the South Atlantic Ocean. Susceptibility indicates the concentration of magnetic grains in the sediment sample, while ARM estimates changes in magnetic mineral grain size: large ARM values indicate finer grain sizes. ARM normalized for magnetic concentration variation (ARM/ξ) increases precision in magnetic grain-size estimation. In the Argentine Basin, the magnetic characteristics reflect broad changes in magnetic mineral concentration rather than a substantial change in magnetic grain size across the basin. Data variations are consistent with deep-water circulation and sediment sources in the basin. The center of the basin acts as a sediment sink and is characterized by uniform magnetic grain concentrations and small grain sizes. Localized sediment input to abyssal depths from the South American continental margin dramatically modifies the magnetic pattern along the western margin of the basin. The data also show a marked downstream decrease in bulk magnetic grain content of the sediment as Antarctic Bottom Water enters the basin to the southeast and moves first westward and then northward along the margin of the basin.