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.     

Thermomagnetic characteristics in late orogenic granites and gneisses of the southern Appalachian Piedmont

Thermomagnetic curves have been obtained for samples from 25 granites and gneisses in Georgia, South Carolina, and North Carolina. This data set can be divided into two distinct curve types: Type I includes rocks which exhibit no distinct Curie points and a linear decrease in magnetization with increasing temperature; Type II curves exhibit a distinct Curie point over a range of temperatures expected for magnetite (500–580°C). Synthetic samples constructed from magnetite and hematite powder exhibit Type I behavior when the hematite to magnetite ratio is high (e.g. 15 : 1). Examination of polished sections shows relatively coarse-grained magnetite only in Type II rocks. We interpret our data to indicate that Type I thermomagnetic curves are dominated by relatively large hematite/magnetite ratios while Type II granites are characterized by relatively coarse-grained magnetite. Type I granite samples have low magnetic susceptibility values (less than 4 × 10^?4 cgs) while most Type II granite samples have higher values. The Type I granites are invariably¹⁸O-enriched whereas Type II granites typically exhibit low¹⁸O/¹⁶O ratios. These relationships are consistent with previously reported correlations of susceptibility (generally indicative of magnetite content) and oxygen isotopic trends in the southern Appalachian Piedmont.     

秦岭造山带晚三叠世糜署岭岩体的岩石磁学及磁组构可靠性约束

近年来,针对秦岭造山带晚三叠世花岗岩体侵位机制的巨大争议,一些研究采用磁组构方法分析了岩体的内部组构特征及其与区域构造的关系,提出了具有重要意义的新认识.然而,目前这些研究均缺乏对岩体磁组构本质意义的分析,利用该方法约束岩体内部组构的可靠性并不十分清晰.针对这一问题,本文以秦岭造山带内具典型代表性意义的晚三叠世糜署岭花岗岩体为例,开展了该岩体的磁组构、岩石磁学、矿物形态组构和显微构造的综合研究.结果表明,糜署岭岩体的磁化率总体较低,属钛铁矿系列花岗岩.绝大部分样品的磁化率受控于顺磁性的黑云母等铁镁硅酸盐矿物,部分高磁化率样品包含了少量多畴磁铁矿等铁磁性组分的贡献,且随磁化率增大,铁磁性组分的贡献更为明显.样品的磁组构也主要是黑云母组构或由黑云母与磁铁矿的亚组构复合而成.由于样品中磁铁矿含量较低且与黑云母密切共生,磁组构与黑云母形态组构基本一致,因此,黑云母与磁铁矿的亚组构基本共轴.糜署岭岩体的磁组构本质上等同于黑云母组构,反映了黑云母等页硅酸盐矿物在岩体中的分布,可以有效的指示岩体的内部构造特征.宏观和显微构造观察还显示,糜署岭岩体的内部组构形成于岩浆侵位的晚期阶段,叠加了同岩浆期区域构造的关键信息,是从岩体构造角度开展区域构造演化的良好载体.     

Development of a magnetic method for reconstructing the paleoclimate of the rock formation time: A case study of the Paleolithic Kostenki-12 site section (the Voronezh region)

A magnetic method is proposed for reconstructing the paleoclimate existing at the formation time of rocks of the Paleolithic Kostenki-12 site section. Results obtained from an experimental study of scalar magnetic characteristics (SMCs) of the rocks (magnetic susceptibility, anhysteretic remanent magnetization, and saturation isothermal remanent magnetization) are presented. These results indicate complex along-section SMC behavior distinguished by alternating larger and smaller values of the parameters. The magnetic minerals of the section are mainly represented by pseudo-single-domain magnetite grains. The size of the magnetite grains in the pseudo-single-domain range varies along the section, increasing in its lower half. The SMCs values plotted on different scales for the upper and lower halves of the section clearly revealed 12 magnetic zones (MZs). Comparison of the MZs with palynological zones obtained in a parallel section showed, on the whole, a convergence of data: MZs with larger (smaller) SMC values correlate with thermomers (cryomers). Comparison of along-section SMC variations with the GRIP (Greenland Ice Core Project) curve of the time dependence of paleotemperatures confirmed or refined the ages of lithologic horizons. As a result, it is established that constraints on paleoclimatic changes in the rock formation time can be gained from SMC variations if the main carriers of magnetization in rocks are pseudo-single-domain magnetite grains even with magnetic grain size varying along the section.     

