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1.
The magnetic properties of samples of the Olivenza chondrite (LL5) obtained from four collections have been investigated. The natural remanent magnetization (NRM) consists of a very stable primary component, which is randomly scattered in direction on a scale of 1 mm3 or less within the samples, and a secondary magnetization widely varying in intensity, and probably also in direction. The origin of the secondary NRM is not clear, and may be of terrestrial origin. It is concluded that the NRM is carried by the ordered nickel-iron mineral, tetrataenite. The origin of the primary NRM could be a magnetic field associated with the solar nebula, out of which the metal grains condensed and acquired a thermo-remanent magnetization (TRM), or Olivenza could be a fine-grained breccia, the constituent fragments possessing randomly directed magnetization. The implications for the origin and evolution of Olivenza and its parent body if the former magnetizing process has occurred are discussed.  相似文献   

2.
Paleomagnetic investigations of marine and subaqueous deposits in the 12-meter marine terrace of a section of the Karadzha Range (Azerbaijan) are performed. These deposits correspond to OI stage 3 and encompass the time interval ～45–20 ka. Four anomalous deviations of the magnetization from the dipole field at the sampling site are recorded in the upper and lower transgressive members of deposits. Investigations of the influence of the anisotropy of the magnetic susceptibility (AMS) on directions of the natural remanent magnetization (NRM) showed that only three of the four identified intervals can actually reflect geomagnetic field changes. The fourth interval of the anomalous NRM behavior is recorded in samples demonstrating the presence of the identified AMS direction pointing to a possible deformation of layers, which could turn the NRM vector toward the direction of the acting factor. Based on the age of the terrace under investigation, three other anomalous horizons could correspond to heavily reduced records of the Mono and Lashamp excursions of the geomagnetic field.  相似文献   

3.
We present results of paleomagnetic and sedimentological studies carried out on three cores Lmor1, Lmo98-1, Lmor98-2 from bottom sediments of Lake Moreno (south-western Argentina), and integrate them with data from our previous studies. Measurements of directions (declination D and inclination I) and mass specific intensity of natural remanent magnetization (NRM intensity), magnetic susceptibility (specific, χ and volumetric, κ), isothermal remanent magnetization (IRM), saturation of isothermal remanent magnetization (SIRM), and back field remanent coercivity (B0CR) were performed. The stability of the NRM was investigated using alternating-field demagnetization. The results show that these sediments meet the criteria required to construct a reliable paleomagnetic record. The cores were correlated very well based on magnetic parameters, such as χ and NRM intensity, as well as with lithological features. Tephra layers were identified from the lithological profiles and magnetic susceptibility logs. We obtained the D and I logs of the characteristic remanent magnetization for the cores as a function of shortened depth. The data from the three cores were combined to form a composite record using the Fisher method. A comparison between stacked inclination and declination records of Lake Moreno and those obtained in previous works on Lake Escondido and Lake El Trébol shows good agreement. This agreement made it possible to transform the stacked curves into time series spanning the interval 12–20 kyr. The results obtained improved our knowledge of SV and the behaviour of the geomagnetic field and also allowed us to determine the range of past inclination variations from −70° to −45° for the southern hemisphere, where data are scarce.  相似文献   

4.
A suite of Sierra Madera Impact deformed rocks was studied and magnetic analyses were performed. We characterized the magnetic signatures of two locations, sites A and B that have different physical characteristics of shock fractured structures as well as the magnetic signatures. Shatter cone at site A has a fine-scale (few to ∼10 mm) distributed array of complete shatter cones with sharp apex. Natural remanent magnetization (NRM) of site A shatter cone is distributed within the plane that is perpendicular to the apexes of the cones. Shatter cone at site B shows no apparent cone shape or apex, instead, a relatively larger scale and multiple striated joint set (MSJS) and sinusoidal continuous peak. NRM of site B shatter cone is clustered along the apexes. The difference in magnetization direction is a likely indicator of the shock pressure where parallel to apex indicates pressures larger than 10 GPa and perpendicular to apex indicate pressures less than 10 GPa. Intensities of NRM and saturation isothermal remanent magnetization (SIRM) contrast and fluctuate within a shatter cone as well as in between two sites. We observed a random orientation of magnetic vector directions and amplitudes changing over small scales leading to the absence of coherent macro-scale signature.  相似文献   

