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1.
The remanent magnetization of iron grains in lunar fines can be studied by allowing a sample of fines to acquire a depositional remanent magnetization (NRM) in the Thellier determination of an ancient lunar field intensity. Although a natural remanent magnetization (NRM) in the Thellier determination of an ancient lunar field intensity. Although a palaeointensity of ca. 1.0 Oe was determined by this method, several factors complicate the interpretation of the result. These include the origin and nature of the iron metal in lunar fines, and the time and acquisition process of its magnetization.  相似文献   

2.
A suite of synthetic titanomagnetites were prepared with compositions Fe2.6?δTi0.4AlδO4 and Fe2.4?δTi0.6AlδO4 (δ = 0, 0.1, 0.2 in both cases). Ball-milling of the synthesized samples produced material in the magnetic monodomain state as indicated by hysteresis loops and the Lowrie-Fuller test. The coercive force of the specimens depends on the Al concentration and lies in the range 1–2 kOe. The TRM induced in the samples is correspondingly “hard”. The low-field (0–1 Oe) TRM acquisition curve is linear. The higher field TRM-H curve is not in agreement with either monodomain or two-domain theoretical models.  相似文献   

3.
Magnetic hysteresis loops and the derived hysteresis ratios RH and RI are used to classify the various natural dilute magnetic materials. RI is the ratio of saturation isothermal remanence (IR) to saturation (IS) magnetization, and RH is the ratio of remanent coercive force (HR) to coercive force (HC). The RH and RI values depend on grain size, the characteristics of separate size modes in mixtures of grains of high and low coercivity, and the packing characteristics. Both RH and RI are affected by thermochemical alterations of the ferromagnetic fraction. Hysteresis loop constriction is observed in lunar samples, chondrite meteorites and thermochemically-altered basaltic rocks, and is due to mixtures of components of high and low coercivity. Discrete ranges of RH and RI for terrestrial and lunar samples and for chondrite meteorites provide for a classification of these natural materials based on their hysteresis properties.  相似文献   

4.
Compressional plastic deformation of a Cu-Co alloy in which Co particles (submicron to a few tens of microns in diameter) were precipitated in a Cu matrix resulted in a systematic rotation of remanent magnetization (IRM atH = 1000Oe) away from the axis of compression with increasing degree of the deformation.  相似文献   

5.
In reexamining the accumulated magnetic data on lunar rocks, several common patterns of magnetic behavior are recognized. Their joint occurrence strongly suggests a new model of lunar rock magnetism, which appeals only to partial preferred textural alignment of the spontaneous moments of magnetic grains, without requiring the existence of ancient lunar magnetic fields. This magnetic fabric, mimetic to locally oriented petrofabric, gives rise to an apparent “textural remanent magnetization” (TXRM). In order to account for the observed intensity of “stable remanence” in lunar rocks, only a minute fraction (10?3 to 10?5) of the single-domain iron grains present need be preferentially aligned. Several mechanisms operating on the lunar surface, including shock and diurnal thermal cycling, appear adequate for producing the required type and degree of magnetic alignment in all lunar rock classes. The model is supported by a wide variety of direct and indirect evidence and its predictions (e.g. regarding anisotropic susceptibility and remanence acquisition) can be experimentally tested.  相似文献   

6.
Parallel studies on rock magnetic properties, petrology and mineralogy were conducted on 16 eclogite samples from the ZK703 hole and magnetic susceptibilities and densities of 41 eclogite samples with different degree of retrograde metamorphism (from fresh eclogite to fully-retrograded eclogite) from the Chinese Continental Scientific Drilling (CCSD) near the ZK703 hole, located at Donghai, southern Sulu ultrahigh-pressure metamorphic belt, eastern China. Results show: (1) that the high-field slopes obtained from the hysteresis loops (the paramagnetic fraction χpara) and density have a positive correlation with the volume concentration of garnet + omphacite, a typical mineral assemblage used to semi-quantify the degree of retrograde metamorphism. The low-field slopes obtained from hysteresis loops (the ferrimagnetic susceptibility fraction χferri), saturation isothermal remanent magnetization Mrs and saturation magnetization Ms have a positive correlation with the volume concentration of symplectite, a mineral related to retrograded metamorphism. Therefore they could be potential indicators for quickly semi-quantifying the degree of retrograde metamorphism of the eclogite units. (2) The dominant magnetic carriers in retrograded eclogites are magnetite particles in pseudo-single domain grain size region. (3) The P–T conditions during the retrograde (decompressional) process could first increase the concentration of magnetite, which can reach up to 3% for extensively retrograded eclogite and then was dissolved for fully-retrograded eclogite. Therefore, change in the magnetite contents during the retrograde process is the major factor controlling the magnetism of retrograde eclogites.  相似文献   

