Remanent Magnetization (remanent + magnetization)

Distribution by Scientific Domains

Kinds of Remanent Magnetization

  • natural remanent magnetization

  • Selected Abstracts

    The influence of Pb-ion irradiation on melt-textured YBa2Cu3Ox crystals

    I. M. Obaidat
    Abstract Melt-textured YBa2Cu3Ox crystals have been irradiated along the c -axis with 208Pb56+ ions corresponding to dose matching fields, B, = 0.5 T and B, = 2.0 T. Magnetization measurements were conducted along the ab plane of the samples. The strength of pinning sites was investigated by measuring magnetization hysterisis and the saturation remanent magnetization MR at several temperatures. We have found that the pinning strength was considerably enhanced after irradiation at both doses. Interestingly, the pinning strength at a Pb-ion irradiation which corresponds to the dose matching field B, = 0.5 T, was found to be significantly larger than that at the dose matching field B, = 2.0 T at all temperatures. ( 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Installation age of limestone masonry determined from its viscous remagnetization

    Graham John Borradaile
    Many rocks passively acquire some time-dependent or "viscous" remanent magnetism (VRM) at ambient temperatures, without any extraordinary energetic intervention. This magnetization overprints existing remanent magnetization so that it is effectively a remagnetization subparallel to the contemporary geomagnetic field, averaging the geomagnetic field orientation. Certain limestone masonry remagnetizes viscously over an archaeologically useful interval (100 to 8000 Ka) so that the degree of remagnetization is monotonically (but not linearly) related to the construction age. The laboratory unblocking temperature (TUB) that removes the viscous magnetization is a simple monotonic measure of relative age. The longer a piece of masonry remained stabilized in a certain orientation, the greater is its viscous remagnetization and the higher is its TUB. Monuments of known age with a similar limestone source permit us to establish a calibration curve of T UB against historical ages. The resulting calibration curve may then be used to predict the ages of otherwise-undated masonry. Viscous remanent magnetism dating provides precision of <50a in medieval monuments in England and <150a precision for classical to Neolithic monuments in Cyprus; precision depends on the remagnetization rate of the limestone in question. Our calibration curves, for the Jurassic Oolitic Limestone of England and for the Lefkara-Pakhna Chalks of Cyprus, allowed us to investigate the authenticity of a medieval English synagogue in Lincoln, England, and of a medieval house in Cyprus. Multiple archaeologic VRMs show that masonry was recycled in historical times. 2006 Wiley Periodicals, Inc. [source]

    Lightning strikes and prehistoric ovens: Determining the source of magnetic anomalies using techniques of environmental magnetism

    David Maki
    Techniques of environmental magnetism were used to examine soil samples from a North American archaeological site in an effort to determine the source of magnetic field gradient anomalies. Testing revealed the source of one anomaly to be lightning-induced remanent magnetization (LIRM). This anomaly had initially been identified as a possible archaeological feature, but excavations were unable to identify a visible source. LIRM appears to be a relatively common source of anomalous signal on archaeological sites and may often be misinterpreted in magnetic imagery. Thermoremanent magnetization (TRM) was also documented and quantified in soil from archaeological hearths at the site, as were changes in the ferrimagnetic mineral concentration and coercivity spectra resulting from high-temperature enhancement within the hearths. 2005 Wiley Periodicals, Inc. [source]

    Tectonic deformation of the Indochina Peninsula recorded in the Mesozoic palaeomagnetic results

