Secondary remanent magnetization carried by magnetite inclusions in silicates: a comparative study of unremagnetized and remagnetized granites

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   

Rock Magnetic Investigation of Historical Lavas Used in Palaeointensity Studies

Five samples from historical lava flows on Mt. Etna, which had previously been used in a palaeointensity study, were examined using a combination of rock magnetic and microscopic techniques to elucidate the causes of failure of palaeointensity determination. The samples were characterised using a combination of low-temperature susceptibility (LT-), Bitter pattern imaging and scanning electron microscope (SEM). High-temperature susceptibility curves and hysteresis loops had been previously measured by Calvo et al. (2001). Of the five samples only one gave an accurate palaeointensity. This sample was deuterically oxidised and consisted of large exsolved ore grains. It was only possible to distinguish this sample either optically or magnetically from a similar sample by the LT- warming curves; the unsuccessful sample displayed alteration in the LT- warming curves, which was not readily observed in the high temperature susceptibility curves. It is proposed that the measurement of LT- curves before and after heating could be a more sensitive method of determining suitability for palaeointensity determination than previous rock magnetic pre-selection techniques.     

Monodomain behaviour in multiphase oxidized titanomagnetite

Large grains, of the order of tens of microns in size, of synthetic titanomagnetite have been oxidized to produce an intergrowth of phases, essentially similar to that produced by the natural process of deuteric oxidation. The scale of the intergrowth is at the limit of the optical range but the electron microscope reveals the characteristic lamella microstructure. The magnetic hysteresis properties of the most highly oxidized material, having probably about 10% of residual spinel phase, are typical of dispersed monodomain magnetite rods. The investigation therefore supports the model in which it is proposed that titanomagnetite grains in a slowly cooled basalt may carry a highly stable component of natural remanent magnetization.     

Single-domain and superparamagnetic titanomagnetite with variable Ti content in young ocean-floor basalts: No evidence for rapid alteration

High resolution electron microscope studies have been carried out on ‘zero-age’ (New Flow) basalts from the Juan de Fuca Ridge and on young (< 20 ka) basalts from the axis of the East Pacific Rise at 12°N. Such data lead to characterization of the magnetic minerals, especially those of smaller grain size, which have been hypothesized by Kent and Gee to have undergone grain size-dependent alteration. In addition to larger titanomagnetite grains, abundant submicrometer titanomagnetite has been observed in globules within a glassy matrix. These grains, likely to be single-domain (SD) or superparamagnetic, are associated with apatite, uncommon pyrrhotite and residual glass. The submicrometer titanomagnetite grains have a wide compositional range (0 < x < 0.8), where x is the fraction of ulvöspinel component, whereas the larger, multi-domain (MD)-sized titanomagnetite grains have a narrow composition range of approximately x = 0.6. This variability in Ti content provides a ready explanation for the thermal rock magnetic properties observed by Kent and Gee and eliminates the need to invoke extremely rapid (< 20 ka) alteration of these young basalts.     

Alteration induced changes of magnetic fabric as exemplified by dykes of the Koolau volcanic range

We studied 93 samples from 8 basaltic dykes of the Koolau volcanic range on the island of Oahu,Hawaii,USA,to determine the influence of hydrothermal alteration on the magnetic fabric as determined by anisotropy of magnetic susceptibility (AMS) measurements. Rock magnetic as well as microscopic investigations show that only ≈25% of the samples have retained their original magnetomineralogical composition of unaltered Ti-poor titanomagnetite. The remaining samples have undergone hydrothermal alteration which transformed the primary magnetic phase into a granular intergrowth of titanomagnetite, titanomaghemite and hematite. In both sample groups,this magnetic phase occurs in coarse (tens of microns),irregularly shaped particles as well as interstitial clusters of smaller (< 5 μm) grains. Our investigations show that hydrothermal alteration does change the bulk susceptibility and the degree of anisotropy but not the directions of principal axes of the AMS ellipsoid which are predominantly corresponding to normal magnetic fabric. The stability of AMS directions, regardless of the degree of alteration, points towards the model of distribution anisotropy as the controlling factor for the observed magnetic fabric.     

Electron probe microanalysis study on processes of low-temperature oxidation of titanomagnetite

Partially oxidized titanomagnetite grains in various kinds of volcanic rocks were investigated by electron probe microanalyzer (EPMA) in order to clarify the process of oxidation at low temperature. The following results were obtained by the present investigation: (1) Primary composition of titanomagnetite is homogeneous in individual grains, although variation in composition among different grains is observed on each thin section. (2) Migration of Fe cations during low-temperature oxidation is clearly seen in all oxidized grains. Some other constituent cations are also bleached and consequently the relative content of the remaining cations becomes large. (3) The detailed internal structures of titanomagnetite grains are observed as backscattered electron images (BEI) with an electronprobe microanalyzer, and it seems likely that the structures depend upon the degree of low-temperature oxidation. (4) The chemical and physical properties of oxidized titanomagnetites imply that low-temperature oxidation is not a simple process but a complex one. Such an oxidation process is correlatable to both the mobility of cation and the oxidation condition such as a circulation of some active hydrothermal materials at low temperature.     