7.
Anhysteretic remanent magnetization (ARM) is used as a means of estimating lunar magnetic field palaeointensities from several Apollo 11 and 16 samples. A value of 1.4 Oe was obtained by this method for an Apollo 16 sample of age 3.9 AE and this value is in close agreement with the value obtained by the conventional Thellier method (1.2 Oe) carried out on the same sample. A further sample which was of age 4.0 AE and which had been reheated at 3.84 AE also showed evidence of a primary magnetization acquired in a field of this magnitude. Determinations on two younger samples (about 3.6 AE) gave palaeofields of about one quarter of this value (0.33 and 0.38 Oe respectively). These estimates of field strengths are considerably higher than previously reported.  相似文献   

8.
A single-heating procedure is presented which makes possible the determination of two partially independent values of paleofield intensity for a given sample, one serving as a check to the other. The approach combines data required for Shaw-type and “ARM-method” determinations and in so doing furnishes a value of the ratio of TRM to ARM acquisition efficiency (f′) corrected for any physicochemical alteration to the magnetic carriers which may have occurred during laboratory heating.

Applicability of the Shaw-method to Fe-bearing samples is favorably demonstrated through simulated paleointensity determinations conducted on synthetic samples containing multi-domain grains. Moreover, coercivity spectra corresponding to anhysteretic remanent magnetization (ARM) are found to be considerably more sensitive to thermally induced alteration when compared with those corresponding to thermoremanent magnetization (TRM).

The combined Shaw-ARM procedure was successfully applied to lunar basalt sample 10017,135 rendering a paleointensity of 0.82 ± 0.11 Oe. The Thellier-Thellier method, however, was not able to provide a meaningful determination on the neighboring chip (number 136). These apparently conflicting findings may be explained by one or more of the following possible interpretations: (1) multiple step-wise heatings cause considerably more damage to the carriers of remanence than does a single-heating procedure; (2) the rock possesses extreme variability in magnetic properties from one sub-sample to the other; (3) the natural remanent magnetization in this lunar basalt is not a simple TRM.  相似文献   


9.
Palaeomagnetic investigation of basic intrusives in the Proterozoic Mount Isa Province yields three groups of directions of stable components of NRM after magnetic cleaning in fields up to 50 mT (1 mT= 10 Oe). The youngest group (IA) includes results from the Lakeview Dolerite, and yields a palaeomagnetic pole at 12°S, 124°E (A95 = 11°). The second group (IB) has a palaeomagnetic pole 53°S, 102°E (A95 = 11°). The third group (IC) is derived from the Lunch Creek Gabbro and contains normal and reversed polarities of magnetization with a palaeomagnetic pole at 63°S, 201°E (A95 = 9°). Some samples from the gabbro have anomalously low intensities of remanent magnetization in obscure directions attributed to the relative enhancement of the non-dipole component of the palaeomagnetic field during polarity reversal. The present attitude of the igneous lamination is probably of primary, not tectonic origin.  相似文献   

10.
Based on the theory of two-phase interacting nanoparticles, the formation of thermoremanent and chemical remanent magnetization in nanosized titanomagnetites is modeled. It is shown that the value of thermoremanent magnetization barely depends on the degree of titanomagnetite exsolution whereas, chemical remanent magnetization which emerges during the exsolution increases up to at most the value of thermoremanent magnetization. The values of the ratio of thermoremanent to ideal magnetization, R t , fall within the limits 0.8 ≤ R t ≤ 1. The analogous ratio of chemical remanent magnetization to the ideal R c are below R t at all stages of the exsolution. Besides, the magnetic interaction between the nanoparticles reduces the values of thermoremanent and chemical magnetization but barely affects the ratio.  相似文献   

