On the theory of depositional remanent magnetization in sedimentary rocks

The effects of various factors such as thermal agitation, coagulation, anisotropy of susceptibility, and shape irregularity on the alignment of magnetic carrier grains during the process of acquisition of depositional remanent magnetization in sedimentary rocks is discussed.     

Introductory notes on shock remanent magnetization and shock demagnetization of igneous rocks

Summary Effects of mechanical shocks of about 0.5 msec in duration on the remanent magnetization of igneous rocks are experimentally studied. The remanent magnetization acquired by applying a shock (S) in the presence of a magnetic field (H), which is symbolically expressed asJ _R (H₊S H_o), is very large compared with the ordinary isothermal remanent magnetization (IRM) acquired in the same magnetic field.J _R (H₊S H_o) is proportional to the piezo-remanent magnetization,J _R (H₊P₊P_o H_o).The effect of applyingS in advance of an acquisition of IRM is represented symbolically byJ _R (S H₊ H_o).J _R (S H₊ H_o) can become much larger than the ordinary IRM, and is proportional to the advance effect of pressure on IRM,J _R(P₊ P₀ H₊ H₀).The effect of shockS applied on IRM in non-magnetic space is represented by the shock-demagnetization effect,J _R(H₊ H₀ S), which also is proportional toJ _R(H₊ H₀ P₊ P₀).Because, the duration of a shock is very short, a single shock effect cannot achieve the final steady state. The effect ofn-time repeated shocks, is represented byJ ₀+J ^*(n), whereJ ₀ means the immediate effect and J ^*(n) represent the resultant effect of repeating, which is of mathematical expression proportional to [1–exp {–(n–1)}].

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Zusammenfassung Die Effekte des mechanischen Stosses mit der Dauer von etwa 0.5 ms auf der remanenten Magnetisierung wurden experimentell nachgesucht. Das erworbene Remanenz der Magnetisierung nach dem Stoss (S) unter dem magnetischen Feld (H), das hier symbolisch alsJ _R(H₊ SH₀) bezechnet wird, ist sehr stark im Vergleich mit der normalen isothermischen remanenten Magnetisierung (IRM) unter demselben magnetischen Feld.J _R(H₊ S H₀) ist im Verhältnis zur piezoremanenten Magnetisierung,J _R(H₊ P₊ P₀ H₀).Der Effekt vom Stoss vor der Erwerbung von IRM wird symbolisch alsJ _R(S H₊ H₀) bezeichnet.J _R(S H₊ H₀) kann viel stärker als die normale IRM werden, im verhältnis zum Effekt des vorausgegebenen Drucks auf IRMJ _R(P₊ P₀ H₊ H₀).Der Effekt des Stosses auf IRM im Raum ohne magnetisches Feld wird mit dem Stossentmagnetisierungseffekt dargestellt,J _R(H₊ H₀ S), der auch proportional zuJ _R(H₊ H₀ P₊ P₀) ist.Da die Dauer einzelnen Stosses sehr kurz ist, kann der Effekt des einmaligen Stosses den endgültigen stabilen Zustand nicht erreichen. Der Effekt nachn-maligen wiederholten Stossen wird alsJ ₀+J ^*(n) bezeichnet, wobeiJ ₀ den unverzüglichen Effekt bedeutet, und J ^*(n) beschreibt den resultanten Effekt der Stosswiederholung, dessen mathematische Darstellung proporational zu [1–exp {–(n–1)}] ist.

     

Flocculation mechanism of the acquisition of remanent magnetization by sedimentary rocks

Coagulation of particles into aggregates during their deposition in a reservoir is numerically simulated with regard for Brownian motion, Van der Waals forces, gravitation, Stokes friction, and magnetostatic interaction, and the effect of this process on the depositional magnetization (DRM) is estimated. Clusters obtained due to random aggregation of smaller clusters have a loose and branching structure. The average fractal dimension of the clusters is d = 1.83 ± 0.23. In the process of coagulation, magnetic particles do not form chains or clusters, as was supposed in a number of preceding works, but become rather uniformly distributed among nonmagnetic particles, which provides an additional argument in favor of the fact that chains of magnetite particles in marine sediments are of biogenic origin rather than a result of mutual attraction of magnetic particles due to magnetostatic interaction. The deposition process is shown to obey a kind of the principle of scale invariance: the number of clusters and their average number of particles do not change if the basin depth H and the surface concentration of initial material c ₀ simultaneously change (provided that temperature and the initial particle size r are constant) in such a way that Hc ₀ = const. Coagulation is the most important factor forming the bottom layer structure and the magnetization of the suspension at a relatively high concentration c ₀ Typical of redeposition conditions, lakes, and shelf seas. Coagulation virtually does not influence oceanic sediments because of the smallness of c ₀.     