Magnetic Susceptibility Screening of Anthropogenic Impact on the Danube River Sediments in Northwestern Bulgaria - Preliminary Results

Several events of anthropogenic impacts on the floodplain river sediments of the Danube along a section near the town of Oryakhovo (NW Bulgaria) have been detected using magnetic technique. In the field, magnetic susceptibility was measured using three susceptibility meters – MS2D, KT-5 and SM30. The differences in the surface susceptibility values measured with the different instruments are ascribed to the various penetration depths which depend on several parameters like sensor diameter, frequency and field strength. This is supported by the detailed laboratory study on penetration depth and sensitivity of the new SM30 susceptibility meter and the comparison with the already existing data for the other two sensors. Boundaries between different flooding events are clearly linked with significant variability of the measured susceptibility values corresponding to one and the same level. Sediment cores, taken at different distances from the water level, show the history of depositional events and corresponding degree of anthropogenic pollution. Identification of the magnetic phases responsible for the signal was carried out by means of thermomagnetic (T) curves. The main carrier is magnetite with Tc of 580°C or oxidized magnetite with Tc of 600°C. A second kink at about 300°C is better expressed for samples with lower susceptibilities. It may correspond to several minerals, e.g. maghemite, pyrrothite, titanomagnetite, as well as different mineral transformations of paramagnetic minerals (decomposition, dehydroxilation, etc. for example siderite, lepidocrocite). The samples showing strong magnetic enhancement are characterized by the predominance of magnetite. Optical microscopy on magnetic extracts shows the presence of small spherical particles, typical for the anthropogenic magnetic phases from high-temperature technological processes. Well expressed susceptibility variations along the cores suggest the presence of several stages of different degree of pollution, covering an estimated period of about 30 years.     

Magnetic mineral diagenesis in anoxic laminated sediments from the Southern Gulf of California

Diagenetic effects on the magnetic mineralogy in marine sediments have long been investigated, including oxidation/reduction reactions, magnetic dilution, formation of iron sulfides and oxides, magnetization acquisition mechanisms and reliability of the paleomagnetic record. This study investigates diagenetic effects in low-oxygen depositional environments characterized by recent and past magnetic mineral dissolution zones. We analyze a marine sequence from the Alfonso Basin in the southern Gulf of California in which the topmost sediments show diagenetic effects marked by high magnetic enhancement factors. The susceptibility logs show high values at the top sediments with well-defined small amplitude low frequency fluctuations down core. Magnetic hysteresis loops indicate low coercivity saturation, characteristic of magnetites and low-tititanomagnetites with varying paramagnetic contributions. Intensity of natural remanent, isothermal and anhysteretic magnetizations and coercivity parameters show similar variation patterns with depth. The anhysteretic remanence intensity-susceptibility ratio shows an inverse correlation to magnetic susceptibility, indicating varying concentration of fine grained single domain and superparamagnetic particles. The magnetic logs record diagenetic changes and magnetite authigenesis, with preserved recent and old dissolution zones marked by enriched single-domain/pseudo-singledomain/multi-domain magnetite in between the dissolution fronts. The oxidation/reduction processes relate to climatic and water/sediment interface factors controlling the dissolution processes, which occur in the Alfonso Basin anoxic conditions.     

