武昌地区街道尘埃磁学特征及其对环境污染的指示意义

对在武昌地区4个功能区域(青山、洪山、东湖风景区、东湖开发区)主要交通街道收集的尘埃样品进行详细的环境磁学测量,包括质量磁化率、频率磁化率、等温剩磁退磁系数以及典型样品的磁滞回线参数等。研究结果表明:尘埃样品中剩磁载体均以亚铁磁性矿物为主,样品中的磁性颗粒呈现低矫顽力的磁铁矿特征,软磁性的磁性矿物是样品剩余磁性的主要载体。尘埃样品χ、SIRM的区域分布特征受区域周围工业、车流量及道路状况等多种环境因素的影响,在4个功能区域中存在明显的磁性差异:青山工业区污染较重,东湖开发区、洪山、东湖风景区次之;硬剩磁(HIRM)的分布可能指示了道路交通来源的磁颗粒特征,东湖开发区和青山工业区由于行驶的货车和重型车辆较多,表现出硬剩磁量值较高。与其他城市(兰州、西安、南京、徐州等)相比,整个武昌地区街道尘埃样品的磁性含量仍较高,说明在城市环境方面还有待于进一步改善和保护。     

红粘土的磁学性质研究

本文对陕西宝鸡剖面红粘土上部进行了详细的岩石磁学研究，确定了该地区红粘土的主要磁性矿物是磁铁矿。赤铁矿相对含量很少，对剩余磁性贡献较小。磁性质不稳定的磁赤铁矿存在于红粘土中，但不影响剩磁稳定性。特征剩磁载体主要是磁铁矿。磁性矿物的粒度是以准单畴为主。红粘土的短时间弛豫粘滞剩磁很强，因此在零磁空间进行退磁和剩磁测量是非常重要的。     

马兰黄土和离石黄土的磁学性质

中国黄土-古土壤系列的磁学性质,如磁化率等被广泛用于古气候和古环境研究,这必须建立在对黄土-古土壤的剩磁的性质和获得剩磁的机制以及古土壤磁性增强的原因有深入了解的基础之上。作者通过对洛川和吉县剖面马兰黄土和离石黄土的磁学性质的深入研究,对黄土与古土壤磁学性质差异的各种表现,对它们剩磁的性质,对古土壤磁性增强的原因以及磁性变化对古气候、古环境研究的意义进行了讨论,并对利用某些磁学性质建立的黄土-古土壤形成的时间模式提出了质疑。     

Preliminary magnetic studies of lagoon and stream sediments from Chascomús Area (Argentina)—magnetic parameters as indicators of heavy metal pollution and some results of using an experimental method to separate magnetic phases

Magnetic properties of lagoon and stream sediments from Chascomús area (Buenos Aires Province) and the relevance of various magnetic parameters as giving pollution status are studied in the present work. This work is focussed on magnetic parameters, such as concentration-dependent (magnetic susceptibility, saturation anhysteric and isothermal remanent magnetisation) and feature-dependent parameters (S-ratio, coercivity of remanence, anhysteric susceptibility/magnetic susceptibility-ratio), as pollution indicators. Firstly and most importantly, different magnetic parameters and chemical measurements were correlated in order to investigate their goodness, obtaining the best results for feature-dependent magnetic parameters. Coercivity of remanence correlate very well with chemical variables, showing correlations at high level of significance up to 0.9094, and the anhysteric susceptibility/magnetic susceptibility-ratio also shows very good correlations (up to 0.8376). Some results and advantages of using a new experimental method in order to discriminate hard and soft magnetic phases are also shown. This method uses alternately backfield isothermal remanent magnetisation and alternating field demagnetisation. From the experimental separation, the presence of hard magnetic phases in some samples was confirmed.     

