Rock magnetic properties and ore microscopy of the iron ore deposit of Las Truchas, Michoacan, Mexico

Iron ore and host rocks have been sampled (90 oriented samples from 19 sites) from the Las Truchas mine, western Mexico. A broad range of magnetic parameters have been studied to characterize the samples: saturation magnetization, Curie temperature, density, susceptibility, remanence intensity, Koenigsberger ratio, and hysteresis parameters. Magnetic properties are controlled by variations in titanomagnetite content, deuteric oxidation, and hydrothermal alteration. Las Truchas deposit formed by contact metasomatism in a Mesozoic volcano-sedimentary sequence intruded by a batholith, and titanomagnetites underwent intermediate degrees of deuteric oxidation. Post-mineralization hydrothermal alteration, evidenced by pyrite, epidote, sericite, and kaolin, seems to be the major event that affected the minerals and magnetic properties. Magnetite grain sizes in iron ores range from 5 to >200 μm, which suggest dominance of multidomain (MD) states. Curie temperatures are 580±5°C, characteristic of magnetite. Hysteresis parameters indicate that most samples have MD magnetite, some samples pseudo-single domain (PSD), and just a few single domain (SD) particles. AF demagnetization and IRM acquisition indicate that NRM and laboratory remanences are carried by MD magnetite in iron ores and PSD–SD magnetite in host rocks. The Koenigsberger ratio falls in a narrow range between 0.1 and 10, indicating the significance of MD and PSD magnetites.     

Magnetic structure of polluted soil profiles from Eastern Ukraine

Our study concerned magnetic properties of soil profiles taken from polluted regions of Eastern Ukraine around the industrial centres Krivyj Rig, Mariupol and Komsomolsk. Soils represent chernozem and podzoluvisol. The low-field magnetic susceptibility shows enhancement in the topsoil caused by contamination by coarse-grain magnetite connected with industrial pollution. Magnetic mineralogy was determined by means of thermal demagnetisation of SIRM, monitoring of susceptibility changes during warming from ?196°C to room temperature and heating to 700°C, and Mössbauer analysis. Granulometry of magnetic particles was investigated by determination of hysteresis parameters, susceptibility, M _s , SIRM and ARM ratios and frequency dependence of susceptibility. The chemical parameters, namely pH, organic carbon and iron content in different pedogenic and lithogenic minerals, measured for particular horizons determined pedogenic characteristic of soil profiles. Our study showed that differences in magnetic parameters of non-polluted and polluted soil profiles are not limited to the topsoil, but reach deep layers of the parent material. Industrial pollution promotes formation of the so-called “pedogenic” SP and SD particles in these layers.     

杭州城区土壤的磁性与磁性矿物学及其环境意义

对杭州城区四个不同功能区块土壤进行了系统的环境磁学测定,结果表明城市土壤的磁化率平均值为128×10^-8m³·kg^-1,频率磁化率平均值3.6%(样品数=182),城市土壤呈现明显的磁性增强. 城市土壤的磁化率与频率磁化率呈极显著指数负相关,表明城市土壤磁性增强明显区别于自然成土过程引起的以超顺磁性(SP)颗粒为主的表土磁性增强机理. 统计分析表明,城市土壤磁化率与软剩磁和饱和等温剩磁(SIRM)呈显著直线正相关,说明亚铁磁性矿物是城市土壤剩余磁性的主要载体. 综合等温剩磁获得曲线、热磁曲线、磁滞回线等岩石磁学测定和SEM/EDX分析,城市土壤的磁性矿物以磁铁矿和赤铁矿为主,磁性矿物以假单畴-多畴（PSD-MD）颗粒存在,粒度明显大于成土过程形成的磁性颗粒,这些磁性颗粒主要来自燃料燃烧、汽车尾气等环境污染物. 因此,城市土壤磁测可作为城市土壤污染监测、污染空间分布和污染物来源判断的新手段.     

A comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials: Some results from lake sediments

A new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility (χ_ARM, i.e. specific ARM obtained in a 1 Oe steady field), which is particularly sensitive to the single domain (SD) and small pseudo-single domain (PSD) grains of the finer magnetite fraction. A second parameter, low-field susceptibility (χ), is relatively more sensitive to the coarser magnetite fraction (larger PSD and smaller multidomain (MD) grains). We can then obtain a measure of the ratio of coarse- to fine-grain magnetite for large numbers of samples by plotting χ_ARversusχ. A simple idealized model based on sized magnetite samples is proposed to explain the use of the χ_ARMversusχ plot for detecting relative grain-size changes in the magnetic content of natural materials. The sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of χ_ARM and χ are used to test the model.The model is used to interpret the magnetic variations observed, and the interpretations are supported by high-field hysteresis measurements of the same sediments. The combination of the high-field hysteresis method of Day et al. [1] and the χ_ARM vs. χ method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite. The χ_ARMvs.χ method should be used as an intial means of identifying distinct groups of samples.The high-field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation.     

Domain structure of small synthetic titanomagnetite particles and experiments with IRM and TRM

Stoichiometric titanomagnetites Fe_3?xTi_xO₄ with compositions between x = 0 (magnetite) and x = 0.72 (a titanomagnetite having a Curie temperature of 60°C) have been synthesised using the double-sintering technique in controlled atmospheres. The quality of these materials was tested by various mineralogical and magnetic measurements. Isolated small multidomain (MD) and pseudo-single-domain (PSD) particles within pores of the bulk material were investigated with respect to their domain structures, and threshold sizes for the transition from the PSD to the SD stage determined for titanomagnetites of various compositions by extrapolation from the domain state of small MD grains. The threshold size was found to be 0.7 and 0.5 μm, respectively, for TM72 (x = 0.72) and TM62 (x = 0.62). The threshold size decreases slightly for smaller x values; however, the experimental data obtained to date are not sufficiently reliable to yield precise results.Preliminary experiments concerning hysteresis loops and TRM generation are also reported.     

Determination of domain structure in igneous rocks by alternating field and other methods

AF (alternating field) demagnetization, ARM (anhysteretic remanent magnetization) and strong-field hysteresis properties of a large collection of mostly continental igneous rocks are reported here. The collection included rocks whose magnetic carriers were believed from previous work to be of one of three types: MD (multidomain); SD/PSD (single-domain/pseudo-single-domain); or a bimodal mixture of MD grains (e.g., discrete opaques) and SD/PSD material (e.g., silicate inclusions). Two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour. SD/PSD rocks have relatively hard inflected AF decay curves (decay rate initially increasing, then decreasing), MD rocks have soft, exponential-like decay curves, and bimodal rocks have a combination of these characteristics. Relative hardnesses of normalized decay curves of remanences acquired in weak, intermediate and strong fields (the Lowrie-Fuller test) are also distinctively different for the three classes, and the results support the theory developed in an accompanying paper [1] that Lowrie-Fuller characteristics are an expression of the shapes of decay curves. The Lowrie-Fuller test, although its result can be expressed as a numerical parameter, is not capable of fine-scale classification of domain structure or grain size. The shape of the ARM induction curve does have a quasi-continuous variation with grain size, however. The parameter χ_ar/J_rs (initial anhysteretic susceptibility normalized to saturation remanence), which is easily measured with standard paleomagnetic instrumentation, is potentially useful for magnetic granulometry, although χ_ar itself was not diagnostic of grain size.     

Magnetic hysteresis in the F.M.R. spectra of fine-grained spherical iron: Possible evidence for a new carrier of hard remanence in lunar soils and rocks