High magnetic susceptibility produced by thermal decomposition of core samples from the Chelungpu fault in Taiwan

We carried out thermomagnetic susceptibility analyses of fault rocks from core samples from Hole B of the Taiwan Chelungpu Fault Drilling Project (TCDP) to investigate the cause of high magnetic susceptibilities in the fault core. Test samples were thermally and mechanically treated by heating to different maximum temperatures of up to 900 °C and by high-velocity frictional tests before magnetic analyses. Thermomagnetic susceptibility analyses of natural fault rocks revealed that magnetization increased at maximum heating temperatures above 400 °C in the heating cycle, and showed three step increases, at 600 to 550 °C and at 300 °C during the cooling cycle. These behaviors are consistent with the presence of pyrite, siderite and chlorite, suggesting that TCDP gouge originally included these minerals, which contributed to the generation the magnetic susceptibility by thermomechanical reactions. The change in magnetic susceptibility due to heating of siderite was 20 times that obtained by heating pyrite and chlorite, so that only a small fraction of siderite decomposition is enough to cause the slight increase of the susceptibility observed in the fault core. Color measurement results indicate that thermal decomposition by frictional heating took place under low-oxygen conditions at depth, which prevented the minerals from oxidizing to reddish hematite. This finding supports the inference that a mechanically driven chemical reaction partly accounts for the high magnetic susceptibility. A kinetic model analysis confirmed that frictional heating can cause thermal decomposition of siderite and pyrite. Our results show that decomposition of pyrite to pyrrhotite, siderite and, to some extent, chlorite to magnetite is the probable mechanism explaining the magnetic anomaly within the Chelungpu fault zone.     

西秦岭温泉岩体的磁组构特征及其侵位机制意义

结合岩石磁学、磁化率各向异性度与区域构造分析了西秦岭温泉岩体的侵位机制及意义.温泉岩体的样品的平均磁化率k_m值总体很大,岩石磁学表明对于磁化率较低的样品,顺磁性矿物（如黑云母等）对磁化率的贡献较大,而少量铁磁性矿物（如磁铁矿等）可能作为剩磁载体.对于磁化率较高的样品,其主要载磁矿物为磁铁矿;花岗岩样品的校正磁化率各向异性度P_J总体小于1.2,显示了岩体为流动磁组构的特征,磁化率椭球体形态参数T总体大于0,扁率E总体大于1,以压扁椭球体为主;岩体的磁面理同磁线理相比更为发育,样品的磁面理普遍表现出围绕岩体边界分布的特点,且倾角较陡;而在岩体内部磁线理与磁面理分布相对散乱,定向性差,这一特征说明温泉岩体的磁组构主要由侵位时的侧向挤压作用形成的;虽然岩体的磁组构特征总体显示了N-NEE和SW向的挤压作用,但岩体侵位时由商丹缝合带闭合所产生的垂直于缝合带方向的挤压作用已相对较弱.本文认为,温泉岩体侵位时是一种弱挤压环境,或者是一种相对稳定的环境甚至可能是一种相对引张的背景,这与温泉岩体形成于后碰撞环境,秦岭造山带已演化至后碰撞拆沉作用发生的伸展阶段所反映的区域构造背景是一致的.     

苏北盆地兴化1#钻孔沉积物的岩石磁学特征

对苏北盆地兴化1#钻孔岩心沉积物进行了质量磁化率、频率磁化率、变化曲线、等温剩磁和磁滞回线等参数的测试分析，结果表明，粘土质沉积物中的主要载磁矿物为磁铁矿和赤铁矿；砂质沉积物中的磁性矿物除了磁铁矿和赤铁矿外，还含有少量的磁赤铁矿和针铁矿.针铁矿普遍存在于钻孔下部的样品中.整个钻孔沉积物中的磁性矿物颗粒都是介于单畴和多畴之间的准单畴颗粒，但粘土质沉积物中的磁性矿物颗粒更趋近于单畴颗粒，而砂质沉积物中的磁性矿物颗粒更趋近于多畴.根据不同的磁性矿物组合特征，选择合理的温度和磁场强度区间对古地磁退磁结果进行分析，得到沉积物可靠的特征剩磁方向，为古地磁年代学提供依据.     