5.
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   

6.
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   

7.
Magnetic carriers in remagnetized Cretaceous granitic rocks of northeast Japan were studied using paleomagnetism, rock magnetism, optical microscopy and scanning electron microscopy (SEM) by comparison with unremagnetized granitic rocks. The natural remanent magnetization (NRM) of the remagnetized rocks is strong (0.3–1.7 A/m) and shows a northwesterly direction with moderate inclination (NW remanence), whereas the unremagnetized rocks preserve weak NRM (<0.5 A/m) with westerly and shallow direction (W remanence). Although thermal demagnetization shows that both NRMs are carried by magnetite, the remagnetized rocks reveal a higher coercivity with respect to alternating field demagnetization (20 mT相似文献   

8.
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   

9.
葛淑兰  L 《地球物理学报》2013,56(2):542-557
沉积物记录的地磁场强度首先提供了模拟地磁场演化的数据约束,其次提供了沉积物的年龄信息. 本文报道了菲律宾海西北部岩芯记录的地磁场相对强度,并结合岩石磁学和沉积学性质探讨了影响强度的各个因素. 除底部红粘土层的局部磁偏角偏转可能揭示了沉积后改造以外,磁化率各向异性和地磁场方向特征表明沉积物为原状沉积. 岩石磁学性质表明沉积物符合磁性均一性,可以记录可靠的地磁场强度. 由于红粘土层及其下部的磁偏角异常,本文讨论其上部约125 ka的结果. 常规归一方法获得的两个地磁场强度参数NRM/ARM(特征剩磁和非磁滞剩磁比值)和NRM/κ(特征剩磁和磁化率比值)与其它记录对比得到时间-深度对比点,对比点之间的年龄为线性内推或者外推. 地磁场强度时间模型上的岩芯氧同位素与全球氧同位素综合曲线一致证明强度结果的有效性和对比的正确性. 磁化率为归一参数的强度大多低于以非磁滞剩磁为归一参数的强度,频谱和相关分析证明NRM/ARM不与ARM和磁性矿物粒度(ARM/κ)相关,也没有轨道周期性,而NRM/κ却与κ和ARM/κ相关,而且有13~12 ka的周期. 由此我们认为NRM/ARM记录的地磁场强度比NRM/κ更好地消除了气候印记. 进一步探讨了超顺磁含量、碳酸钙含量、磁性矿物组成以及磁性矿物粒度变化与地磁场强度差值的关系,发现末次间冰期较高的超顺磁含量和磁性矿物粒度的较大范围变化造成了地磁场强度差值,后者至少造成了90%差异. 中等含量的碳酸钙和较小的磁性矿物组成变化不是磁场强度差值产生的原因. 如何校正磁性矿物粒度变化的影响将是下一步工作的重点.  相似文献   

10.
The natural remanent magnetization (NRM) in individual chondrules from the Allende meteorite was measured. These had previously been oriented relative to each other. The NRM directions of the chondrules are not initially random, but they become scattered after either alternating field (AF) or thermal demagnetization. The NRM is less stable than anhysteretic remanent magnetization (ARM) against AF-demagnetization.

The bulk of the NRM in the matrix is erased by 300°C. For the larger chondrules it is erased by 550°C, but for the smaller chondrules and the white inclusion a substantial decrease in NRM occurs by 350°C leaving about 20% up to 600°C. The behavior of the laboratory-induced ARM and the NRM under alternating field demagnetization suggest that the NRM of the chondrules consists of at least two components of TRM. One is a high-temperature component which was acquired when the individual chondrules were cooled through the Curie temperature and before they were assembled into the Allende meteorite. The other is a low-temperature component which was probably acquired in a field of about 1 Oe when the meteorite experienced thermal metamorphism or during the assembly of the meteorite.  相似文献   


11.
Summary The natural remanent magnetization (NRM) of basalts from Argentina and Iceland and dolerite from the Great Whin Sill exhibit reversed magnetization. In order to test whether this was due to a self-reversing property of the rocks, samples from these three suites have been examined byx-ray and thermomagnetic techniques. No correlation between the properties of rocks and the sense of the NRM was found. The thermal stability of the magnetic extracts from these rocks was related to their lattice parameters and Curie points.  相似文献   