    Kazuhiro Takemoto
    SUMMARY In order to describe features of tectonic deformation in the Indochina Peninsula, Early Jurassic to Early Cretaceous red sandstones were sampled at three localities in the Shan-Thai and Indochina blocks. Stepwise thermal treatment of most samples revealed the presence of characteristic remanent magnetization, which is generally unblocked by 680 C. This component from Phong Saly (21.6N, 101.9E) and Borikhanxay (18.5N, 103.8E) localities yield positive fold tests with Late Jurassic,Early Cretaceous directions of Dec/Inc = 28.8/32.1 (ks= 15.4, ,95= 8.8, N= 22) and Dec/Inc = 42.1/46.9 (ks= 20.1, ,95= 7.9, N= 18), respectively. Additionally, a syn-folding mid-Cretaceous characteristic magnetization is observed in the samples of Muang Phin locality (16.5N, 106.1E), which gave a mean direction of Dec/Inc = 30.8/39.9, k= 102.6, ,95= 3.0, N= 23. This reliable Late Jurassic to Mid-Cretaceous palaeomagnetic directions from three different localities are incorporated into a palaeomagnetic database for Shan-Thai and Indochina blocks. Based on these compilations, tectonic deformation of the Shan-Thai and Indochina blocks is summarized as follows: (1) the Shan-Thai and Indochina blocks experienced a clockwise rotation of about 10 as a composite unit in the early stage of India,Asia collision and (2) following this, the Shan-Thai Block underwent an internal tectonic deformation, whereas the Indochina Block behaved as a rigid tectonic unit during the same period. Comparison of our palaeomagnetic results with seismic tomographic images suggests that the strength of continental lithosphere beneath these blocks played an important role in the process of deformation rather than any other tectonic regime. In contrast to the Shan-Thai Block, an existence of continental roots beneath the Indochina Block prevented its internal deformation. [source]

    The Tortonian reference section at Monte dei Corvi (Italy): evidence for early remanence acquisition in greigite-bearing sediments

    S. K. Hsing
    SUMMARY The reliability of primary natural remanent magnetization (NRM) signals in greigite-bearing sediments has been frequently questioned. Here, we show that the stable NRM in the deep marine Middle to Late Miocene sediments at Monte dei Corvi, northern Italy, is mainly carried by greigite. Combined rock magnetic experiments and scanning electron microscopy successfully enabled discrimination between two greigite populations. One fine-grained and relatively well-dispersed greigite population (grain size between 60 and 200 nm) is most likely of magnetotactic origin. The second greigite population with larger grain sizes (typically 700 nm to 1 ,m) is most likely of authigenic (bacterially mediated) origin, and is related to post-depositional sulphidization processes. Greigite is the main magnetic remanence carrier in the older part of the section (12.8 to 8.7 Ma), whereas greigite and fine-grained (presumably magnetotactic) magnetite are present in the younger part of the section (8.7 to 6.9 Ma). Similar remanent magnetization directions of the magnetite and greigite components, and the likelihood of a magnetotactic origin, suggests that the NRM is of syn-depositional age. We suggest that moderate methane seepage from the underlying sediments may have occurred that resulted in the preservation of pristine greigite. This corroborates the reliability of the previously established magnetostratigraphy at Monte dei Corvi. [source]

    Measuring remanence anisotropy of hematite in red beds: anisotropy of high-field isothermal remanence magnetization (hf-AIR)

    Dario Bilardello
    SUMMARY The potential of using high-field anisotropy of isothermal remanence magnetization (hf-AIR) measurements for determining the origin of natural remanent magnetization in red beds and for identifying and correcting possible red-bed inclination shallowing was investigated for specimens of the Carboniferous Shepody Formation of New Brunswick and Nova Scotia, Canada. The technique makes it possible for a typical paleomagnetic laboratory to measure the remanence anisotropy of high-coercivity hematite. High-field (hf) AIR was used in conjunction with 100 mT alternating field (af) and 120 C thermal demagnetization to separate the contribution of hematite to the remanence anisotropy from that of magnetite/maghemite and goethite, respectively. A 5-T impulse DC magnetic field was used for the hf-AIR to reset the magnetic moment of high-coercivity hematite so that demagnetization between AIR orientations was not necessary. The ability of a 5-T field to reset the magnetization was tested by generating an isothermal remanent magnetization acquisition curve for hematite by using impulse DC magnetic fields up to 5 T in one orientation and followed by applying a field in the opposite direction at each step. Each field application was treated by 120 C heating and 100 mT af demagnetization before measurement. At 5 T, the difference between the magnetizations applied in opposite directions disappeared indicating that no magnetic memory persisted at this field strength. We performed a validity and reproducibility test of our hf-AIR measurement technique by measuring three specimens multiple times along two orthogonal coordinate systems. The method yielded highly reproducible results and, on rotating the specimen's coordinates, the fabric rotated by 90 as expected, showing that it is not an artifact of the technique. We also measured hf-AIR on samples that had previously been chemically demagnetized in 3N HCl to remove the secondary, chemically grown pigmentary hematite. The hf-AIR fabric of leached samples is similar to that of untreated samples, but shows a better-defined magnetic lineation and imbrication. We interpret the fabric observed for the Shepody Formation to be a compactional fabric that has been reoriented by strain during folding following a flexural-slip model. [source]