Magnetic susceptibility variations of carbonates controlled by sea-level changes

This paper presents magnetic susceptibility results of 578 samples from three Devono-Carboniferous carbonate successions in the Yangtze Block. They indicate that the magnetic susceptibility variations in carbonates are controlled by ancient sea-level changes. It is particularly so if the supply of detrital materials was low, when the magnetic susceptibility was mainly controlled by the content of authigenic minerals. If the supply of detrital materials was high, the average susceptibility value would be higher, but there is still a correlation between susceptibility and sea-level curves. A sea level rise would lead to a decrease in the content of the detrital materials, and therefore to a decrease in the susceptibility values. On the top of HST, the uplift and exposure events may lead to elevated magnetic susceptibility values. Carbonate susceptibilities can therefore be considered as one of the environmental proxy data for the research of sequence stratigraphy.     

Magnetic susceptibility variations of carbonates controlled by sea-level changes--Examples in Devonian to Carboniferous strata in southern Guizhou Province, China

This paper presents magnetic susceptibility results of 578 samples from three Devono-Carboniferous carbonate successions in the Yangtze Block. They indicate that the magnetic susceptibility variations in carbonates are controlled by ancient sea-level changes. It is particularly so if the supply of detrital materials was low, when the magnetic susceptibility was mainly controlled by the content of authigenic minerals. If the supply of detrital materials was high, the average susceptibility value would be higher, but there is still a correlation between susceptibility and sea-level curves. A sea level rise would lead to a decrease in the content of the detrital materials, and therefore to a decrease in the susceptibility values. On the top of HST, the uplift and exposure events may lead to elevated magnetic susceptibility values. Carbonate susceptibilities can therefore be considered as one of the environmental proxy data for the research of sequence stratigraphy.     

Magnetic mineralogy and paleomagnetism of serpentinized ultramafic rocks from the Braszowice-Brzeźnica fragment of sudetic paleozoic ophiolite

Complex paleomagnetic, rock-magnetic and mineralogical studies were performed on serpentinized utramafic rocks from Braszowice-Brze?nica massif (BB) situated at the southern extremity of the Niemcza Shear Zone, close to the Sudetic Marginal Fault. Studies of magnetic minerals revealed presence of several varieties of magnetite grains, and partly altered Cr-spinels. Paleomagnetic studies revealed stable component HS of natural remanence carried by magnetite formed probably during the initial serpentinization. The directions of HS have good grouping within each locality, but differ between localities. Studies of anisotropy of magnetic susceptibility (AMS) have shown that directions of anisotropy axes are steep instead of horizontal as is common within the Niemcza Shear Zone. Taking advantage of the directions of AMS we rotated the directions of HS and brought them to the reference direction for the Sudetes for 372 Ma. We suggest that after acquiring AMS and HS during the Upper Devonian, the BB became divided into multiple units due to tectonic activity in the region.     

Effect of hydrocarbon-contaminated fluctuating groundwater on magnetic properties of shallow sediments