Magnetic Fabric Studies of High-Grade Metamorphic Rocks from the Juiz de Fora Complex,Ribeira Belt,Southeastern Brazil

Granulite from 66 sites along the Além-Paraiba dextral shear zone were collected for magnetic analyses. The rocks were affected by the Braziliano orogeny, which was responsible for the present structural pattern. Magnetic fabrics were determined applying anisotropy of low—field magnetic susceptibility (AMS, all sites) and anisotropy of remanence magnetization (ARM, in 21 sites). The ferromagnetic minerals are magnetite, titanohematite, and in some samples, minor pyrrhotite. Hysteresis curves show that both para— and ferromagnetic minerals are the carriers of AMS. Thus AMS is due to the preferred crystallographic orientation of paramagnetic matrix minerals and titanohematite, to the shape anisotropy of magnetite grains, or to a combination of all three. ARM was performed imposing both anhysteretic remanence (AAR) and isothermal remanence (AIRM). The AMS, AAR, and AIRM fabrics are coaxial and are tectonic in origin. Their parallelism indicates that both ferromagnetic and paramagnetic minerals recorded the same metamorphic event. A passive—marker model is suggested for ferromagnetic minerals at the outcrop scale. The magnetic foliation is very close to the strike of the Além Paraíba shear zone, suggesting that this generated the local rock fabrics during the Braziliano orogeny.     

Palaeoenvironmental application of magnetic measurements from inter-drumlin hollow lake sediments near Dungannon, Co. Tyrone, Northern Ireland

Measurements of palaeomagnetic horizontal remanent intensity, saturation isothermal remanence, coercivity of remanence, high field remanence and magnetic susceptibility, have been made on lateglacial and postglacial sediments from two small, inter-drumlin hollow lakes from central Northern Ireland. These magnetic measurements have been compared with pollen and chemical analyses from the same profiles and with complementary mineral magnetic data from local soils. Representative sediment samples have also been divided into a range of particle size fractions and each fraction has been subjected to magnetic investigation. Five distinctive stratigraphic horizons have been identified on the basis of the magnetic mineralogy. Two horizons relate to phases of pronounced erosion. One of these is connected with solifluction processes in the lateglacial and the other to an intensive period of farming activity which started in Medieval times. Two horizons are associated with periods of very high iron and manganese deposition and are possibly related to the precipitation of magnetic minerals within the lake. Hydrological changes during periods of local anthropogenic activity appear to produce magnetic mineral assemblages indicative of stream bed/bank substrate sources. The particle size data also support the suggestion that downcore changes in the mineral magnetic record are mainly caused by changes in the sediment source rather than particle size or sedimentological effects.     

试论前寒武纪变质岩古地磁研究的可行性：以闽北地区的研究为例

本文从变质作用与岩石矿物获得剩磁的关系和剩磁获得时间与同位素记年的相关性，岩石的形变对磁化方向的影响等方面讨论了前寒武纪变质岩古地磁研究的可行性；并列举部分国外前寒武纪早期岩石的例证；同时重点以闽北地区前寒武纪变质岩的古地磁结果为例，讨论了获得磁性可信性及其在地质构造方面的意义。     

Investigating the potential of using environmental magnetism techniques as pollution proxy in urban road deposited sediment

The use of mineral magnetic techniques as pollution proxy for road deposited sediment was explored using various statistical approaches. Standard techniques were adopted for measurement of mineral magnetic and geochemical parameters. The analyses of magnetic parameters revealed that the samples were dominated by ferrimagnetic minerals and multidomain grains. This implied that the magnetic fractions in the samples might be of anthropogenic origin. Results also indicate that the samples were dominated by low coercive, magnetically soft minerals. Thermomagnetic curves confirmed magnetite as the remanence bearing magnetic mineral having a Curie point temperature of ~580 °C. The strong association observed between magnetic susceptibility, susceptibility of anhysteric remanent magnetization and saturation isothermal remanent magnetization and aluminum, titanium, manganese, iron, chromium and lead demonstrated that these metals occurred as ferrimagnetic particles of technogenic origin resulting from vehicular sources. Assessment of pollution status of the road deposited sediment identified silicon and lead as the priority pollutants of concern. Generally, pollution load index was <1 (mean, 0.66 ± 0.14), indicating that the samples were not polluted in the overall, but the metals were in the buildup stage requiring constant monitoring. The sources of pollutants from principal component and cluster analyses identified the sources of pollution to be mainly from vehicular emissions such as brake linings, exhaust materials, tire wear, corroded metal parts, abrasion of lubricating oil and road construction materials. This study found that mineral magnetic techniques offer great potential as pollution proxy for soil pollution studies.     

