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1.
Summary The anisotropy of susceptibility which is induced in rocks and dispersed powdered magnetite by the application of a strong alternating field is removed by heating more rapidly than is thermoremanence in the same specimens. The anisotropy associated with thermoremanence induced in the earth's field is negligible, but in fields greater than 5 oersteds it becomes appreciable. It is concluded that the domain structure associated with low field thermoremanence is only a very slight perturbation of the zero field structure. However this is much less simple than has been supposed and involves two- or three-dimensional domain closure. An appropriate minor revision of the theory of multidomain thermoremanence is presented.  相似文献   

2.
Summary The changes of the anisotropy of magnetic susceptibility of igneous rocks, induced by a magnetic field, are studied. It is proved that changes in the degree of anisotropy of susceptibility and of the orientation of the susceptibility ellispoid of specimens occur due to the configuration of the domain structure under the effect of the magnetic field. The influence of this effect on the total anisotropy of rocks depends on the degree of anisotropy due to the shape factor and on the stability of the domain structure. A model concept is presented, explaining the qualitatively different pattern of the changes of the anisotropy of susceptibility under the effect of the magnetic field in various directions of the specimens.  相似文献   

3.
Magnetic anisotropy and fabric of some foliated rocks from S.E. Australia   总被引：1，自引：0，他引：1  
Summary The magnetic anisotropy of foliated rocks of several types has been measured by the torque-meter method, and shows that the alignment of long axes of magnetic grains in rocks normally follows the pattern of foliation evident in field observations. In a sharp fold in a lit-par-lit formation the magnetic anisotropy indicated an otherwise undetected lineation independent of the bedding and superimposed upon the foliation determined by the layering. In two adamellites, each with two alignment patterns separated by an angle of 30° the magnetic data are shown to be consistent with two foliations but not with one foliation plus a lineation. Magnetic anisotropy data can be ambiguous for rocks in which two or more grain alignment processes have operated, but combined with other observations magnetic measurements can provide a valuable new tool in the study of rock fabrics.  相似文献   

4.
A phenomenological model based on a linear relationship between the magnetic coercivity field and the reciprocal of the grain diameter is applied to explain the anhysteretic remanent magnetization (ARM) imparted to artificial samples with different concentrations of a very well characterized magnetite powder. By analyses of scanning electron microscopy images, the spherically shaped single domain synthetic magnetite is found to follow a lognormal grain size distribution with ~86 nm of mean diameter. The proposed model, fitted to ARM measurements up to a peak alternating field of 100 mT, yields a very good agreement. The coercivity behaviour predicted by micromagnetism theory disagrees with the experimental results of this work. A likely explanation for the discrepancy is that the magnetite particles, which consist of a mixture of grains in coherent rotation and curling modes, produce similar observations as domain processes.  相似文献   

5.
The anisotropy of magnetic susceptibility (AMS) and mean particle size in the silt fraction have been measured in surface sediments and down selected cores from the Vema Channel. Results show that a function (Fs) which represents magnetic grain long axis alignment is highly correlated with variation in mean size of the silt fraction. An increase in alignment is caused by an increase in bottom water velocity which corresponds to an increase in mean particle size. Best alignments and coarsest particle sizes are found in the axis of the Vema Channel. In contrast, poor particle alignment and fine silt mean characteristics the shallower region east of the channel axis, consistent with lower current velocities.Core CH 115-61 was collected near the Level of Least Motion at the transition between northward-flowing Antarctic Bottom Water (AABW) and southward-flowing North Atlantic Deep Water. A positive relationship exists between silt size and AMS in the core. Fluctuations in particle alignment and mean silt size indicate three or four periods of increased velocity during the past 150,000 years.Geographic orientation of the maximum AMS axes in cores CH 115-60 and -61 was accomplished using the declination of remanent magnetism and an axial dipole as the reference azimuth. The data show that the larger magnetic grains in these sediments are aligned normal to the bathymetric trend of the channel which suggests that these grains are aligned by traction transport. The result indicates that AABW in the Vema Channel has flowed along an azimuth of approximately N30°E for the past 150,000 years.  相似文献   

6.
The behaviour of some magnetic properties of natural and synthetic haematite of different grain size is examined. The natural haematite was obtained from the hydrothermal deposit Kada (Czech and Slovak Federal Rep.). Six grain-size fractions ranging from 120 to 40 μm were prepared by means of sieving and two further fractions down to 5 μm by wet ultrasonic sieving. Since the behaviour of the fractions is similar, that of only four representative samples is reported. In addition, the behaviour of one submicron synthetic haematite fraction (0.5 μm) prepared by oxidation of ferrous sulphate (uniform in size and shape) was investigated.