Magnetic hysteresis effects have been observed in ferromagnetic resonance (FMR) spectra obtained at 9 and 16 GHz for certain simulated lunar glasses which were reduced by H₂ in the melt and rapidly quenched. Transmission electron microscopy has revealed that these samples contained spherical particles in the size range ～0.01–0.5 μm. FMR spectra obtained at 35 GHz (applied field ～ 12.5 kOe) exhibited a line shape characteristic of spherical, single-domain (SD) iron particles with no hysteresis. Computer simulations of the latter spectra confirmed that the average deviation from sphericity must be ?3% and that (2K₁/M_s) ≈ + 600 Oe for the precipitated magnetic phases. The principal features of the spectra obtained at all three frequencies have been explained on the basis of a simple theoretical treatment for spherical iron particles which have 2 domains in applied fields ?7 kOe, but become saturated at higher fields. Isothermal remanent magnetization (IRM) of these samples has been studied by both FMR and standard static techniques; the mean coercive force measured by the former (～4 kOe) contrasts with the mean value determined by the latter (～550 Oe). Apparently, FMR singles out and even amplifies the contributions of two-domain particles (which are magnetically hard), while the static measurement is more sensitive to the average of all particles present. The intensity of the FMR hysteresis of typical lunar soils is found to be ～1% of the total FMR intensity. In spite of this seemingly small value, two-domain iron particles may be important carriers of natural remanent magnetization (NMR) in certain lunar rocks.     

First-order reversal curve (FORC) diagrams for pseudo-single-domain magnetites at high temperature

The recently developed first-order reversal curve (FORC) technique for rapidly examining magnetic domain state has great potential for paleomagnetic and environmental magnetic investigations. However, there are still some gaps in the basic understanding of FORC diagrams, in particular the behavior of pseudo-single-domain (PSD) grains and the contribution of magnetostatic interactions. In this paper we address some of these problems. We report the first FORC diagrams measurements on narrowly sized and well-characterized synthetic PSD through multidomain (MD) magnetite samples. The FORC diagrams evolve with grain size from single-domain (SD)-like to MD-like through the PSD grain size range. Since each sample contains grains of essentially a single size, individual PSD grains evidently contain contributions from both SD-like and MD-like magnetic moments, in proportions that vary with grain size; the evolving FORC diagrams cannot be due to physical mixtures of SD and MD grains of widely different sizes. The FORC diagrams were all asymmetric. Small PSD samples have FORC diagrams with a distinctive closed-contour structure. The distributions of the larger MD grains display no peak, and lie closer to the interaction-field axis. To assess the effect of magnetostatic interactions, we measured FORC diagrams between room temperature and the Curie temperature. On heating the FORC distributions contract without changing shape until ∼500°C. Above this temperature the diagrams become more MD-like, and in addition become more symmetric. The temperature dependence of the interaction-field parameter is proportional to that of the saturation magnetization, in accordance with Néel’s interpretation of the Preisach diagram. The decrease in asymmetry with heating suggests that the origin of the asymmetry lies in magnetostatic interactions. The magnetic hysteresis parameters as a function of temperature were determined from the FORC curves. As the grain size decreased the normalized coercive force was found to decrease more rapidly with temperature.     

Rock-magnetic properties of Neogene sediments, northern flank of Tianshan Mountains

Analyses of rock-magnetic properties of Neogene sediments of the Taxihe section, northern Tianshan Mountains, show that the section can be classified into three categories including lacustrine facies, fluvial facies and alluvial facies, which correspond to the lower, middle and upper of the Taxihe section respectively. The magnetic minerals of the lacustrine facies may be affected by the process of weath- ering, lithogenesis and biolithogenesis besides the source of the sediments. The natural remanence intensities are between 10-3 A/m and 10-2 A/m. The minerals are dominated by magnetite and the high coercive magnetic mineral may be goethite. The magnetic grains are the mixture of PSD SD or SD SP. The natural remanence intensities of the strata of fluvial facies are between 10-2 A/m and 10-1 A/m, about ten times that of the lacustrine facies. The magnetic minerals are mainly magnetite and hematite, and the magnetic grains are mainly PSD. The characteristic remanence (ChRM) carriers are magnetites. In the alluvial facies, the natural remanence intensities are mostly less than 1×10-2 A/m. The magnetic minerals of the series are dominated by magnetite and hematite, almost the same as the fluvial facies. But the difference is that most of the stepwise demagnetization can reveal two components and the ChRM carriers are hematites. The magnetic grains are PSD in terms of the hysteresis parameters.     