    Palaeointensity and palaeodirectional studies of early Riphaean dyke complexes in the Lake Ladoga region (Northwestern Russia)

    V. V. Shcherbakova
    SUMMARY Results of palaeointensity and palaeomagnetic studies for the volcanic rocks of 1450 Ma, from Early Riphaean Baltic shield dyke complex sampled in Lake Ladoga region (Karelia, Northwestern Russia) are reported. Electron microscope observations, thermomagnetic and hysteresis measurements indicate the presence of single domain (SD) to pseudo-single domain (PSD) titanomagnetite (TM) with low Ti content as the main magnetic mineral. Stepwise alternating field (AF) and/or thermal demagnetization revealed a two-component natural remanent magnetization (NRM) for most of the samples. The characteristic remanent magnetization (ChRM) component was isolated between 440 and 590 C. Note that the ChRM amounts to 95 per cent of the NRM intensity. The geographic position of the ChRM palaeopoles does not contradict the ,key poles' of the [1270; 1580] Myr time interval, testifying anticlockwise rotation of whole East Europe Craton between 1450 and 1500 Ma. Palaeointensity determinations were performed by Coe-modified Thellier procedure. 35 samples passed our palaeointensity selection criteria and show large linear segments on Arai-Nagata plots. The site mean virtual dipole moment (VDM) varies from 2.00 to 3.91 ( 1022 A m2). Based on these and other observations, we suggest that the Palaeo- and MezoProterozoic was dominated by low VDMs. [source]

    Tectonic deformation around the eastern Himalayan syntaxis: constraints from the Cretaceous palaeomagnetic data of the Shan-Thai Block

    Kenji Tanaka
    SUMMARY Lower to Middle Cretaceous red sandstones were sampled at four localities in the Lanpin-Simao fold belt of the Shan-Thai Block to describe its regional deformational features. Most of the samples revealed a characteristic remanent magnetization with unblocking temperatures around 680 C. Primary natures of magnetization are ascertained through positive fold test. A tilt-corrected formation-mean direction for the Jingdong (24.5N, 100.8E) locality, which is located at a distance of 25 km from the Ailaoshan,Red River Fault, revealed northerly declination with steep inclination (Dec./Inc. = 8.3/48.8, ,95= 7.7, N= 13). However, mean directions obtained from the Zhengyuan (24.0N, 101.1E), West Zhengyuan (24.0N, 101.1E) and South Mengla (21.4N, 101.6E) localities indicate an easterly deflection in declination; such as Dec./Inc. = 61.8/46.1, ,95= 8.1 (N= 7), Dec./Inc. = 324.2/,49.4, ,95= 6.4 (N= 4) and Dec./Inc. = 51.2/46.4, ,95= 5.6 (N= 13), respectively. The palaeomagnetic directions obtained from these four localities are incorporated into a palaeomagnetic database for the Shan-Thai Block. When combined with geological, geochronological and GPS data, the processes of deformation in the Shan-Thai Block is described as follows: Subsequent to its rigid block clockwise rotation of about 20 in the early stage of India,Asia collision, the Shan-Thai Block experienced a coherent but southward displacement along the Red River Fault prior to 32 Ma. This block was then subjected to a north,south compressive stresses during the 32,27 Ma period, which played a key role in shaping the structure of Chongshan-Lancang-Chiang Mai Belt. Following this some local clockwise rotational motion has occurred during the Pliocene-Quaternary time in central part of the Shan-Thai Block as a result of internal block movements along the reactivated network of faults. [source]

    Magnetic investigation of heavy metals contamination in urban topsoils around the East Lake, Wuhan, China