We investigate magnetic phase (trans)formation in the presence of petroleum hydrocarbons and its relation to bacterial activity, in particular in the zone of remediation driven fluctuating water levels at a former military air base in the Czech Republic. In a previous study an increase of magnetite concentration from the groundwater table towards the top of the groundwater fluctuation zone (GWFZ) was reported, however with limited reliability as there was no control on small-scale effects. To recognize statistically significant magnetic signatures versus depth, we obtained multiple sediment cores from three locations in January 2011 and April 2012, penetrating the unsaturated zone, the GWFZ and the uppermost one meter below the groundwater level (～2.3 m depth at the time of sampling). Magnetic concentration variation versus depth was determined by measuring magnetic susceptibility (MS) and remanence parameters. Small-scale features were identified and eliminated by statistical processing of multiple cores. A trend of increasing MS values from the lowermost position of the groundwater table upward was verified and highest magnetic concentration was found at the top of the GWFZ. Magnetic mineralogy indicates that newly formed fine-grained magnetite in the single domain to small pseudo-single domain range is responsible for the MS enhancement confirming previous results. There is no correlation with the depth variation of hydrocarbon (HC) concentrations; however, total organic carbon is linked to MS and may represent a degradation product of HC that is bioavailable for microorganisms. Bacterial activity is likely responsible for magnetite formation as indicated by most probable number (MPN) results of iron-metabolizing bacteria. The comparison of our results with an earlier study conducted at the same site revealed that magnetic concentration clearly decreased since remediation was terminated in 2008, possibly due to dissolution of magnetite.     

Magnetic studies of basalt fragments recovered by deep drilling in Iceland,and the “magnetic layer” concept

The magnetic susceptibility of 1300 samples of igneous rock drill cuttings obtained from eight deep drill holes in Iceland has been measured, in order to directly provide limits on the thickness of the layer which is the source of the magnetic anomalies over Iceland. The remanent magnetism of some of the material has also been studied, and the variation of magnetic susceptibility in 740 lava flows from eastern Iceland has been analysed as a function of depth of burial.All the results indicate no systematic change of susceptibility with depth up to 2.0 km. The Curie point of all deeply buried basalts in Iceland appears to be close to that of magnetite, so that the magnetic layer may be 5 km or more in thickness when susceptibility contrasts are considered; lateral contrasts in primary remanence may reach to 3 km depth. Derivation of a magnetic layer thickness in Iceland from analyses of magnetic anomalies, using methods which have been conventionally applied to marine magnetic anomalies could, on the other hand, yield much lower apparent thickness values (less than 1 km).We therefore argue that estimates of the magnetic layer thickness in oceanic regions should be based on considerations of magnetite Curie point isotherm behaviour, rather than on anomaly analysis.     

USEFULNESS OF WELL REGISTRATION DATA

Abstract. This investigation was to determine whether data on irrigation-well registration forms as reported by well drillers can be used in hydrologic studies. Transmissivity maps were prepared for Hamilton County, Nebraska using only specific capacity values computed from yield and drawdown data, as reported on irrigation well-registration forms. On one map each control point was the mean of the transmissivity values calculated for all wells in a section. On a second map each control point was the mean of the means for four contiguous sections sharing a common corner. The second map was similar to that of a previously prepared map based on control point values computed from specific capacities of selected wells, test hole logs, and geologic interpretations. These results suggest that reported registration data for a given area have a normal population distribution. In situations where individual data are unverified the data base can be treated as samples of a population where the mean of the means of several samples for adjacent areas is representative of actual field conditions.     

临汾市树叶磁性的时空变化特征及其对大气重金属污染的指示

粉煤灰经工厂废气排放进入大气,对人类健康和生态系统都造成了无法弥补的破坏.本文选取具有高空间分辨率优势的树叶作为收集粉煤灰的载体,对临汾市大气中可吸入颗粒物进行磁学参数和重金属含量监测.结果表明,磁化率最大值出现在工厂污染源附近,磁化率空间分布呈现随污染源距离增加而降低的趋势.工业区收集到的磁性颗粒以低矫顽力、粗粒度的磁铁矿为主.夏季磁性矿物来源单一,主要为人为影响.冬季大气中悬浮的磁性颗粒有部分来自于西北风/北风的自然尘降.同一采样点磁化率随时间变化特征表明,树叶的磁学性质可以灵敏和有效地反映较短时期内大气污染的现状.统计分析表明磁化率和重金属元素(铁,铬,镍,铜,铅,钴)之间存在显著相关性.污染负荷指数用于评估研究区域内重金属各元素综合污染的程度.结果显示,在废弃的旧工业区附近无大气污染指示,但在运营中的工厂集中的地区,大气均受到严重污染.污染负荷指数与表征磁性矿物含量的磁化率呈相关性(r2=0.66),因此树叶的磁性参数可以作为大气重金属污染的替代指标.     