钢铁厂周围不同污染介质的磁学性质及环境意义

为了通过钢铁厂周围不同污染介质的磁学性质研究,实现对环境污染的有效监测,本文系统地采集了河北兴隆和湖南娄底两个钢铁厂周围的树皮和草表面的飞灰、土壤及不同树种的年轮样品.其中前三者用来反映现今污染源,而树木年轮用来追踪长时间尺度的污染历史.通过综合岩石磁学实验分析（包括热磁曲线、磁滞性质和低温实验等）、电子扫描电镜(SEM)和能谱(EDX)以及X射线衍射(XRD)分析,发现钢铁厂周围现今污染源中的主要粗颗粒磁性矿物为磁铁矿、赤铁矿和纯铁.其中8~50 μm的球形磁铁矿颗粒和呈棱角状的铁颗粒是钢铁厂污染物的特征磁性矿物.对不同树种来说,其中香樟树和枫树树木年轮中的磁性矿物以顺磁性物质为主, 不适合用于环境磁学的污染监测.而柳树中的磁性矿物（主要为磁铁矿）与现今污染源的磁性矿物相符.以上研究表明多参数岩石磁学测量并辅以微观分析等手段,可以精确地提取磁性矿物的种类及粒度特征, 判别重工业区污染源,为应用磁学方法来监测环境污染提供依据.     

Magnetic methods in tracing soil erosion,Kharkov Region,Ukraine

Magnetic measurements of soils are an effective research tool in assessing soil erosion. This approach is based on detecting layers showing different magnetic properties in vertical soil profiles and lateral catenas. The objective of this research is to compile data on magnetic susceptibility (MS) of soils in Eastern Ukraine to assess the soil erosion rates. The chernozems of Tcherkascy Tishki (Kharkov Region, Ukraine) have undergone a field crop rotation without proper soil conservation technologies being applied. We conducted an intrinsic element grouping of the magnetic susceptibility values and demonstrated that they can be used as MS cartograms in soil erosion mapping. The study showed a strong correlation between the MS values and the erosion index. MS and the erosion index were found to correlate with the humus content. Magnetic mineralogical analyses suggest the presence of highly magnetic minerals (magnetite and maghemite) as well as weakly magnetic goethite, ferrihydrite, and hematite. Stable pseudosingle-domain (PSD), single-domain (SD), and superparamagnetic (SP) grains of pedogenic origin dominate in the studied chernozems. Being an effective, quick and low cost alternative, magnetic methods can be successfully used in the soil erosion investigations.     

Magnetic survey of topsoils in Windsor–Essex County, Canada

Windsor–Essex County is a major cross-border truck and transportation route, with significant localized industrialization as well as rural and farming areas. Magnetic property measurements (in-field and laboratory susceptibility, frequency-dependent susceptibility, hysteresis properties, thermomagnetic and thermosusceptibility curves, anhysteretic and isothermal magnetizations) were made in order to determine the potential for using such variables to distinguish between natural and anthropogenic pollutants. In-field magnetic susceptibility measured on 324 soil sampling sites on a 0.5–2 km grid spacing through Windsor–Essex County ranged from 3.7 × 10^− 6 to 305.2 × 10^− 6 SI (average 36.2 ± 35.8 × 10^− 6 SI), and showed that high magnetic susceptibility values were obtained on soil sampling sites in and around the cities/towns of Windsor, Harrow, Olinda and Oakland and near the beaches of Point Pelee National Park (PPNP) and Deerbrook, whereas lower susceptibility values were observed in near the towns of Lakeshore and Essex. On this grid spacing, Highway 401 (the major truck route) did not show anomalous susceptibility values; however, closer (1–3 m) sampling on other roads did show anomalously high values, suggesting that the coarser grid spacing may have missed anomalies. Laboratory measurements indicated that the dominant magnetic mineral in the Windsor–Essex County soils is magnetite; however, the grain size is variable. Pseudo-single domain (PSD)–multidomain (MD) magnetite is generally found on beaches and in PPNP, whereas single domain (SD)–PSD magnetite has been found near the City of Windsor and other towns. While certain correlations exist between some anthropogenic activities and the measured magnetic susceptibility and magnetic property values, no overall correlation can be made. A variety of geologic and anthropogenic factors must be considered when interpreting the origin of the magnetic signal in a particular area.     