    Tao Yang
    SUMMARY Magnetic measurements and heavy metal analyses were performed on 133 samples from the urban soils around the East Lake in Wuhan, China. Samples were collected from four areas with different environmental settings: a heavy industrial area well known for thermal power generation and steel works; villages located in the downwind area of the industrial area; a main road with heavy traffic and roads around the East Lake. Results show that concentrations of magnetic particle and heavy metals in urban topsoils are significantly elevated due to the input of coarser-grained magnetite from industrial (e.g. power generation and steel production) and other anthropogenic activities (e.g. vehicle emissions). Concentration-related magnetic parameters, for example, magnetic susceptibility, saturation isothermal remanent magnetization and anhysteretic remanent magnetization, significantly correlate with the concentration of heavy metals. Moreover, in terms of grain sizes, the magnetic particles of different origins can be efficiently discriminated at the studied region. Therefore, magnetic measurements provide a basis for discrimination and identification of different contamination sources, and can be used as an economic alternative to chemical analysis when mapping heavy metal contamination in urban soil around the East Lake region, Wuhan, China. [source]

    Magnetic quantification of urban pollution sources in atmospheric particulate matter

    S. Spassov
    SUMMARY A new method is presented for fast quantification of urban pollution sources in atmospheric particulate matter (PM). The remanent magnetization of PM samples collected in Switzerland at sites with different exposures to pollution sources is analysed. The coercivity distribution of each sample is calculated from detailed demagnetization curves of anhysteretic remanent magnetization (ARM) and is modelled using a linear combination of appropriate functions which represent the contribution of different sources of magnetic minerals to the total magnetization. Two magnetic components, C1 and C2, are identified in all samples. The low-coercivity component C1 predominates in less polluted sites, whereas the concentration of the higher-coercivity component C2 is large in urban areas. The same sites were monitored independently by Hglin using detailed chemical analysis and a quantitative source attribution of the PM. His results are compared with the magnetic component analysis. The absolute and relative magnetic contributions of component C2 correlate very well with absolute and relative mass contributions of exhaust emissions, respectively. Traffic is the most important PM pollution source in Switzerland: it includes exhaust emissions and abrasion products released by vehicle brakes. Component C2 and traffic-related PM sources correlate well, which is encouraging for the implementation of non-destructive magnetic methods as an economic alternative to chemical analysis when mapping urban dust pollution. [source]

    A lock-in model for the complex Matuyama-Brunhes boundary record of the loess/palaeosol sequence at Lingtai (Central Chinese Loess Plateau)

    S. Spassov
    SUMMARY In most marine sedimentary records, the Matuyama-Brunhes boundary (MBB) has been found in interglacial oxygen isotope stage 19. In the magnetostratigraphic records of most Chinese loess/palaeosol profiles the MBB is located in loess layer L8, which was deposited during a glacial period. The MBB at Lingtai (central Chinese Loess Plateau) also occurs in L8 and is characterized by multiple polarity flips. The natural remanent magnetization is mainly carried by two coexisting components. The higher coercivity (harder) component dominates in loess layers and is thought to be of detrital origin. The lower coercivity (softer) component prevails in palaeosols and was most probably formed in situ by (bio-)chemical processes. A lock-in model for the Lingtai MBB record has been developed by extending the lithologically controlled PDRM model of Bleil & von Dobeneck (1999). It assumes two lock-in zones. The NRM of the magnetically harder component is physically locked by consolidation shortly after loess deposition, whereas the softer component is formed at greater depth by pedogenesis and acquires a chemical remanent magnetization of younger age. At polarity boundaries, grains carrying reversed and normal directions may therefore occur together within a single horizon. The model uses ARM coercivity spectra to estimate the relative contributions of the two components. It is able to explain the observed rapid multiple polarity flips and low magnetization intensities as well as the stratigraphic shift of the Lingtai MBB with respect to the marine records. [source]

    Palaeomagnetic study of the El Quemado complex and Marifil formation, Patagonian Jurassic igneous province, Argentina