The initial remanence value (Jr) seems to increase with decreasing grain size. During alternating field (AF) demagnetization, all fractions behaved similarly, except for the submicron fraction which is considerably softer than the others. Normalized (isothermal remanent magnetization) IRM acquisition curves were similar for all fractions.

Parameters of the anisotropy of magnetic susceptibility (AMS) display significant changes, mainly during IRM acquisition. During AF demagnetization, the anisotropy degree P exhibits a slight increase (some %), while the behaviour of the shape factor T is complicated. The anisotropy ellipsoid exhibits a tendency to rotate. Significant changes in the AMS parameters occur during IRM acquisition. Curves of P and T vs. IRM acquisition field, for various grain-size fractions, show no coherent pattern. For all the samples studied, the T vs. H curve exhibits a threshold value at which change in the type of arrangement of easy axis of magnetization occurs. For the IRM acquisition fields higher than some 320 kA m−1, the minimum susceptibility axis parallels the direction of the IRM acquisition field.

Hysteresis curves of the fractions are similar to each other. The Preisach distribution function was determined and it indicates that the reversible part of the magnetization process plays an important role comparatively. Based on the coercivity data presented no unambiguous conclusion could be drawn from the single-domain (SD)-multidomain (MD) transition, associated with a coercivity maximum.  相似文献   


7.
Summary Evidence for flow structure within hexagonal columns of basalt has been sought, using measurements of the anisotropy of magnetic susceptibility. The observed magnetic grain alignment indicates that the flow was essentially uniform and horizontal across the hexagons which were sampled. The flow probably occurred during emplacement of the lava; there is no evidence of convection within the hexagons, either from the degree of grain alignment or its orientation.  相似文献   

8.
Low-field anisotropy of magnetic susceptibility (AMS) has been determined for a total of 248 basaltic specimens taken from cross sections between the cooling interfaces of 6 subaerial lavas, 6 deep-sea lavas, and 6 intrusives (5 dikes and 1 sill). Statistically significant AMS clusters are exhibited by all the dikes examined and, based upon these clusters, derivation of emplacement direction becomes possible. Two lavas are observed to have statistically significant AMS clusters which can be used for flow direction determinations. The methods of emplacement and flow direction analysis are discussed as well as the statistics used. It is concluded that most of the dikes examined have low angle emplacement directions. A classification scheme for AMS data distributions is presented.The AMS analysis shows that intrusives and deep-sea lavas can be distinguished from subaerial lavas approximately 80% of the time by the random AMS ellipsoid orientations exhibited in subaerial lavas. Contrasts in the fluid properties, degassing, wall effects with subsequent distortion of the fluid, and grain interaction during the extrusion of subaerial lavas can be expected to distort magnetic grain alignment. Further effects such as convection and secondary processes contribute to yield the random distributions observed for most of these bodies.  相似文献   

9.
Anisotrophy of magnetic susceptibility (AMS) results from 27 specimens drilled from the top and two sides of a single columnar basalt segment are presented. The magnetic foliation plane is nearly horizontal for all parts of the column, which is consistent with a primary magma flow pattern, without evidence of local convection or differentiative processes. The shape of AMS ellipsoids is however predominantly prolate, which may be indicative of increased magnetic grain elongation due to crystal growth or grain realignment normal to a vertical stress field (due to thermal contraction). Apparent systematic variations related to column shape are found in bulk susceptibility, anisotropy degree and degree of lineation and foliation; some of the variation may also be related to weathering effects. The results are consistent with a primary AMS pattern resulting from thermal contractive stresses during column formation. Comparison of results from previous studies of columnar basalts reveals that there is a relatively large variation in AMS properties. There appears to exist a number of factors which may locally control the magnetic anisotropy of columns and very likely some of their other characteristics.  相似文献   