12.
Thermal demagnetization of red and drab sandstones from the St. Bees Sandstone shows a wide range of directional stability. After the removal of a metastable secondary magnetization at temperatures up to 300°C red sandstones may show stable or unstable magnetization. Experiments indicate that both the stable and unstable magnetization is carried by coarse haematite particles (specularite).Drab sandstones, which have been subject to reduction and dissolution of haematite are generally unstable but specimens with a stable NRM occur and this must be carried by specularite because the pigment has been removed from these specimens.The stable magnetization is believed to have developed during deposition and early diagenesis by the oxidation of detrital iron oxides. Pole positions correspond to known Triassic poles and there are abundant normal and reversed zones typical of the Lower Triassic.The unstable magnetization of the red sandstones is apparently due to the development of authigenic overgrowths of haematite on the detrital specularites. This phase of authigenesis may have taken place over a long time, and after significant changes in the ambient geomagnetic field resulting in complex magnetizations in individual grains and hence whole rocks.  相似文献   

13.
The ferromagnetism of irons, stony-irons, E-, H-, L- and LL-chondrites and achondrites is due to a metallic phase comprising mostly Fe and Ni and small amounts of Co and P. The ferromagnetic constituent in non-metamorphosed C-chondrites is magnetite, but some metamorphosed C-chondrites contain FeNi metallic grains too.

Among the stony meteorites, the content of metals as determined by their saturation magnetization (IS) sharply decreases in the order E → H → L → LL → achondrites, whereas the IS value for magnetite and additional metals in C-chondrites ranges from the IS value of achondrites to that of L-chondrites.

With an increase of Ni-content in the metallic phase in chondrites of the order E → H → L → LL → C, the relative amount of Ni-poor kamacite magnetization, IS(), in the total IS decreases in the same order, from IS()/IS 1 for E-chondrites to IS()/IS 0 for C-chondrites. Thus, E-, H-, L-, LL- and C-chondrites and achondrites are well separated in a diagram of IS()/IS versus I, which could be called a magnetic classification diagram for stony meteorites.

As the surface skin layer of all meteorites is anomalously magnetized, it must be removed and the natural remanent magnetization (NRM) of the unaltered interior only must be examined for the paleomagnetic study. The NMR of C-chondrites is highly stable and that of achondrites is reasonably stable against AF-demagnetization, whereas the NMR of E-chondrites and ordinary chondrites as well as stony-iron meteorites is not very stable in most cases. Although the NRM of iron meteorites is reasonably stable, it is not attributable to the extraterrestrial magnetic field.

The paleointensity for Allende C3-chondrite is estimated to be about 1.0 Oe assuming that its NRM is of TRM origin. The paleointensity for other reasonably reliable C-chondrites (Orgueil, Mighei, Leoville and Karoonda) is also around 1 Oe.

The paleointensity for two achondrites has been determined to be about 0.1 Oe. The NRM of other achondrites also suggests that their paleointensity is roughly 0.1 Oe.

The NRM of ordinary chondrites is less stable than that of C-chondrites and achondrites so that the estimated paleointensity for ordinary chondrites is less reliable. The paleointensity for comparatively reliable ordinary chondrites ranges from 0.1 to 0.4 Oe.

The paleointensity values of 1 Oe for C-chondrites and 0.1 Oe for achondrites may represent the early solar nebula magnetic field about 4.5 × 109 years ago. A possibility that the paleomagnetic field for achondrites was a magnetic field attributable to a dynamo within a metallic core of their parent planet may also not be rejected.  相似文献   


14.
This study presents rock magnetic properties along with magnetic field measurements of different stratigraphic and lithologic basalt units from Reykjanes, the southwestern promontory of the Reykjanes peninsula, where the submarine Reykjanes Ridge passes over into the rift zone of southwestern Iceland. The basaltic fissure eruptions and shield lava of tholeiitic composition (less than 11500 a old) show a high natural remanent magnetization (NRM, Jr) up to 33.6 A/m and high Koenigsberger ratio (Q) up to 52.2 indicating a clear dominance of the NRM compared to the induced part of the magnetization. Pillow basalts and picritic shield lava show distinctly lower Jr values below 10 A/m. Magnetic susceptibility (κ) ranges for all lithologies from 2.5 to 26×10−3 SI.  相似文献   