11.
Hysteresis loops to 1200 oersteds (9.55×104 A m?1) are measured between 295 K and 105 K for two deep-sea basalts (DSDP, Leg 34 and 37) containing large (～200 μm) unexsolved titanomagnetite grains. The Curie points, electron microprobe analyses and saturation magnetizations of the magnetic grains are the same as for unoxidized synthetic titanomagnetite (xFe2TiO4·(l ? x)Fe3O4) with x=0.6.As temperature is lowered from 295 to 190 K, coercive force Hc slowly rises from ～40 Oe to ～95 Oe approximately in proportion to the rise in the magnetostriction constant λ. Presumably, Hc is controlled by λ through internal stresses impeding domain wall motion. As expected of multidomain grains, the ratio of saturation remanence to saturation magnetization (in 1200 oersted cycles) jR/jS rises approximately in proportion to Hc, with a constant of proportionality consistent with titanomagnetite (x=0.6).As temperature is lowered from 190 to 120 K, Hc rises rapidly to ～400 Oe as a roughly linear function of the magnetocrystalline anisotropy constant K1. Perhaps Hc is now controlled by K1 through non-magnetic inclusions impeding domain wall motion.As temperature is lowered from 120 to 105 K, Hc rises even more rapidly to ～600 Oe. The control over Hc seems to have changed again, though most of the titanomagnetite is in grains large enough to still contain a few domains. The ratio jR/jS reaches 0.7 by 105 K and appears to be saturating towards the theoretical limit of 0.83.  相似文献   

12.
An iron ore which behaves as a permanent magnet is lodestone. The intrinsic magnetic properties and the microstructual characteristics which are responsible for the permanent-magnet properties of the lodestone have never been described or explained. Iron ores capable of being charged sufficiently strongly to behave as permanent magnets are defined here as proto-lodestones and fall into two categories — Class I which contains Fe as the only significant cation, and Class II which contains considerable Ti, Mg and Al as well. Proto-Lodestones are magnetically hardened by oxidation and precipitation processes which produce the microstructure responsible for the permanent-magnet properties. Lodestones are charged proto-lodestone iron ores. Lodestones have RH values (ratio of remanent coercive force, HR, to coercive force, HC) between 2.0 and 2.5; RI values (ratio of saturation remanence, ISR, to saturation magnetization, IS) >0.1–0.25 and the ratio of NRM/SIRM (natural remanence to saturation remanence) is 0.15–0.7. The intrinsic magnetic properties and microstructural characteristics of proto-lodestones and other iron ores are described and explained. The mechanism of charging the proto-lodestone appears to be either transient magnetic fields associated with lightning-discharge currents or presently obscure aspects of magnetization intensity enhancement associated with maghemitization of massive iron ores.  相似文献   

13.
Sized fractions of x = 0.6, 0.4, 0.2 and 0.0 titanomagnetites were studied with a vibration magnetometer. In the course particles (d > 150 μm), no compositional dependence of hysteresis parameters was found. HC was less than 50 Oe, HR/HC > 4 and JR/JS < 10?2, reflecting multi-domain behaviour. In contrast, fine particles (d ? 0.1 μm) revealed systematic grain-size dependence of parameters with coercive force as high as 2,000 Oe in x = 0.6 titanomagnetite. Grain-size dependence studies revealed broad transition sizes for the onset of true multi-domain behaviour depending upon which factor is chosen. In magnetite it varies from 10 to 20 μm. The experimental critical size for single-domain behaviour for magnetite is about 0.1 μm and for x = 0.6 titanomagnetite 1–2 μm.  相似文献   