10.
Summary Measurements have been made of the dieletric anisotropies of a number of rocks for which magnetic anisotropy data have been obtained previously. The purpose was to examine the possible usefulness of dielectric anisotropy as a physical property indicative of rock fabrics. Its advantage over the magnetic method is that it measures an average alignment of crystals of the dominant minerals, whereas magnetic anisotropy is due only to the ferromagnetic grains. Disadvantages are an extreme sensitivity to specimen shape and difficulty in distinguishing the several types of alignment which can give rise to dielectric anisotropy. In a number of strongly foliated rocks the axes of dielectric anisotropy were found to coincide with the axes of magnetic anisotropy. Specimens from a magnesian-pyroxene rich layer in a Tasmanian dolerite sill and from the olivine rich layer of the Palisades dolerite sill, New York, were found to have no systematic anisotropy. The pyroxenes in the Tasmanian dolerite are elongated crystals (about 2: 1) so that the dielectric measurements show that they do not have a preferred horizontal alignment and therefore have probably not settled as individual crystals. Most of the olivines in the Palisades dolerite are more nearly equidimensional so that the absence of measurable anisotropy in this rock is less conclusive evidence against crystal settling.  相似文献   

11.
Holocene records documenting variations in direction and intensity of the geomagnetic field during the last about seven and a half millennia are presented for Northwest Africa. High resolution paleomagnetic analyses of two marine sediment sequences recovered from around 900 meter water depth on the upper continental slope off Cape Ghir (30°51′N, 10°16′W) were supplemented by magnetic measurements characterizing composition, concentration, grain size and coercivity of the magnetic mineral assemblage. Age control for the high sedimentation rate deposits (∼60 cm/kyr) was established by AMS radiocarbon dates. The natural remanent magnetization (NRM) is very predominantly carried by a fine grained, mostly single domain (titano-)magnetite fraction allowing the reliable definition of stable NRM inclinations and declinations from alternating field demagnetization and principal component analysis. Predictions of the Korte and Constable (2005) geomagnetic field model CALS7K.2 for the study area are in fair agreement with the Holocene directional records for the most parts, yet noticeable differences exist in some intervals. The magnetic mineral inventory of the sediments reveals various climate controlled variations, specifically in concentration and grain size. A very strong impact had the mid-Holocene environmental change from humid to arid conditions on the African continent which also clearly affects relative paleointensity (RPI) estimates based on different remanence normalizers. To overcome this problem the pseudo-Thellier RPI technique has been applied. The results represent the first Holocene record of Earth’s magnetic field intensity variations in the NW Africa region. It displays long term trends similar to those of model predictions, but also conspicuous millennium scale differences.  相似文献   

12.
The anisotropy of magnetic susceptibility (AMS) and rock magnetic properties were measured on specimens from a basalt plate that was cut from a vertical section of a basalt column from Hrepphólar, Iceland. Macroscopic structures are clearly distinguishable in the plate, including banding inferred to represent viscous fingering parallel to the vertical axis of the column. Rock magnetic experiments indicate that the dominant ferromagnetic (sensu lato) mineral is titanomagnetite, Fe 3?x Ti x O4, with a Ti-composition of x?=?~0.6. Magnetic properties are related to the position within the plate and reveal a dominant volume fraction of single domain titanomagnetite in the center of the basalt column, with multidomain titanomagnetite away from the center. The AMS determined by low-field measurements shows an inconclusive relationship with the visual structures, which arises from variation of the grain size (i.e., single domain versus multidomain) across the column. In contrast, the AMS measured with a high-field torsion magnetometer avoids the complication of magnetic domain state, as is demonstrated in this contribution, and additionally allows for the separation of ferrimagnetic from paramagnetic sub-fabrics. Both sub-fabrics display a clear relationship with the macroscopic structures and support the hypothesis that vertical flow of melt took place during development of the Hrepphólar columnar basalt. Maximum susceptibility axes of the ferrimagnetic sub-fabric are grouped near the vertical axis of the column. The paramagnetic sub-fabric varies systematically across the column in coincidence with internal structure. The shape of the magnetic susceptibility ellipsoid varies across the basalt column, showing an increasingly prolate fabric toward its center.  相似文献   