辽宁复县金伯利岩的磁性特征及其意义

利用磁滞回线、磁畴状态分析、剩磁磁化率和感磁磁化率测量等，对辽宁复县金伯利岩岩体群的磁性矿物成分和成因机制进行了分析．结果表明，该金伯利岩中的磁性矿物成分主要是单畴、似单畴颗粒磁铁矿，它们主要是在金伯利岩岩浆后期，即汽水热液期，在相对快速冷却的环境下形成的原生磁性矿物．在古地磁研究中，利用主成分分析，结合同位素测年结果，给出了该区金伯利岩的古地磁结果，为中国东部早古生代的古地磁研究提供了补充和验证．     

典型沙漠绿洲城市表土磁性特征及环境指示意义

选取典型沙漠绿洲城市（乌海、石嘴山、银川）表土为研究对象,对其环境磁学性质、土壤质地与重金属含量分布进行系统研究.结果表明,乌海市和石嘴山市表土为典型砂土,银川市为砂壤土.三个城市表土样品均以较粗的MD-PSD颗粒的亚铁磁性矿物（磁铁矿）为主导,磁化率均值分别为152×10^-8、104×10^-8和117×10^-8 m³·kg^-1.乌海市表土磁性矿物含量和磁性颗粒大小都显著高于石嘴山和银川市.重金属（Cr、Cu、Pb、Zn、Fe）的污染负荷指数（PLI）均显示,三个城市表土均存在轻度污染特征.同时磁化率和污染负荷指数在非工业区表现出点状高值分布,在工业区表现为面状高值分布特征.乌海市表土磁参数与重金属的相关系数显著高于石嘴山和银川,表明磁参数对于污染来源单一地区的污染程度评估更具优势.尽管三个城市表土磁学性质存在较大差异,但其磁化率均随污染负荷指数的增加呈现相同程度的递增.磁化率可以作为宁夏平原地区表土重金属污染程度评估的统一有效的代用指标.     

Magnetic survey of topsoils in Windsor–Essex County,Canada

Windsor–Essex County is a major cross-border truck and transportation route, with significant localized industrialization as well as rural and farming areas. Magnetic property measurements (in-field and laboratory susceptibility, frequency-dependent susceptibility, hysteresis properties, thermomagnetic and thermosusceptibility curves, anhysteretic and isothermal magnetizations) were made in order to determine the potential for using such variables to distinguish between natural and anthropogenic pollutants. In-field magnetic susceptibility measured on 324 soil sampling sites on a 0.5–2 km grid spacing through Windsor–Essex County ranged from 3.7 × 10^− 6 to 305.2 × 10^− 6 SI (average 36.2 ± 35.8 × 10^− 6 SI), and showed that high magnetic susceptibility values were obtained on soil sampling sites in and around the cities/towns of Windsor, Harrow, Olinda and Oakland and near the beaches of Point Pelee National Park (PPNP) and Deerbrook, whereas lower susceptibility values were observed in near the towns of Lakeshore and Essex. On this grid spacing, Highway 401 (the major truck route) did not show anomalous susceptibility values; however, closer (1–3 m) sampling on other roads did show anomalously high values, suggesting that the coarser grid spacing may have missed anomalies. Laboratory measurements indicated that the dominant magnetic mineral in the Windsor–Essex County soils is magnetite; however, the grain size is variable. Pseudo-single domain (PSD)–multidomain (MD) magnetite is generally found on beaches and in PPNP, whereas single domain (SD)–PSD magnetite has been found near the City of Windsor and other towns. While certain correlations exist between some anthropogenic activities and the measured magnetic susceptibility and magnetic property values, no overall correlation can be made. A variety of geologic and anthropogenic factors must be considered when interpreting the origin of the magnetic signal in a particular area.     

北京市朝阳区大气降尘磁学特征及对空气污染物浓度的指示

应用环境磁学方法研究了2008年奥运会前后北京市朝阳区大气降尘及对照点表土样品的磁学特征.结果表明,所有样品中载磁矿物主要为低矫顽力的亚铁磁性矿物(主要为磁铁矿).但大气降尘样品的磁性颗粒粒度较表土偏粗;磁性矿物含量高于表土样品,大气降尘除来自于自然源外,更多为来自于人为因素的结果.大气降尘磁化率与空气污染物浓度随时间...     