14.
Summary Measurements of bulk magnetic properties, including the natural remanent magnetization (NRM), susceptibility and the Königsberger ratio, on over 250 samples of Tertiary basalts from Disko and Nûgssuaq, West Greenland are reported.The NRM intensities in basalts (geometric mean value 3.3 A/m in SI units) were on average three to four times as large as the induced magnetization intensities. The susceptibilities (geometric mean value 2.1×10–2 SI units) were much more uniform than the NRM intensities. In the majority of samples, the NRM was predominantly of reverse (R) polarity, but samples from a few sites showed a remanence of normal (N) polarity.The NRM of both polarity classes (N, R) was very stable against alternating field (AF) demagnetization with median destructive fields of the order of 20,000–30,000 A/m (250–350 Oe), comparable to those for many stable continental and oceanic basalts. The viscous remanence intensity, as studied by storage tests on some specimens, was found to be an insignificant fraction of the original NRM, except in few cases.The low field hysteresis loops (Rayleigh loops) were studied for some specimens. A qualitative association was noted between wide hysteresis loop and relatively low AF stability, but no correlation was apparent between the loop type and the Königsberger ratio (Q n) of a specimen.Contribution no. 6 Institute of Geophysics, University of Copenhagen.  相似文献   

15.
Thermally acquired remanent magnetization is important for the estimation of the past magnetic field present at the time of cooling. Rocks that cool slowly commonly contain magnetic grains of millimeter scale. This study investigated 1-mm-sized magnetic minerals of iron, iron–nickel, magnetite, and hematite and concluded that the thermoremanent magnetization (TRM) acquired by these grains did not accurately record the ambient magnetic fields less than 1 μT. Instead, the TRM of these grains fluctuated around a constant value. Consequently, the magnetic grain ability to record the ambient field accurately is reduced. Above the critical field, TRM acquisition is governed by an empirical law and is proportional to saturation magnetization (Ms). The efficiency of TRM is inversely proportional to the mineral's saturation magnetization Ms and is related to the number of domains in the magnetic grains. The absolute field for which we have an onset of TRM sensitivity is inversely proportional to the size of the magnetic grain. These results have implications for previous reports of random directions in meteorites during alternating field demagnetization, or thermal demagnetization of TRM. Extraterrestrial magnetic fields in our solar system are weaker than the geomagnetic field by several orders of magnitude. Extraterrestrial rocks commonly contain large iron-based magnetic minerals as a common part of their composition, and therefore ignoring this behavior of multidomain grains can result in erroneous paleofield estimates.  相似文献   

16.
The grain size dependence of the ratio of saturation remanent magnetization to saturation magnetization (J R :J S ), weak field susceptibility (X 0), thermoremanent magnetization (TRM) and its stability against AF demagnetization are interpreted in terms of nucleation theory. It is concluded that each of these parameters exhibits grain size dependence due to two effects. The first is the increasing difficulty with which domain walls are neucleated as grain size decreases. The second is an intrinsic grain size dependence of the parameters in multidomain particles.  相似文献   

17.
Results of a detailed petromagnetic study of sediments of the Koshak section, including the Mesozoic/Cenozoic (K/T) boundary, are presented. The rocks are shown to have a very low magnetization. A relatively high magnetization is characteristic of two thin clayey beds, one at the K/T boundary and the other 0.6 m above it: x up to 2.5 × 10?9 m3/kg, M s up to 0.6 × 10?3 A m2/kg, and M rs up to 0.4 × 10?3 A m2/kg. This is related to relatively high concentrations of hemoilmenite (up to 0.2%), magnetite (up to 0.01%), and goethite (up to 0.24%) in these beds. It is evident that the distribution of these magnetic minerals is lithologically controlled (the predominant occurrence in clayey beds), which is expressed, in particular, in the relation between the paramagnetic (clayey) and diamagnetic (carbonate) contributions to the magnetization of the sediments. Thus, clayey interbeds are sharply distinguished by the value of the paramagnetic magnetization (M p = (83–86) × 10?5 A m2/kg) as compared with purely diamagnetic chalk (M d = ?(26–35) × 10?5 A m2/kg). Minor concentrations of metallic iron (up to ～0.002%) discovered in the sediments have a lithologically uncontrolled distribution (metallic iron is more often observed near the K/T boundary rather than in clayey beds). Most probably, magnetite, hemoilmenite, and goethite were accumulated mostly with clay and other terrigenous material, while fine particles of iron are likely to have been dispersed by air. The whole set of the data of this work suggests that the K/T boundary is not distinguished by characteristic magnetomineralogical and magnetolithologic features.  相似文献   