断裂岩岩石磁学研究进展

断裂岩的岩石磁学研究可以揭示地震断裂作用的物理和化学环境,对于探讨地震断裂作用机制具有重要作用。本文在断裂岩岩石磁学最新文献的基础上,结合笔者及所在研究团队在龙门山断裂带获得的研究成果,综述了断裂岩的岩石磁学研究进展。大量研究发现断层泥和假玄武玻璃通常具有磁化率值或剩磁强度异常特征。顺磁性矿物在摩擦热或流体作用下形成新的铁磁性矿物是断层泥和假玄武玻璃高磁化率值或高剩磁强度的主要原因;地震断裂摩擦熔融作用中形成的单质铁是假玄武玻璃中高磁化率值或高剩磁强度异常的另一个重要原因。蠕滑断裂和出露于浅地表的断裂带中可见一些具有低磁化率值异常的断层泥,原因可能是流体作用或断裂带未经历高温摩擦热。断裂岩的岩石磁学研究为地震断裂带的应力应变、形成温度、摩擦热效应、流体作用、形成深度和氧化还原特征等提供了重要信息,可用于分析地震断裂的孕震和发震环境。综合岩石磁学测试和微米至纳米尺度的超显微学研究,并辅助地震断裂岩的摩擦实验、高温热模拟实验等研究可以更好地获得断裂岩的岩石磁学信息。     

Magnetic properties of the remagnetized Middle-Ordovician limestones of the Ponón Trehué Formation (San Rafael Block,central-western Argentina): Insights into the Permian widespread Sanrafaelic overprint

The widespread Sanrafaelic remagnetization reset most of the early Cambrian to mid-Ordovician carbonate platform of the Argentine Precordillera and the calcareous units of the San Rafael Block. We conducted a detailed rock-magnetic study on the Middle-Ordovician limestones of the Ponón Trehué Formation at both limbs of a tight anticline exposed in the San Rafael Block (Mendoza province, central-western Argentina) that are carriers of a syntectonic magnetization of Permian age. We found that the magnetic overprint in the Ponón Trehué Formation is carried by both pyrrhotite and magnetite, with goethite and subordinate haematite likely related to weathering. Hysteresis parameters, frequency dependence of magnetic susceptibility, Cisowski and modified Lowrie-Fuller tests suggest the presence of ultrafine particles of chemical origin. Demagnetization of natural remanent magnetization and of three-axis isothermal remanence confirm pyrrhotite and magnetite as important contributors to the remanence. Both minerals carry the same magnetic syntectonic component suggesting a coeval or nearly coeval remanence acquisition and therefore mineral formation. This and the results of the magnetic fabric analyses indicate an authigenic origin of the magnetic minerals during folding associated with the Sanrafaelic tectonic phase (ca. 280 Ma). Although the chemically active (oxidizing?) fluids expelled from the orogen as it developed in the early Permian is a viable explanation for the Sanrafaelic remagnetization, the role of the nearly coeval magmatism in Precordillera and the San Rafael Block remains to be properly evaluated.     