    Maria P. Iglesia Llanos
    SUMMARY The upper Jurassic El Quemado Complex was sampled at 36 sites from five localities in the cordilleran foothills of southern Patagonia between Lago Argentino and Lago Posadas,Sierra Colorada, and the middle Jurassic Marifil Formation at 12 sites in the Somuncur Massif near Camarones. The main lithology was ignimbrite, with minor tuff and lava. Petrographical and SEM observation show that the El Quemado rocks suffered an intense, high-temperature alteration which resulted in transformation of most primary Ti-magnetite in pseudobrookite, rutile and minor Ti-haematite and Fe hydrated oxides. A similar, less pronounced alteration occurred in the Marifil rocks. 40Ar/39Ar dating of El Quemado was possible for one sample from Sierra Colorada and yielded an age of 156.5 1.9 Ma. Magnetic mineralogy measurements (isothermal remanence, hysteresis loop, Curie balance) show that the remanent magnetization is dominated by PSD low-Ti magnetite, often associated to a minor high-coercive mineral (haematite). Secondary magnetization components are usually absent or weak at El Quemado sites, strong at Marifil. They were completely erased by thermal and AF demagnetization and a characteristic remanence (ChRM) stable up to temperatures higher than 550C or peak-field values of 100 mT was successfully isolated. The virtual geomagnetic pole (VGP) from the Marifil Formation (83S, 138E) is in agreement with the literature data for Jurassic rocks from stable South America. The El Quemado VGPs fall in two groups. The localities to the north of latitude 48S (Lago Posadas, Sierra Colorada) yield a VGP (81S, 172E) close to that of Marifil, whereas those south of latitude 49S (Lago San Martn, Lago Argentino) show a highly elongated VGP distribution consistent with counter-clockwise block-rotation about vertical axes. These rotations were likely caused by thrust sheets which were rotating counter-clockwise at the same time they were advancing towards the foreland. The amount of rotation varies according to the location of the sampling sites in the thrust and fold belt. [source]

    The reliability of emplacement temperature estimates using palaeomagnetic methods: a case study from Santorini, Greece

    Leon Bardot
    The mode of origin of volcaniclastic deposits can be difficult to determine from field constraints, and the palaeomagnetic technique of emplacement temperature (Te) determination provides a powerful discriminatory test for primary volcanic origin. This technique requires that the low-blocking-temperature (Tb) component of remanence in the direction of the Earth's field in inherited lithic clasts is of thermal origin and was acquired during transport and cooling in a hot pyroclastic flow; otherwise, the Te determination may be inaccurate. If the low- Tb component is not of thermal origin it may be a viscous remanent magnetization (VRM) or a chemical remanent magnetization (CRM). The acquisition of a VRM depends on the duration of exposure to an applied magnetic field, and thus the laboratory unblocking temperature (Tub) of a VRM of a certain age imposes a minimum Te that can be determined for that deposit. Palaeointensity experiments were carried out to assess the magnetic origin (pTRM, CRM, or a combination of both) of the low- Tb component in a number of samples from pyroclastic deposits from Santorini, Greece. Seven of the 24 samples used in these experiments passed the stringent tests for reliable palaointensity determination. These values demonstrated, for six of the samples, that the low- Tb component was of thermal origin and therefore that the estimate of Te was valid. In the other 17 samples, valuable information was gained about the characteristics of the magnetic alteration that occurred during the palaeointensity experiments, allowing assessment of the reliability of Te estimates in these cases. These cases showed that if a CRM is present it has a direction parallel to the applied field, and not parallel to the direction of the parent grain. They also show that, even if a CRM is present, it does not necessarily affect the estimate of Te. Two samples used in these experiments displayed curvature between their two components of magnetization. Data from this study indicate that the curvature in these vector plots is probably due to the presence of MD grains and not CRM, although CRM is not ruled out as the cause of curvature in other samples with two vector components. [source]

    A precisely dated Proterozoic palaeomagnetic pole from the North China craton, and its relevance to palaeocontinental reconstruction

    Henry C. Halls
    A palaeomagnetic pole position, derived from a precisely dated primary remanence, with minimal uncertainties due to secular variation and structural correction, has been obtained for China's largest dyke swarm, which trends for about 1000 km in a NNW direction across the North China craton. Positive palaeomagnetic contact tests on two dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions. The age of one of these dykes, based on U,Pb dating of primary zircon, is 1769.1 2.5 Ma. The mean palaeomagnetic direction for 19 dykes, after structural correction, is D = 36, I = , 5, k = 63, ,95 = 4, yielding a palaeomagnetic pole at Plat=36N, Plong=247E, dp = 2, dm = 4 and a palaeolatitude of 2.6S. Comparison of this pole position with others of similar age from the Canadian Shield allows a continental reconstruction that is compatible with a more or less unchanged configuration of Laurentia, Siberia and the North China craton since about 1800 Ma [source]