Analysis of the natural remanent magnetization of rocks by measuring the efficiency ratio through alternating field demagnetization spectra

The REM(AF) method is a new tool for the analysis of the origin and alternating field demagnetization coercivity spectra of the remanent magnetization. We applied this method on precambrian Gila diabase sheets from Arizona in order to identify the high coercivity magnetic carrier, and on artificially shocked Rowley Regis basalt from UK in order to analyze the effect of the shock on the natural remanent magnetization. In the Gila diabase the high coercivity magnetic component was identified to be most likely represented by the acicular magnetite (increase in the efficiency ratio in the high coercivity region). In the Rowley Regis basalt, the REM(AF) analysis revealed that comparing to NRM, the shock produced a different distribution of the AF demagnetization coercivity spectra due to the occurrence of the Shock Remanent Magnetization.     

On the theory of depositional remanent magnetization in sediments

Summary A more precise solution of equation, describing process of orientation of isometric ferromagnetic particles at their deposition in liquid under influence of external homogeneous magnetic field is presented. It is shown, that this moreprecise investigation of the problem doesn't lead to qualitative new results for describing processes of particles orientation, than that given byT. Nagata.     

On the use of anhysteretic remanent magnetization in paleointensity determination

The relative intensities of anhysteretic remanent magnetization (RAM) and thermoramanent magnetization (TRM) are strongly dependent on grain size, blocking temperature and applied field, and are poorly predicted by existing theories. Analog techniques that substitute ARM for TRM probably yield adequate relative paleointensities in suites of mineralogically similar rocks, but they yield uncertain estimates of absolute paleointensity.     

Changes of stability of remanent magnetization with the concentration of vacancies in titanomagnetites

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On the abnormally high natural remanent magnetization of certain lavas

Summary The rock specimens found to have natural remanent magnetization of abnormally high intensity, have been generally from hill sides or tops, or from ridges high up from the local surroundings. A field of several hundred oersteds has been found sufficient to produce in some of the artificially demagnetized specimens an isothermal magnetization of the same order as the abnormal natural ones, and this magnetization has shown a similar degree of stability as the natural one. Variations in the direction and magnitude of the natural magnetic vector have been found over a distance of a few centimetres. Laboratory tests indicate a normal chemical composition for the specimens. The lightning discharge seems to be the plausible cause of abnormally high magnetization of rocks, which is generally isothermal and might have originated in the recent past, but the magnetization is sometimes complicated probably by the thermal effect of the discharge.     

Measurement of the effect of particle size variation on the detrital remanent magnetization to anhysteretic remanent magnetization ratio in some abyssal sediments

Deposition experiments have been carried out to measure the effect of particle size variation on the relationship between detrital remanent magnetization (DRM), anhysteretic remanent magnetization (ARM), and geomagnetic field intensity in sediments.Foraminiferal ooze from a box core taken in the Columbian Basin south of Jamaica was separated into several particle size ranges and redeposited in the laboratory in known magnetic fields. The intensity and alternating field (AF) demagnetization characteristics of the DRM and a subsequently applied ARM were compared for the various particle size ranges.The results show a variation of DRM/ARM ratios with particle size. The DRM intensities and directions indicate that particle sizes greater than 38 μm do not contribute significantly to the DRM of the total sediment. ARM intensities for larger particle sizes and particle size analysis of the whole sediment indicate that the fraction greater than 38 μm does make a significant contribution to the total ARM of a sample. Use of the DRM/ARM ratio in experimental measurements of magnetic paleointensity indicates that the method is unsatisfactory for sediments having a significant fraction of magnetic particles larger than 38 μm. It is also shown that, for sediments having a significant fraction of high-coercivity magnetic grains, the relative orientation of the ARM and DRM will affect ARM intensities, making necessary the use of corrected ARM and DRM intensities for ratio calculations.     