Secondary Fe–Mn-oxides in minerals heavily damaged by α-recoil: possible implications for palaeomagnetism

Sub-micron Fe,Mn-oxides in columbite–tantalite minerals are bound to metamict domains in the host. These nano-oxides are secondary minerals as the metamict zones formed through accumulation of damages from α-recoil, each of which in a small volume destroys the crystal lattice of the U and Th bearing columbite–tantalite host. Transmission electron microscope investigations demonstrate that the oxides fall in the compositional range of magnetite–jacobsite–iwakiite (Fe,Mn)₃O₄ and of hematite-type minerals along the Fe₂O₃–Mn₂O₃ join, i.e., minerals that are known to be major carriers of magnetic remanence. Measured magnetic properties of isolated columbite–tantalite crystals demonstrate in some samples magnetic remanence, which is bound to sub-micron pseudosingle-domain crystals rather than to the host. Thus, the magnetic remanence is not related to the formation of columbite–tantalite, but to the crystallization of the nano-oxides, which occur in metamict domains of the host and therefore are secondary. Depending on the content and distribution of U and Th, the nano-oxides may form diachronously within an individual and among several host crystals. Magnetic secondary nano-oxides are not restricted to columbite–tantalite minerals, but may occur in other minerals where high contents of Fe and Mn are associated with high contents of U and Th. Rocks that show the same spatial distribution for U and Fe, as for instance some red sandstones, may display magnetic properties that are controlled by recoil-induced growth of secondary Fe-oxides.

Magnetic carriers and remanence mechanisms in magnetite‐poor sediments of Pleistocene age,southern North Sea margin

Pleistocene sedimentary sequences in the East Anglian region of Britain record both major and minor climatic oscillations, and the impact of isostatic and eustatic variations. Intensively studied in terms of their lithology and biostratigraphy, the sequences have been difficult to place in an absolute timeframe. Dating and correlation by magnetostratigraphy has been attempted over a number of years. However, these sediments are difficult to date by palaeomagnetic means because they are poor in detrital magnetite, are subject to post‐depositional deformation and diagenesis, and have unknown rates of sedimentation. Determining whether their natural remanence magnetisation (NRM) directions are reliable thus requires information on the mode and timing of remanence acquisition. Here, we apply palaeomagnetic, rock magnetic and mineralogical analyses to identify the NRM carriers in these sediments and hence their palaeomagnetic reliability. Within oxidised fluvial sediments (the Kesgrave Formation), the magnetic carriers appear to be relict magnetic minerals (ferrian ilmenites, chromites, haematite and goethite), which sometimes carry a reliable primary depositional remanence (DRM) but often an overprinting viscous (time‐varying) remanence (VRM). Within some reduced marine and intertidal sediments (within the Crag basin), the iron sulphide, greigite, has been found to carry a reliable, ‘syn’‐depositional chemical remanence (CRM). In all the sediments, magnetic inclusions within silicates are abundant, are significant for the mineral magnetic signal but contribute little to any recoverable palaeomagnetic information. Copyright © 2005 John Wiley & Sons, Ltd.     

A preliminary magnetic study on lacustrine sediments from Zoigê Basin,eastern Tibetan Plateau,China: Magnetostratigraphy and environmental implications

A preliminary magnetic study was conducted on Core RM (310 m long), drilled in the Zoigê Basin (33 °57′N, 102 °21′E), on the eastern Tibetan Plateau. Greigite in SD state exists as a main magnetic contributor along the whole core. In addition, magnetite and haematite are detected between 160-45 m. NRM directions show a complex record with many apparent changes in inclination, for which greigite or the remanence acquisition process may be responsible. Nevertheless, the Matuyama/Brunhes boundary and, possibly, the Blake Event are identified. Between 310 and 160 m (as well as between 45 and 4.5 m), predominant anoxic conditions favour the formation of greigite, leading to higher susceptibility (χ) and remanence peaks. However, higher carbonate content during dry periods in these zones, may dilute the concentration of magnetic minerals, resulting in lower values of magnetic parameters. Between 160 and 45 m, weak oxic conditions preserve detrital magnetic minerals. The increasing χ background in this zone may indicate changes in the concentration of magnetite and haematite related to the uplift of the Tibetan Plateau, while higher χ peaks may indicate fluctuations to anoxic conditions, which enhance greigite concentration.     