13.
The influence of magnetic interactions on the anisotropy of magnetic susceptibility (AMS) have been largely studied by several theoretical models or experiments. Numerical models have shown that when magnetostatic interactions occur, the distributions of particles over the volume rather than their individual orientations control the AMS. We have shown recently from a comprehensive rock magnetic study and from a theoretical 2-dimensional (2-D) model that single domain particles closely packed in globule aggregates could produce strong local random interaction magnetic fields which could influence the magnetic susceptibility and decrease the degree of anisotropy. In this paper, we first present in detail this 2-D theoretical model and then we extend it to the 3-D case. The possible distribution function of the magnetostatic interaction fields comprises two extreme states: it is either isotropic or ordered. The former case corresponds to the thermal-demagnetized state while the second case corresponds to the alternating field (AF) demagnetized state. We show that when easy axes of magnetization are not uniformly distributed, the degree of anisotropy decreases as the interaction field increases in both AF- and thermal-demagnetized states in 2-D and 3-D geometry. Thus we conclude that random magnetic fields generated by a random arrangement of magnetic particles over the sample volume decrease the degree of anisotropy of AMS and may alter the magnetic fabric.  相似文献   

14.
North-seeking bacteria (NSB) with 1 μm diameters migrate to the S pole only. They were applied to identify the S pole determination on a polished surface of magnetite-rich pyroxenite whose natural remanent magnetization (NRM) intensity was 5.64 × 10−3 Am2 kg−1. The microscopic observations were performed under dark-field illumination in a controlled magnetic field to 10 μT. The NSB formed clusters on limited areas of magnetite grains and scattered over the whole magnetite grains.

The NRM decreased to 1.02 × 10−5 Am2 kg−1 by alternating field (AF) demagnetization to 60 mT but no clusters appeared, while small populations of the NSB scattered on each grain. These scattered bacteria may gather toward the S pole resulting from magnetic domain walls.

When the sample acquired saturation isothermal remanent magnetization (SIRM) to 1 T, the NSB formed dense clusters at the opposite side to the applied field direction on the many grains as expected. This evidence indicated that the NSB can be useful micro-organisms for the determination of fine magnetic structures. Some grains also had NSB clusters at the edge of the grains toward the field direction or did not exhibit any clusters. The complicated distribution of the clusters (the S poles) may be explained by shape anisotropy of the magnetic grains.  相似文献   


15.
This paper reports low-temperature cycling (LTC) through the Verwey transition of anhysteretic remanence (ARM), partial ARMs and partially demagnetised saturation isothermal remanence (SIRM) induced at room temperature in pseudo-single-domain and multidomain (MD) magnetite. The remanences were cooled in zero field to 50 K and then heated back to room temperature. By inducing partial ARMs over different field ranges and by partially alternating field demagnetising SIRMs, it was possible to isolate both low-coercive-force and high-coercive-force fractions of remanence. On cooling through the Verwey transition, a sharp increase in the remanence was observed. The relative size of the jump increased as the high-coercive-force fraction was increasingly isolated. This behaviour is interpreted as being due to both an increase in the single-domain/multidomain threshold size on cooling through the Verwey transition and to the reduction or elimination of closure domains in the low-temperature phase. In addition, the memory ratio, i.e. the fraction of remanence remaining after LTC divided by the initial remanence, was found to be higher for the high-coercive-force fraction than the low-coercive-force fraction. In our interpretation, the high-coercivity fraction behaviour is associated with reversible domain re-organisation effects, whilst the low-coercive force fraction’s behaviour is associated with irreversible domain re-organisation and (de-)nucleation processes. Due to the decrease in magnetocrystalline anisotropy on cooling to the Verwey transition, the high-coercive-force fraction is likely to be magnetoelastically controlled. Thus, a rock displaying high-coercive-force behaviour is likely to carry a palaeomagnetically meaningful remanence with high unblocking temperatures. In addition, LTC analysis can be used to identify the domain state dominating the natural remanence in magnetite-bearing rocks.  相似文献   

16.
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   

17.
The electromagnetic fields induced by hydroacoustic waves, propagating in a liquid conducting medium in an alternating magnetic field, have been considered. The equation, relating the induced magnetic field to the undisturbed antenna field and acoustic wave parameters, has been obtained. The spatial—temporal pattern of the induced field has been constructed in the case when acoustic wave propagates along a direct line with an alternating current.  相似文献   