Microscopy and rock magnetism of fine grain-size titanomagnetite from the Jacupiranga Alkaline Complex,Brazil: unearthing Ti-magnesioferrite nanoparticles

Very fine samples from the mineralized zones of the Jacupiranga complex at the Cajati mine were selected for crystallographic identification of Ti-magnesioferrite (TMf) nanostructures embedded in titanomagnetite (TM) using high-resolution transmission electron microscopy (TEM). A magnetic concentrate obtained of pyroxenite samples (sites 4 to 7) was reduced and divided into fractions of distinct range sizes: 26±2 μm, 19±1 μm, 13±1 μm, 9±1 μm, 6±1 μm and 6-0.1 μm. The mineralized samples of carbonatite and pyroxenite were characterized by X-ray diffraction, transmitted and reflected light microscope, and scanning electron microscope with multielemental analysis. The finest magnetic concentrate sample (MC₆) was analyzed under high-resolution transmitted electron microscopy (TEM) and high angle annular dark field and Raman spectroscopy. Magnetic properties were measured for the distinct granulometric fractions, showing drastic changes when grain sizes go beyond the frontier from micro to nanometer sizes. Frequency-dependent magnetic susceptibility percentage (÷_fd%) report higher values (10.2%) for the finer fractions (6±1 μm and 6-0.1 μm) attributed to dominant fractions of superparamagnetic particles. Nanometer and < 6 μm grain size TMf in TM particles require a magnetic field up to 249 mT to reach saturation during the isothermal remanent magnetization experiment. Coercivity and remanent magnetization of these samples increase when the particle size decreases, probably due to parallel coupling effects. Magnetic susceptibility versus temperature experiments were conducted two times on the same (< 35 nm) sample, showing that the repetition during the second heating is probably due to the formation of new TMf nanoparticles and growth of those already present during the first heating process.     

Rock magnetic and petrographical–mineralogical studies of the dredged rocks from the submarine volcanoes of the Sea-of-Okhotsk slope within the northern part of the Kuril Island Arc

The rock magnetic properties of the samples of dredged rocks composing the submarine volcanic edifices within the Sea-of-Okhotsk slope of the northern part of the Kuril Island Arc are studied. The measurements of the standard rock magnetic parameters, thermomagnetic analysis, petrographical studies, and microprobe investigations have been carried out. The magnetization of the studied rocks is mainly carried by the pseudo-single domain and multidomain titanomagnetite and low-Ti titanomagnetite grains. The high values of the natural remanent magnetization are due to the pseudo-single-domain structure of the titanomagnetite grains, whereas the high values of magnetic susceptibility are associated with the high concentration of ferrimagnetic grains. The highest Curie points are observed in the titanomagnetite grains of the igneous rocks composing the edifices of the Smirnov, Edelshtein, and 1.4 submarine volcanoes.     

Magnetic studies and scanning electron microscopy — X-ray energy dispersive spectroscopy analyses of road sediments,soils and vehicle-derived emissions

Human health and environmental problems related to particulate matter emission from vehicles has become a topic of research interest in recent years. These airborne particles can not only be directly inhaled, but are also present as suspended and deposited particles on paved areas and roadside soils. Here we report on magnetic studies, scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS) and chemical analyses of vehicle-derived particles collected from both primary sources and as deposited particles on roads and soils. Preliminary results, recently published by the authors, have revealed that the magnetic signal of such particles is controlled by a magnetite-like phase with magnetic grain size ranging between 0.1 μm and 5 μm. An enrichment of some trace elements: Ba, Cr, Cu, Zn and Pb was also found. In this study we focus on SEM and EDS complementary studies of magnetic extracts. SEM observations showed small individual particles or spherulites, small aggregates in the form of chains or clusters, large aggregates of spherules, flake-like bodies, fibre-like particles, sheet-like particles, irregular debris and large particle agglomerates, i.e. a wide variety of shapes. Grain size distribution is also in agreement with magnetic grain size estimations. Additionally the following elements: C, O, Na, Mg, Al, Si, S, K, Ca, V, Ba, Ti, Cr, Mn, Fe, Cu, Zn and Pb were detected by EDS analysis.     