Ferrimagnetic minerals in red paleosols of Pleistocene Epoch, eastern China

The type,grain size and origin of ferimagnetic minerals separated from red paleosols of pleistocene Epoch(Q2)in eastern China ,were studied by using mineral magnetic measurement,X-ray powder diffraction and electron microscopy.Results showed that the iron oxider in red paleosols were composed of hematite(α-Fe2O3),maghemite(γ-Fe2O3) and goethite(α-FeOOH),Mineral magnetic parameters and X-ray diffraction patterns indicated that maghemite was the dominant remanence carrier in red paleosols,which is characterized by superparamagnetic(SP) and stable single domain(SSD) grains,The variations of magnetic susceptibility(χ） ,anhysteretic magnetic susceptibility(χRAM）and saturation isothermal remanent magnetization(SIRM) for red paleosols following heating to various temperatures showed two peak values at 700℃ and 900℃.The spherulitic magnetic particles measuring 250-1000μm in diameter in red paleosols were separated by the magnetic separation method,indicating that these magnetic particles were an assemblage of superparamagnetic and stable single domain ferrimagnetic grains,It is suggested that the ferrimagnetic minerals of red paleosols be a pedogenic ferrimagnetic component under high temperature and high humid conditions in the Pleistocene Fpoch(Q2).It is concluded that the magnetism characteristics of red paleosols can be used to evaluate the environmental changes of Quaternary in eastern China.     

塞浦路斯特罗多斯型洋壳最上部500米剖面(CY—1钻孔)中的不透明矿物、磁性及其蚀变作用的研究

引言近十年来为了鉴别大洋盆地中线性磁异常模式的起源,对洋壳磁化强度的研究显示了极大的兴趣。目前了解洋壳(尤其在深部)可以通过几种方法,如在推测为构造断崖的附近进行海底拖曳打捞、通过大洋基底钻探     

Composite detrital and thermal remanent magnetization in tuffs from Aeolian Islands (southern Tyrrhenian Sea) revealed by magnetic anisotropy

This paper reports on the complex relation between rock emplacement and remanence acquisition in tuffs deposited by pyroclastic density currents, disclosed by systematic measurements of the anisotropy of magnetic susceptibility and natural remanent magnetization (NRM). Thermal demagnetization shows that the NRM consists of two components with different blocking-temperature spectra. The direction of the low-temperature component is consistent with the geocentric axial dipole value, whereas the high-temperature component has dispersed directions. The magnetic fabric is oblate, the magnetic foliation is close to the bedding and the lineations are generally dispersed along a girdle within the foliation plane. The directions of the magnetic lineation and the high-temperature remanence component of individual specimens are close to each other. This correspondence suggests that the high blocking-temperature grains acquired a remanence aligned to their long dimension before deposition, while cooling within the explosive cloud and the moving pyroclastic current. Thereafter, during deposition, the traction processes at the base of the current oriented the grains along the flow direction and affected both fabric and high-temperature remanence. This NRM component results from mechanical orientation of previously magnetized grains and is thus detrital in origin. A second, thermal component was then acquired during the cooling of the low blocking-temperature grains after deposition. These results show that NRM in fine-grained pyroclastic rocks is affected by the Earth’s magnetic field as well as the emplacement processes and that magnetic fabric data are essential to unravel its complex nature.     

The geochemistry of iron in Recent tidal-flat sediments of the Wash area,England: a mineralogical,Mössbauer,and magnetic study