Possibilities of remote estimation of the natural remanent magnetization of rocks in the zones of geomagnetic field reversal

The possibilities of the remote estimation of the distribution character of the rock natural remanent magnetization in the zones of geomagnetic field reversal are considered. The Faraday effect—rotation of the signal polarization plane in the magnetic field—is used for this purpose in the process of radar impulse sounding of the magnetic field.     

Determination of the paleomagnetic field intensity by the dispersion of the remanent magnetization vector of the sediment rocks

Summary It is shown that the intensity of the initial paleomagnetic field can be determined by the empirical distribution of the vector of the sediment rock magnetization.     

Application of A. C. demagnetization to investigate the stability of rock magnetization caused by lightning currents

Summary An apparatus is described which enables progressive demagnetization of rock specimens under alternating fields in order to remove the unstable components of magnetization while retaining a measurable fraction of the stable component.The apparatus is utilized to study the stability of magnetization created in rock specimens by artificial lightning currents. It is shown that the remanent magnetization due to strong lightning currents could be fairly stable and in certain cases peak a. c. fields of intensity as high as 1700 Oe may fail to destroy completely the effect of this hard component.Possible means for avoiding the magnetic noise effect of lightning on paleomagnetic investigations are discussed.

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Zusammenfassung Es wird eine Apparatur beschrieben die es ermöglicht, Gesteinsproben mit Hilfe von Wechselfeldern schrittweise zu Entmagnetisieren. Dabei werden die unstabilen Komponenten der Magnetisierung entfernt, während ein Teil der stabilen Magnetisierung erhalten bleibt.Die Apparatur wurde verwendet, um die Stabilität der in den Gesteinsproben durch künstliche Beblitzung erzeugten Magnetisierung zu untersuchen. Es wird gezeigt, dass die durch starke Blitze erzeugte remanente Magnetisierung recht stabil sein kann und dass es in gewissen Fällen selbst Wechselfeldern mit Feldstärken bis zu 1700 Oe nicht gelingt, diese harte Magnetisierung zum Verschwinden zu bringen.Möglichkeiten, um den störenden Einfluss der Blitze bei päleomagnetischen Untersuchungen zu vermeiden, werden besprochen.

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Part II of the dissertation Theoretical study of the magnetic attraction due to rock bodies and experimental investigation of the stability of rock magnetism submitted to the Swiss Federal Institute of Technology (ETH), Zurch, for the degree of Doctor of Natural Sciences.     

On the role of Brownian motion in the control of detrital remanent magnetization of sediments

Summary The field dependence of magnetic remanence in laboratory deposited sediments is re-examined in terms of the average orientation of small grains of magnetite subject to the combined effect of an aligning field and thermal agitation. The classical (Langevin) formula for paramagnetic susceptibility is generalized to describe an assembly of grains with magnetic moments having a uniform distribution of values between zero and an upper limit, yielding a simple analytical expression in excellent accord with the pioneering measurements of Johnson, Murphy and Torreson. Comparison of theory and observation shows that the grain moments are intermediate between the values expected for single domains and multidomains, confirming the existence of pseudo-single domain effects, as deduced from observations of thermoremanence.     

鲁科一井上白垩统沉积岩岩石磁学和天然剩磁分析:兼论利用剩磁方向恢复钻孔岩芯原始方位的可行性

本文对"鲁科一井"（CCSD-LK-Ⅰ）768.9~1112.3 m之间的上白垩统沉积岩样品进行了岩石磁学、磁化率各向异性（AMS）以及天然剩磁组分的研究.在此基础上,分析了利用特征剩磁（ChRM）和黏滞剩磁（VRM）方向恢复岩芯原始方位的可行性.三轴等温剩磁热退磁曲线、磁滞回线、反向场退磁曲线、一阶反转曲线等岩石磁学测量结果表明,沉积岩的主要载磁矿物为磁铁矿和赤铁矿.335块样品的AMS测量结果表明磁化率椭球主轴的最大轴K1和中间轴K2与水平面夹角较小,最小轴K3接近垂直于水平面分布,说明沉积岩保留了原始沉积磁组构特征.系统热退磁实验表明,多数样品在25~350℃和500~690℃温度段分别获得VRM和ChRM分量.利用ChRM偏角方向,并考虑构造旋转量校正,对VRM偏角方向进行恢复,Fisher统计得到DVRM=-1.3°,IVRM=59.6°,与当地现代地磁场方向（D=-6.7°,I=53.9°）基本一致.用ChRM偏角方向对磁化率主轴K1偏角方向进行校正,校正的结果为：Dch_K1=349.2°,Ich_K1=-0.7°.本文研究结果对于地质勘探中利用古地磁学方法恢复钻孔岩芯原始方位具有一定参考意义.     