    Magnetic field intensity study of the 1960 Kilauea lava flow, Hawaii, using the microwave palaeointensity technique

    Mimi J. Hill
    It is extremely valuable to study historic lava flows where the geomagnetic field at their time of extrusion is well known. In this study, two vertical sections, 16 m apart, have been sampled from the approximately 1 m thick 1960 Kilauea lava flow, Hawaii. Variations are seen in the rock-magnetic and palaeomagnetic properties between and within the two sections, indicating that there are small-scale lateral and vertical variations in the lava flow. The two sections showed different responses to microwave palaeointensity analysis. Section H6001 generally gave ideal linear behaviour on plots of natural remanent magnetization (NRM) lost against microwave-induced thermoremanent magnetization (TM,RM) gained, whilst the majority of samples from H6002 showed anomalous two-slope behaviour. When all plots were interpreted by taking the best-fitting line through all points, the flow mean intensity for H6001 was 31.6,,3.6 ,T and that for H6002 was 37.1,,6.4 ,T, compared with the expected intensity of 36 ,T. Additional historic flows need to be studied in order to ascertain whether this behaviour is typical of all lava, and whether it is best to always interpret NRM lost/TM,RM gained plots by taking the line of best fit regardless of shape. [source]

    Rock magnetism and paleomagnetic stratigraphy of forearc sediments of the Japan Trench, ODP Sites 1150 and 1151

    ISLAND ARC, Issue 1 2004
    Toshiya Kanamatsu
    Abstract Magnetic measurements were carried out to investigate rock magnetic properties and paleomagnetic directions of late and middle Miocene sediments recovered from the land side of the Japan Trench during the Ocean Drilling Program Leg 186. Because the low coercive component in natural remanent magnetization (NRM) normalized by anhysteretic remanent magnetization shows that the drilling-induced magnetization is severe in the sections obtained by the advanced hydraulic piston coring method, careful analyses of demagnetization of NRM using the ,demagnetization plane' were carried out to decompose the direction and intensity. Magnetostratigraphic correlation down to the upper Miocene, supplemented by biostratigraphic data, revealed that the sedimentation rates are characterized by drastic changes, with the early Pliocene having the highest rate. This high sedimentation rate is related to the subsidence of the southern deep-sea terrace of the Japan Trench. [source]

    Hardened foliated fault gouge from the Nojima Fault zone at Hirabayashi: Evidence for earthquake lightning accompanying the 1995 Kobe earthquake?

    ISLAND ARC, Issue 3-4 2001
    Yuji Enomoto
    Abstract Two anomalous features were found in the Nojima Fault zone at Hirabayashi in Awaji Island, south-west Japan: (i) hard foliated gouge between weathered granitic fault breccia and weakly consolidated mudstone of the Osaka Group; and (ii) mudstone near the gouge showing anomalous magnetization behavior. Roots of herbaceous vegetation near the foliated gouge were extraordinarily charred. In order to understand the nature of the gouge, shallow drillings were made to a depth of 3,14 m across the fault zone. Various physicochemical measurements of the gouge at depths and charred roots of herbaceous vegetation were conducted. The main results were: (i) Using electron spin resonance (ESR) analysis, the carbon radical peak (g = 2.006) of the charred roots was found to be 25 times larger than that of the non-charred roots of the same vegetation taken near the fault, indicating that the charred roots were subjected to baking; (ii) the hard foliated gouge clearly showed a lamellar structure consisting alternately of gray and black layers; (iii) the black layers in most of the foliated gouge showed flow structures almost parallel to the fault, but the gray layers rarely showed flow patterns; (iv) natural remanent magnetization (NRM) of the foliated gouge was 430 times greater than that of the granitic fault breccia and approximately 70 times greater than that of the mudstone; (v) the NRM intensity of the mudstone near the fault was highest near the ground level and decreased as the depth increased, although the magnetic susceptibility of the mudstone was almost constant and independent of depth; (vi) the high-coe civity magnetization component vectors of both the mudstone and the foliated gouge in a Schmidt equal-area projection was quite different from that of the present direction of the Earth's field; and (vii) using a magnetic force microscope, intense magnetic force lines were found in the black parts of the foliated gouge. It is suggested that these anomalies were possibly caused by earthquake lightning that accompanied the 1995 Kobe earthquake. In a spark plasma sintering test, which was conducted to simulate the possibility of earthquake lightning-induced sintering of the gouge, weakly altered gouge was successfully sintered within 10 s. The hardness of sintered sample was comparable to that of the hard foliated gouge. [source]