Undisturbed core samples of Recent sediments from the Wash tidal flats, East Anglia, England, obtained using a Delft corer, were studied with special reference to the diagenesis and geochemical behaviour of iron. The Mössbauer effect in ⁵⁷Fe was used to monitor the distribution of Fe between different phases as a function of depth, together with the magnetic mineralogy and palaeomagnetic properties.The cores consist of, successively downwards: 0.36 m brown clay; 1.5 m finely laminated silts and fine sands, and 7.14 m homogeneous fine sands. The dominant minerals are quartz, feldspar, calcite and clay minerals, and chemical analysis for Al, Si, Mg, Mn, Ca, Fe, Na, K showed variations closely linked to lithological changes. Illite is the most abundant clay mineral (mean 48%), followed by mixed layer illite-montmorillonite and montmorillonite, kaolinite and chlorite. Chlorite is the major iron-bearing clay mineral and represents 4 to 10% of the <2 μm fraction throughout the core. Sulphide minerals are present throughout the core, including framboidal pyrite.Computer fit analysis of the Mössbauer spectra of best quality showed contributions from Fe²⁺ and Fe³⁺ in clay minerals (essentially chlorite), low-spin Fe²⁺ in pyrite, and magnetically ordered iron in greigite (Fe₃S₄). Systematic variations, as a function of sample depth, indicate a relative increase in the amount of Fe in pyrite at the expense of the clay minerals.Magnetite and titanium-bearing magnetite are the carriers of natural magnetic remanence in these sediments.The direction and intensity of natural remanence in the samples compare well with the known secular variation of the Earth's magnetic field derived from the historic-archaeomagnetic record and this enables the samples to be dated and sedimentation rates to be determined (1.5 mm yr^?1 for the upper 2 m and ~7.7 mm yr^?1 for the lower 7 m).     

Palaeomagnetism applied to magnetic anomaly interpretation: a new twist to the search for mineralisation in northern Chile

The Chilean Iron Belt is an important location for Fe and Cu(Fe)-Au deposits and includes the recently developed Candelaria deposit, which is located some 20 km south of the city of Copiapó in northern Chile. This mine is now a major Cu producer in Chile but its discovery in the late 1980s was relatively fortuitous. The exploration programme included a ground magnetic survey from which lessons can be learnt in the search for further such deposits. Palaeomagnetic studies throughout the northern part of the Chilean Iron Belt indicate major crustal rotation of the region, probably, related to oblique convergence and transpression at the Andean Margin. The application of palaeomagnetic techniques to a magnetite-apatite deposit, Mina Fresia, indicates that magnetite-rich ores in this area are capable of maintaining a significant remanence component which will contribute to their magnetic anomaly. As with the volcanics, intrusives and sediments of the region this, remanence is clearly rotated clockwise. Using 2.5D and 3D magnetic modelling, it is demonstrated that the magnetic anomaly associated with the Candelaria deposit is also dominated by a remanence component which is significantly rotated but of reversed polarity. Recognition of clockwise-rotated, remanence-dominated anomalies should provide a new key to the search for deposits in this part of Chile and elsewhere. An example of an unexplored anomaly showing this key feature of a clockwise-rotated remanence component dominating over the induced component is presented. Modelling of this anomaly indicates a pair of sources which are either ten times as big or ten times as magnetic as the Candelaria deposit. It is suggested that the low cost of palaeomagnetic study of an exploration target should be a prerequisite to magnetic anomaly interpretation of targets in tectonic areas where vertical axis crustal rotations may be a significant element of the overall deformation pattern. Received: 9 April 1999 / Accepted: 5 October 1999     

The use of the anisotropy of magnetic remanence in the resolution of the anisotropy of magnetic susceptibility into its ferromagnetic and paramagnetic components

The anisotropy of magnetic susceptibility (AMS) is often controlled by both ferromagnetic (sensu lato) and paramagnetic minerals. The anisotropy of magnetic remanence (AMR) is solely controlled by ferromagnetic minerals. Jelínek (Trav. Geophys. 37 (1993)) introduced a tensor derived from the isothermal AMR whose normalized form equals the normalized susceptibility tensor provided that the ferromagnetic fraction is represented by multi-domain magnetite. The present paper shows the close correlation between these tensors for a collection of strongly magnetic specimens containing multi-domain magnetite. In addition, acceptable correlation between the tensors was also found for a collection of specimens containing single-domain magnetite. A new method is developed for the AMS resolution into ferromagnetic and paramagnetic components using the AMR. Some examples are presented of this resolution in mafic microgranular enclaves in granodiorite and in gneisses of the KTB borehole.