On the nature of cosmic dust in sedimentary rocks

In the paper, the chemical composition and origin of cosmic dust, as well as its distribution in the geological past, are discussed. Special attention is given to the problem of the difference in cosmic dust from volcanic dust on the basis of indicators such as cosmogenic helium and iridium.     

Chemical remanent magnetization due to deep-burial diagenesis in oolitic hematite-bearing ironstones of Alabama

Oolitic hematite-bearing ironstones of the Silurian Red Mountain Formation of Alabama are shown to carry a single-component remanence stable enough to have survived major folding (of probable Permian age). Nevertheless, the remanence direction (ten sites yielding a paleopole at 38.0°N, 132.4°E with dm = 3.6°, dp = 1.9°), its reverse polarity and a negative intraformational conglomerate test show that the remanence was very likely acquired during the Pennsylvanian—some 130 Ma after deposition. This remanence is likely a chemical remanent magnetization (CRM) acquired during diagenesis induced by heating due to deep burial under a Pennsylvanian clastic wedge. Two possible mechanisms for acquisition of CRM during deep-burial diagenesis are considered. In hypothesis I, the oolitic hematite transformed from original geothite when heated to about 80°C, acquiring CRM. In hypothesis II, the oolitic hematite originated from ferrihydrite and was too fine-grained to acquire stable CRM until heat raised the solubility of hematite allowing grain growth. Hypothesis I explains the timing of remanence acquisition better, but there is some evidence that oolitic goethites may be stable to considerably more than 80°C. Hypothesis II has some difficulty explaining preliminary paleomagnetic results from oolitic hematite-bearing ironstones of the Silurian Clinton Group, New York State. We prefer hypothesis I but both hypotheses remain plausible. Both hypotheses warn that continental red beds may also acquire CRM during diagenesis induced by deep-burial heating, long after deposition but before folding.     

On the theory of depositional remanent magnetization in sediments: Influence of fluid motion upon the magnetic orientation of sediments

Summary It has examined the influence of fluid laminar motion upon the magnetic orientation of ferromagnetic particles with different form: sphere, rod-like, disk-like, ellipsoid. It is shown, that the inclination error depends on the form and the magnitude of particles, on the value of their remanent magnetization and on the size of vertical velocity gradient of the fluid.     

On the theory of depositional remanent magnetization in sediments: Influence of brownian motion of rotation upon the magnetic orientation of sediments

Summary It has examined theoretically the question of the influence of Brownian motion of rotation upon the orientation of small ferromagnetic particles, settling in a liquid in the presence of the Earth's magnetic field. It has given a criterion of this influence depending on the particle's size and their magnetic momert.     

河西走廊早新生代红层次生剩磁及其对原生剩磁影响评价

系统研究了河西走廊火烧沟组陆相红层200个采点岩石的热退磁行为和17个代表性样品的岩石磁学特征,结果表明有102个采点可以分离出A、B、C三个剩磁分量,有82个采点只有一个分量(C分量),16个采点只能分离出A、B分量.A、B分量分别由针铁矿和磁赤铁矿携带,C分量在砂岩中由磁铁矿携带,在泥质砂岩和泥岩中由磁铁矿和赤铁矿共同携带.A、B分量剩磁方向随机分布,不能获得置信水平的古地磁平均方向,为次生剩磁.它们的存在并不影响岩石原生剩磁的分离,也不影响原生剩磁信号的稳定性和获得的古地磁数据的可靠性.16个砂岩采点中不能分离出C分量,是由于后生的赤铁矿彻底改变了由磁铁矿携带的原生剩磁组分,在高密度采样的情况下剔除这些采点并不影响古地磁极性柱的构建和解释.