18.
During revisiting the Upper Pleistocene Pekla loess-soil section located on the Sea of Azov coast of the Taman Peninsula, its lower 6 m were continuously sampled, which led to an increase in the age range from ∼50 to 400 ka. The detailed rock magnetic study of the structure, grain-size, and concentrations of magnetic mineral (natural remanent magnetization (NRM) carrier) in the collected rock samples revealed regular changes in rock magnetic characteristics along the section and their correlation with climatic fluctuations. Magnetite and hematite both deposited during the transport of sedimentary material and formed during pedogenesis, which involved the entire section to a varying extent, represent the main magnetic minerals in the examined rocks. Automorphic paleosoils that were formed during warm and humid periods corresponding to odd stages of the MIS scale are characterized by elevated concentrations of magnetic mineral (NRM, magnetic susceptibility (K lf), saturation isothermal remanent magnetization (SIRM), and anhysteresis (ideal) remanent magnetization (ARM)) parameters and share of superparamagnetic particles (up to 80%, according to elevated values of the frequency-dependent magnetic susceptibility K td) as well as by lowered rigidity parameter (B cr) and grain size (ARM/K parameter). Such changes in the paleosoils may be explained by the occurrence of newly formed fine-grained magnetite particles close in size to its superparamagnetic and single-domain varieties due to the activation of bio/geochemical processes during warm stages. The growth of the above-mentioned rock magnetic parameters in automorphic soils may be considered as serving a quantitative criterion for defining the boundary between warm and cold periods even in poorly developed soils.  相似文献   

19.
A summary of experiments and analyses concerning electromagnetic induction in the Moon and other extraterrestrial bodies is presented. Magnetic step-transient measurements made on the lunar dark side show the eddy current response to be the dominant induction mode of the Moon. Analysis of the poloidal field decay of the eddy currents has yielded a range of monotonic conductivity profiles for the lunar interior: the conductivity rises from 3·10?4 mho/m at a depth of 170 km to 10?2 mho/m at 1000 km depth. The static magnetization field induction has been measured and the whole-Moon relative magnetic permeability has been calculated to be μμ0 = 1.01 ± 0.06. The remanent magnetic fields, measured at Apollo landing sites, range from 3 to 327 γ. Simultaneous magnetometer and solar wind spectrometer measurements show that the 38-γ remanent field at the Apollo 12 site is compressed to 54 γ by a solar wind pressure increase of 7·10?8 dyn/cm2. The solar wind confines the induced lunar poloidal field; the field is compressed to the surface on the lunar subsolar side and extends out into a cylindrical cavity on the lunar antisolar side. This solar wind confinement is modeled in the laboratory by a magnetic dipole enclosed in a superconducting lead cylinder; results show that the induced poloidal field geometry is modified in a manner similar to that measured on the Moon. Induction concepts developed for the Moon are extended to estimate the electromagnetic response of other bodies in the solar system.  相似文献   

20.
Anhysteretic remanent magnetization (DC field = 1 Oe, peak AC field = 1000 Oe) was given to an artificial sediment consisting of a kaolinite matrix and a 0.03% magnetic fraction of needle-shaped magnetite grains. The mean grain size of the clay flakes and the magnetite needles was approximately 0.5 μm. This sediment was subjected to plane strain while maintaining constant volume. The axis of maximum compression was shortened by as much as 331/3%, which produced no significant change in the direction of magnetization with a decrease in intensity of 28%. A continuous deformation model, in which the magnetite needles are embedded in a plastically deforming linearly homogeneous medium, failed to explain these results. A discontinuous deformation model was more successful. In this model, deformation occurs mainly in shear zones bounding blocks which translate along the shears. Only those magnetite grains in the shear zones rotate, and from symmetry considerations no net change in the direction of magnetization is predicted. Because magnetite grains rotate in opposite directions in complementary sets of shears, the model predicts a decrease in intensity comparable to that which was observed.  相似文献   

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