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...     

Magnetic signature of hydrocarbon-contaminated soils and sediments at the former oil field H?nigsen, Germany

Magnetic properties of hydrocarbon (HC) containing soils and sediments from two sites (Site A and B) of the former oil-field H?nigsen were analyzed in order to determine whether magnetic methods can be employed to delineate HC contamination of soils and sediments. Magnetic parameters such as magnetic susceptibility and induced isothermal remanent magnetizations, as well as soil and sediment properties such as pH, iron content and water content, HC content and most probable number counts of iron-metabolizing microorganisms were determined. The magnetic concentration-dependent parameters for HC contaminated samples were 25 times higher in soils from Site A than in sediment samples from Site B. However, at Site B the magnetic susceptibility was still four times higher in comparison to lithologically similar non-contaminated sediment samples from a third Site C. Newly formed magnetite containing mainly single domain particles was responsible for the magnetic enhancement, whereas superparamagnetic grains represented only a minor component. Site A had an acidic pH compared to neutral pH at Site B, and a higher crystalline and bioavailable total iron content. Nevertheless, Site B samples contained significant numbers of both iron(II)-oxidizing and iron(III)-reducing microorganisms indicating that microbial iron cycling might have taken place at this site and potentially played a role for iron mineral transformation, including magnetite (trans)formation. The content of total non-polar hydrocarbons (TNPH) at Site A was one order of magnitude higher than at Site B. Only at Site A magnetic susceptibility correlated well with TNPH. Our results demonstrate that HC contaminated samples had an enhanced magnetite content compared to non-contaminated soils and sediments. Therefore, magnetic methods may provide a quick and cost-effective way to assess HC contamination in soils and sediments. However, more field sites and laboratory investigations are needed to reveal the complex nature of the processes involved.     

Ficus benjamina leaves as indicator of atmospheric pollution: a reconaissance study

We present a diagnostic study to evaluate the suitability of Ficus benjamina tree leaves as a captor of heavy metal particles from atmospheric dusts in urban areas. Leaf samples were taken at 16 localities within three areas in Morelia, state of Michoacán (Mexico??s medium size city, 830000 inhabitants). Measurements of magnetic susceptibility were conducted to determine the magnetic enhancement using samples from green, relatively unpolluted area, as a reference. The samples collected at areas with heavy traffic (main avenues) yielded values almost ten times higher than the values obtained for the unpolluted reference. Isothermal Remanent Magnetization curves are proportional to the degree of pollution. Associations of almost pure magnetite with heavy metals were revealed by scanning electron microscope.     

北京密云水库表层沉积物磁性矿物的鉴别

本文对密云水库表层沉积物中的磁性矿物进行了岩石磁学和透射电子显微学的综合研究.本实验建立的磁选方法实现将70%~85%左右的磁性矿物从沉积物中分离出来.岩石磁学研究表明,密云水库沉积物中的磁性矿物以多畴和单畴磁铁矿为主,还含有少量高矫顽力弱磁性载磁矿物(可能为赤铁矿).对磁选矿物的透射电镜观测表明,样品中部分单畴磁铁矿具有纳米尺寸和化学纯度高等特点,为拉长的立方-八面体磁铁矿,是趋磁细菌产生的化石磁小体;多畴磁铁矿多数具有微米尺寸,形状不规则,为碎屑成因;超顺磁磁铁矿粒径约为5~20nm,且含硅、铝等元素,可能为自生成因.研究结果表明,岩石磁学和透射电子显微学的综合应用可以更全面、准确地分析沉积物中磁性矿物的成分、含量、粒径和化学成分等信息,为环境磁学、生物地磁学和古地磁学研究提供依据.