    Magnetic characterization of Cretaceous-Tertiary boundary sediments

    Vctor Villasante-Marcos
    Studied sections' locations vary in distance to the Chicxulub structure from distal (Agost and Caravaca, Spain), through closer (ODP Hole 1049A, Blake Nose, North Atlantic), to proximal (El Mimbral and La Lajilla, Mexico). A clear magnetic signature is associated with the fireball layer in the most distal sections, consisting of a sharp increase in susceptibility and saturation isothermal remanent magnetization (SIRM), and a decrease in remanence coercivity. Magnetic properties in these sections point to a distinctive ferrimagnetic phase, probably corresponding to the reported Mg- and Ni-rich, highly oxidized spinels of meteoritic origin. At closer and proximal sections magnetic properties are different. Although there is an increase in susceptibility and SIRM associated with a rusty layer placed on top of the siliciclastic deposit in proximal sections, and with a similar limonitic layer on top of the spherule bed that defines the boundary at Blake Nose, the magnetic properties indicate a mixture of iron oxyhydroxides dominated by fine-grained goethite. Based on previous geochemical studies at Blake Nose and new geochemical and PGE abundance measurements performed in this work at El Mimbral, this goethite-rich layer can be interpreted as an effect of diagenetic remobilization and precipitation of Fe. There is not enough evidence to assert that this Fe concentration layer at proximal sections is directly related to deposition of fine meteoritic material. Magnetic, geochemical, and iridium data reject it as a primary meteoritic phase. [source]

    Paleomagnetism and petrophysics of the Jnisjrvi impact structure, Russian Karelia

    J. Salminen
    The impactites (tagamites, suevites, and lithic breccias) are characterized by increased porosity and magnetization, which is in agreement with observations performed at other impact structures. Thermomagnetic, hysteresis, and scanning electron microscope (SEM) analysis document the presence of primary multidomain titanomagnetite with additional secondary titanomaghemite and ilmenohematite. The characteristic impact-related remanent magnetization (ChRM) direction (D = 101.5, I = 73.1, ,95 = 6.2) yields a pole (Lat. = 45.0N, Long. = 76.9E, dp = 9.9, dm = 11.0). Additionally, the same component is observed as an overprint on some rocks located in the vicinity of the structure, which provides proofs of its primary origin. An attempt was made to determine the ancient geomagnetic field intensity. Seven reliable results were obtained, yielding an ancient intensity of 68.7 7.6 ,T (corresponding to VDM of 10.3 1.1 times 1022 Am2). The intensity, however, appears to be biased toward high values mainly because of the concave shape of the Arai diagrams. The new paleomagnetic data and published isotopic ages for the structure are in disagreement. According to well-defined paleomagnetic data, two possible ages for magnetization of Jnisjrvi rocks exist: 1) Late Sveconorwegian age (900,850 Myr) or 2) Late Cambrian age (,500 Myr). However, published isotopic ages are 718 5 Myr (K-Ar) and 698 22 Myr (39Ar- 40Ar), but such isotopic dating methods are often ambiguous for the impactites. [source]

    Archaeogeophysical investigations around the Bilge Qagan monument in Khosho Tsaidam, Mongolia

    Abdullah Ates
    Abstract Ancient Turkish monuments in Mongolia are historically important owing to the earliest Turkish and Chinese inscriptions written on them. In this paper, magnetic and conductivity meter surveys conducted in the vicinity of the Bilge Qagan monument are presented. Magnetic and conductivity maps show rectangular shaped anomalies, which could be caused by a buried wall-like structure or floor tiles to protect the Bilge Qagan monument. The Altar Stone, which is also described as a base for a pagoda, gives a strong magnetic anomaly. This anomaly is processed by advanced geophysical methods and it is demonstrated that the actual rock body has some form of remanent magnetization, which causes the magnetic anomaly. By means of conductivity surveying, it also can be suggested that the near-surface formations have significant water content. Copyright 2002 John Wiley & Sons, Ltd. [source]