18.
Relative directions of magnetization have been measured within individual pillow basalts collected from the Atlantic Ocean and Caribbean Sea. The angle between the magnetic directions was determined and is referred to as the directional difference. Although one pillow contained a directional difference of 44°, the remaining ten pillows had differences less than 14°. The maximum orientation and measurement error was 7°. Dispersion on the scale found in these fine-grained pillow basalts would not appreciably affect the magnetic anomaly pattern on the sea floor. We detected no reversals of magnetization despite the sometimes large and variable low-temperature oxidation. Comparison of directions within homogeneous segments of the pillow, viscous remanent magnetization (VRM) acquisition experiments, and alternating field (AF) demagnetization indicate a large portion of the dispersion was due to the acquisition of a viscous component in the larger grained, less oxidized portion of the pillows. Evidence from one variably weathered pillow suggests that extreme low-temperature oxidation may lead to the acquisition of a secondary component with high coercivities (20–80 mT). We could not determine whether this was a chemical remanent magnetization (CRM) or a VRM acquired by single domain grains near the superparamagnetic threshold. Hysteresis properties confirmed by microscopic examination indicated that the magnetic grain size in all the pillows was at least as small as pseudo-single domain.  相似文献   

19.
The Shear-Wave Experiment at Atomic Energy of Canada Limited's Underground Research Laboratory was probably the first controlled-source shear-wave survey in a mine environment. Taking place in conjunction with the excavation of the Mine-by test tunnel at 420 m depth, the shear-wave experiment was designed to measure the in situ anisotropy of the rockmass and to use shear waves to observe excavation effects using the greatest variety of raypath directions of any in situ shear-wave survey to date. Inversion of the shear-wave polarizations shows that the anisotropy of the in situ rockmass is consistent with hexagonal symmetry with an approximate fabric orientation of strike 023° and dip 35°. The in situ anisotropy is probably due to microcracks with orientations governed by the in situ stress field and to mineral alignment within the weak gneissic layering. However, there is no unique interpretation as to the cause of the in situ anisotropy as the fabric orientation agrees approximately with both the orientation expected from extensive-dilatancy anisotropy and that of the gneissic layering. Eight raypaths with shear waves propagating wholly or almost wholly through granodiorite, rather than granite, do not show the expected shear-wave splitting and indicate a lower in situ anisotropy, which may be due to the finer grain size and/or the absence of gneissic layering within the granodiorite. These results suggest that shear waves may be used to determine crack and mineral orientations and for remote monitoring of a rockmass. This has potential applications in mining and waste monitoring.  相似文献   

20.
晚更新世以来以一系列地磁漂移事件是区域地层对比和确定沉积物年代框架的重要工具.但是在海相沉积物中,识别这些地磁漂移事件多依赖相对磁场强度(RPI)的变化,在地磁方向异常方面鲜有报到,从而影响对这些事件可靠性的讨论.本文选择南海南部巽他陆架附近重力活塞钻孔BKAS2PC的沉积物,进行系统的岩石磁学和古地磁研究,获得该钻孔沉积记录的RPI和磁倾角变化信息.岩石磁学及扫描电镜结果表明沉积物的原生主要载磁矿物为单畴(SD)和假单畴(PSD)钛磁铁矿.在一些层位,沉积后发生的还原作用形成胶黄铁矿等铁硫化物.磁性矿物的浓度和粒度等参数呈现显著的两阶段变化特征,上部(约220 cm以上)含量低而粒度细,下部含量高但粒度相对较粗,整体上均在一个数量级范围内波动.样品的交变退磁特征显示当交变场超过60 mT时,受胶黄铁矿影响产生旋转磁化,剩磁强度反而上升.因此,在20～60 mT之间确定原生特征剩磁,并把RPI定义为NRM_((20-40)mT)/ARM_((20-40)mT).通过~(14)C限定,并与其他有良好年代控制的相对磁场强度曲线进行对比,建立了钻孔的年代框架.结果显示,钻孔沉积物记录了几次显著的地磁漂移事件,这为联合应用RPI与磁场方向异常构建海相沉积物年代学框架提供了新的依据.  相似文献   

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