Correlation between magnetic and mineralogical properties of some tertiary basaltic rocks from qatrani-fayum area,Egypt

This paper reports clear correlation between the magnetic and mineralogical properties of some basaltic rocks from Egypt. Although all the measured samples are reversely magnetized lavas, yet clear differences were found between them. A four-fold classification based on the nature of both titanomagnetite and discrete ilmenite grains is given. The classification is based mainly on the differentiation between the different stages of oxidation and these were found to be in accordance with the intensity of magnetization and susceptibility values.     

Enrichment of sediments in iron hydroxides at the Mesozoic-Cenozoic boundary: A synthesis of petromagnetic data

Results of petromagnetic studies of epicontinental sediments at the Mesozoic-Cenozoic (K/T) boundary are generalized. Their analysis shows that an abrupt rise in the concentration of iron hydroxides is confined to this boundary. Their concentration is highest at the base of the boundary layer and drops by more than two times at it top. As distinct from iron hydroxides, other magnetic minerals were variously accumulated, depending on their origin and local deposition conditions of terrigenous material (for example, accumulation of particles of metallic iron and nickel, aerial transport of titanomagnetite grains of volcanic origin, and deposition of terrigenous particles of magnetite and ilmenite).     

Magnetic properties of the boundary layer at the Cretaceous/Tertiary boundary in the Gams section,Eastern Alps,Austria

The paper is concerned with the detailed sectional petromagnetic study of boundary clay in four 1A, 1B, 2A, and 2B Gams sections (in Austria). The composition of basic magnetic minerals in the boundary clay of all sections is similar. They are composed of iron hydroxides, hemoilmenite, titanomagnetite, magnetite, hematite, and iron. The difference is the presence in the Gams-1 section of metallic nickel, which is absent in the Gams-2 section, and the presence in the latter of iron sulfides of the pyrite type. Grains of titanomagnetite and ilmenite, connected with volcanic activity, are nonuniformly distributed in the boundary layer, which indicates their irregular precipitation in time. The ensemble of magnetic grains is characterized by high coercitivity. The boundary layer is characterized by an increased content of iron hydroxides. This effect is a global phenomenon and is irrelevant to the local physicogeographical conditions. Such a characteristic of impact events as the particles of metallic iron is almost absent in the boundary layer.     

Effect of Stress on the Magnetic Memory of Induced Magnetic Anisotropy of Rocks and Its Mathematical Model

When re-heated to temperatures below the Curie temperature and subsequently cooled in a constant magnetic field (H _T), rock samples which contain magnetic minerals can acquire an induced magnetic anisotropy (IMA). As the result of acquiring the IMA, a constriction develops in the hysteresis loop of the magnetization of these rocks at the values of the magnetizing field close or equal to the HT. Thus the IMA is capable of retaining the information on the palaeointensity of the geomagnetic field, i.e., if IMA was created in a rock in the geomagnetic field in a past geological epoch, it preserves the information on the intensity of that field. Investigations have shown, that when IMA is created in a rock under external stress, the stress has an impact on the magnetic memory. Here we also deal with the issue of how stress affects the magnetic memory of IMA. A mathematical model for the effect of stress on magnetic memory phenomena related to induced magnetic anisotropy in rocks containing multidomain magnetite and titanomagnetite grains is proposed herewith. The effect of temperature on the magnetic memory of rocks is discussed also.     

Magnetic properties of basalts and geodynamics of the rift zone in the southern Red Sea

Magnetic properties of young rift basalts in the southern Red Sea are studied in detail. It is found that basalt samples dredged in the rift zone are characterized by a large spread in magnetic parameters. The magnetic properties of the basalts are shown to indicate a complex evolution of the Red Sea rift zone. Titanomagnetite grains of the basalts are mostly affected by single-phase oxidation processes and have preserved paleomagnetic information. However, basalts discovered near the rift axis yield evidence of multiphase oxidation of titanomagnetite grains, which is untypical of young oceanic basalts. These basalts have high Curie points and large values of the natural remanent magnetization and Koenigsberger parameter. The corresponding samples were taken in nontransform zones where rocks have experienced the action of significant tectonic forces and, moreover, anomalous geomagnetic field patterns correlate with the position of these zones. Using the magnetic properties of the basalts, the northern segment of the rift axis is relocated.     