15.
Deep-sea sediments, comprising small magnetic grains in coarse and fine fabrics, were reconstituted and deposited in laboratory fields. Both the magnitude and the direction of the natural remanent magnetization (NRM) were accurately reproduced. Only the middle coercivity fraction, however, gave a faithful representation of the laboratory field. This same fraction originally held the stable NRM component. The results were interpreted on a model of post-depositional realignment based on the physical characteristics of the sediment. Laboratory redeposition was found to be a closer analogue to the NRM than were anhysteretic or other laboratory-induced magnetizations. Guidelines are given by which the techniques could be utilised to estimate the intensity of the ancient geomagnetic field.  相似文献   

16.
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17.
Magnetic properties of samples from Bell Island sedimentary rocks have been studied. X-ray analysis indicates that the main magnetic mineral is hematite in all samples. The other iron-bearing minerals identified are siderite and chamosite. Microscope observations of thin sections suggest that the rocks consist of oolitic hematite in a matrix of siderite or calcite. The intensity of natural remanent magnetization (NRM) varies in the range of (0.03–0.4 A m?1), depending on the percentage of hematite. The thermal demagnetization curves of NRM show in some cases a sharp increase in magnetization at temperatures in the range 500–600°C. The peaks that occur in these demagnetization curves are due to a chemical change of siderite during repeated laboratory heating. X-ray analysis confirmed that the newly formed material is magnetite. Since the original NRM has been masked by the new intergrown material, this would result in a serious error in the determination of paleomagnetic pole positions. The samples showing this behaviour were not considered for paleomagnetic study. The samples containing oolitic hematite in a calcite matrix exhibit very high stability of NRM, including directional stability until almost 670°C. For these samples, a virtual pole position based on N = 6 samples (32 specimens) demagnetized to 665°C is 34°N, 114°E, not far from published Ordovician poles for the North American craton.  相似文献   

18.
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   


19.
To further evaluate the potential of magnetic anisotropy techniques for determining the origin of the natural remanent magnetization (NRM) in sedimentary rocks, several new remanence anisotropy measurement techniques were explored. An accurate separation of the remanence anisotropy of magnetite and hematite in the same sedimentary rock sample was the goal.In one technique, Tertiary red and grey sedimentary rock samples from the Orera section (Spain) were exposed to 13 T fields in 9 different orientations. In each orientation, alternating field (af) demagnetization was used to separate the magnetite and hematite contributions of the high field isothermal remanent magnetization (IRM). Tensor subtraction was used to calculate the magnetite and hematite anisotropy tensors. Geologically interpretable fabrics did not result, probably because of the presence of goethite which contributes to the IRM. In the second technique, also applied to samples from Orera, an anisotropy of anhysteretic remanence (AAR) was applied in af fields up to 240 mT to directly measure the fabric of the magnetite in the sample. IRMs applied in 2 T fields followed by 240 mT af demagnetization, and thermal demagnetization at 90°C to remove the goethite contribution, were used to independently measure the hematite fabric in the same samples. This approach gave geologically interpretable results with minimum principal axes perpendicular to bedding, suggesting that the hematite and magnetite grains in the Orera samples both carry a depositional remanent magnetization (DRM). In a third experiment, IRMs applied in 13 T fields were used to measure the magnetic fabric of samples from the Dome de Barrot area (France). These samples had been demonstrated to have hematite as their only magnetic mineral. The fabrics that resulted were geologically interpretable, showing a strong NW-SE horizontal lineation consistent with AMS fabrics measured in the same samples. These fabrics suggest that the rock's remanence may have been affected by strain and could have originated as a DRM or a CRM.Our work shows that it is important to account for the presence of goethite when using high field IRMs to measure the remanence anisotropy of hematite-bearing sedimentary rocks. It also shows that very high magnetic fields (>10 T) may be used to measure the magnetic fabric of sedimentary rocks with highly coercive magnetic minerals without complete demagnetization between each position, provided that the field magnetically saturates the rock.  相似文献   

20.
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