黄土高原西北缘末次冰期晚期以来黄土沉积物的岩石磁学性质

以黄土高原西北缘的靖远和古浪剖面(包含黄土层L1上部和占土壤层SO)作为研究对象,选取代表性样品进行磁化率、频率磁化率、热磁曲线、等温剩磁获得曲线和磁滞回线等测定.结果表明,靖远和古浪L1黄土和SO古土壤具有相似的岩石磁学特征.磁性矿物含量相对较低,载磁矿物均以磁铁矿为主,同时含有磁赤铁矿和赤铁矿,且SO占土壤中的磁赤...     

有机质对纳米级磁铁矿热稳定性的影响

单畴磁铁矿颗粒是地质样品中最重要的磁性载体,其稳定性一直备受关注.为了认识有机质对纳米级磁铁矿颗粒热稳定性的影响,本文对比研究了趋磁细菌AMB-1合成的单畴磁铁矿分别在全细胞中和经去胞提纯后的纯化磁小体中的热磁性质,以及热处理后样品的磁滞参数和低温磁性的变化.发现仅有磁小体膜包裹的纯化磁小体中单畴磁铁矿热稳定性极强,而全细胞中的单畴磁铁矿加热过程中发生了显著的热变化:磁铁矿在约270℃即开始转化,400℃以前几乎完全被有机质还原为顺磁性物质;同时在400℃以前,有机质的还原作用与有机质热分解引起磁小体链的坍塌,共同导致了样品矫顽力(B_c)、剩磁矫顽力(B_cr)和剩磁比(M_rs/M_s)的减小,以及矫顽力比(B_cr/B_c)的增加. 我们的实验结果清楚地表明,当地质样品中含有较多有机质组分并受热事件影响时,其中的单畴磁铁矿难以得到保存.     

Mineral magnetic variation of the Jingbian loess/paleosol sequence in the northern Loess Plateau of China: Implications for Quaternary development of Asian aridification and cooling

A high-resolution mineral magnetic investigation has been carried out on the Jingbian loess/paleosol sequence at the northern extremity of the Chinese Loess Plateau. Results show that the magnetic assemblage is dominated by large pseudo-single domain and multidomain-like magnetite with associated maghemite and hematite. Variations in the ratios of SIRM_100mT/SIRM, SIRM_100mT/SIRM_30mT and SIRM_100mT/SIRM_60mT (SIRM is the saturation isothermal remanent magnetization; SIRM_nmT represents the residual SIRM after an n mT alternating field demagnetization) have been used to document regional paleoclimate change in the Asian interior by correlating the mineral magnetic record with the composite δ¹⁸O record in deep-sea sediments. The long-term up-section decreasing trend in those ratios in both loess and paleosol units has been attributed to a long-term decrease in the relative contributions of eolian hematite during glacial extrema and of pedogenic hematite during interglacial extrema, respectively, which reveals a long-term decreasing trend in chemical weathering intensity in both glacial-stage source region (the Gobi and deserts in northwestern China) and interglacial-stage depositional area (the Loess Plateau region). We further relate this long-timescale variation to long-term increasing aridification and cooling, during both glacial extrema in the dust source region and interglacial extrema in the depositional area, over the Quaternary period. Changes in those ratios are most likely due to Quaternary aridification and cooling driven by ongoing global cooling, expansion of the Arctic ice-sheet, and progressive uplift of the Himalayan–Tibetan complex during this period.     

巴黎盆地下侏罗统的岩石磁学研究及其意义

通过下佛罗统磁性地层学研究,获得了一些能记录岩石形成时的地磁场的样品,根据常规的古地磁方法,等温剩磁和热退磁实验,说明这些岩石的主要载磁矿物为磁铁矿.磁滞回线参数的测量表明多数载磁矿物磁铁矿为假单磁畴,少量样品则可能为假单磁畴与多磁畴的混合物.通过与北美古生代重磁化岩石的磁滞回线参数比值对比,说明重磁化与非重磁化岩五磁滞回线参数比值的分布趋势有明显差别,这一差别可能用于判定岩石的重磁化和非重磁化性质.