Magnetic petrology of the 1991–1995 dacite lava of Unzen volcano,Japan: Degree of oxidation and implications for the growth of lava domes

To understand the oxidation state and process of oxidation of lava domes, we carried out magnetic petrological analyses of lava samples obtained from domes and block-and-ash-flow deposits associated with the 1991–1995 eruption of Unzen volcano, Japan. As a result, we recognize three different types of magnetic petrology, each related to deuteric high-temperature oxidation during initial cooling. Type A oxides are characterized by homogenous titanomagnetite and titanohematite, indicating a low oxidation state and high titanomagnetite concentrations. Type B oxides are weakly exsolved and contain titanohematite laths and rutile lenses, indicating a higher oxidation state. Type C oxides, which represent the highest oxidation state, are completely exsolved and composed of Ti-poor titanomagnetite, titanohematite, rutile, and pseudobrookite, indicating high hematite concentrations. Some grains in Types A and B show indications of reduction, which was related to interaction with volcanic gases subsequent to high-temperature oxidation. In terms of geological occurrence, the oxidation processes probably differed for endogenous and exogenous domes. Endogenous dome lavas are oxidized concentrically and are classified into the three types according to their location within the dome: samples from the surface are strongly oxidized and classified as Type C, while the inner part is unoxidized and classified as Type A. Exogenous dome lavas are unoxidized and assigned to Type A. Some samples show signs of reduction, which may have occurred around fumaroles. We propose that location within the dome and the process of dome growth are the factors that control oxidation.     

青海湖晚更新世沉积物中胶黄铁矿的发现及其环境指示意义

对青海湖南盆沉积中心钻取的一根长18.6 m的高取芯率沉积岩芯(1F)进行磁化率测试显示,该岩芯在8.15～8.96 m和15.50～17.28m两沉积段具有非常高的磁化率.详细的岩石磁学测量和矿物分析表明这两段沉积物中有大量的胶黄铁矿存在,其磁畴特征主要以单畴(SD)为主,并显示少量超顺磁(SP)颗粒特征,这是导致磁...     

Computation of magnetic gradients due to three-dimensional bodies

Thc expressions of magnetic gradients due to 3-D homogeneous magnetized polyhedra are systematically derived and presented, fronr which the forward problem of magnetic gradients of an arbitrary shaped geological body is solved. It is shown that in the rotation of coordinate systems there is an essential difference between the transformation of magnetic fields and that of their gradients. In a 2-D coordinate system a unified transformation formula of any order gradients can he derived, hut cannot in the 3-D case. The calculations of synthetic models show the correctness of the expressioils or magnetic gradients. Project supported by the National Natural Science Foundation of China.     

The Use of Field Dependence of Magnetic Susceptibility for Monitoring Variations in Titanomagnetite Composition - A Case Study on Basanites from the Vogelsberg 1996 Drillhole,Germany

In high Ti basanites from the Vogelsberg 1996 drillhole distinct variations in the field dependence of AC magnetic susceptibility correlate with compositional variations of titanomagnetite, as determined by temperature dependence of magnetic susceptibility. Curie temperatures for the basanites are in the range of 240 to 525°C. The field dependence reaches up to 20% for measurements in 30 A/m and 300 A/m AC field amplitude. It is demonstrated that two-field magnetic susceptibility measurements can prevail information about compositional changes of titanomagnetite and therefore support the interpretation of magnetic susceptibility logs from drillcores of basaltic rock suites.     

苏南江阴-嘉定断裂带断层泥扫描电镜研究及其地震地质意义

本文在分析苏南江阴-嘉定北西向断裂带宏观构造特征的基础上,对断层泥中石英碎砾表面溶蚀结构进行了扫描电子显微镜研究,并与郯-庐断裂带山东段断层泥进行了电镜图象的对比分析,从而确定了江阴-嘉定断裂带的相对(最新)活动年代及其地震活动性;此外,就江阴-嘉定断裂带不同区段活动年代差异及其纵向迁移模式、断层泥石英碎砾表面结构电镜图象聚类的概率分布特征及其地震地质意义等有关问题展开了讨论     

Absolute geomagnetic paleointensity as recorded by ∼1.09 Ga Lake Shore Traps (Keweenaw Peninsula, Michigan)

Absolute geomagnetic paleointensity measurements were made on 255 samples from 38 lava flows of the ～1.09 Ga Lake Shore Traps exposed on the Keweenaw Peninsula (Michigan, USA). Samples from the lava flows yield a well-defined characteristic remanent magnetization (ChRM) component within a ～375°C–590°C unblocking temperature range. Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo-single-domain magnetite and/or low-Ti titanomagnetite. Scanning electron microscopy reveals that the (titano)magnetite is present in the form of fine intergrowths with ilmenite, formed by oxyexsolution during initial cooling. Paleointensity values were determined using the Thellier double-heating method supplemented by low-temperature demagnetization in order to reduce the effect of magnetic remanence carried by large pseudosingle-domain and multidomain grains. One hundred and two samples from twenty independent cooling units meet our paleointensity reliability criteria and yield consistent paleofield values with a mean value of 26.3 ± 4.7μT, which corresponds to a virtual dipole moment of 5.9 ± 1.1×10²² Am². The mean and range of paleofield values are similar to those of the recent Earth’s magnetic field and incompatible with a “Proterozoic dipole low”. These results are consistent with a stable compositionally-driven geodynamo operating by the end of Mesoproterozoic.