Magnetic anisotropy of calcite at room-temperature

The intrinsic room temperature magnetic properties of pure calcite were determined from a series of natural crystals, and they were found to be highly dependent on the chemical composition. In general, dia-, para-, and ferromagnetic components contribute to the magnetic susceptibility and the anisotropy of magnetic susceptibility (AMS). With a combination of magnetic measurements and chemical analysis these three contributions were determined and related to their mineralogical sources. The intrinsic diamagnetic susceptibility of pure calcite is − 4.46 ± 0.16 × 10^− 9 m³/kg (− 12.09 ± 0.5 × 10^− 6 SI) and the susceptibility difference is 4.06 ± 0.03 × 10^− 10 m³/kg (1.10 ± 0.01 × 10^− 6 SI). These diamagnetic properties are easily dominated by other components. The paramagnetic contribution is due to paramagnetic ions in the crystal lattice that substitute for calcium; these are mainly iron and manganese. The measured paramagnetic susceptibility agrees with the values calculated from the known concentration of paramagnetic ions in the crystals according to the Curie law of paramagnetic susceptibility. Substituted iron leads to an increase in the AMS. The paramagnetic susceptibility difference was found to correlate linearly with the iron content for concentrations between 500 and 10,000 ppm. An empirical relation was determined: (k₁ − k₃)^para (kg/m³) = Fe-content (ppm) × (1 ± 0.1) × 10^− 12 (kg/m³/ppm). The maximum susceptibility difference (Δk = k₁ − k₃) was found to be unaffected by iron contents below 100 ppm. Ferromagnetic contributions due to inclusions of ferromagnetic minerals can dominate the susceptibility. They were detected by acquisition of isothermal remanent magnetization (IRM) and their contribution to the AMS was separated by high-field measurements.     

Anisotropy of magnetic susceptibility as an indicator for palaeocurrent analysis in folded turbidites (Outer Western Carpathians,Poland)

The anisotropy of magnetic susceptibility is a well-known geological proxy in revealing the directional tectonic and sedimentological features of rocks, although it can be ambiguous in situations where these two factors co-occur. This paper demonstrates the usefulness of the anisotropy of magnetic susceptibility in determining palaeotransport directions in turbiditic rocks that underwent subsequent thrusting and folding. This study demonstrates that the magnetic lineation is largely unsuitable as a palaeocurrent direction proxy, and suggests that the imbrication of magnetic foliation is better in such cases. Moreover, the anisotropy of magnetic susceptibility results were analyzed in reference to a joint and fold study within the framework of the regional structural geology. Magnetic fabric investigations were conducted in the eastern part of the Outer Western Carpathians (south-east Poland). During the study, a total of 191 oriented palaeomagnetic samples were collected from three outcrops (Nasiczne, Dwernik and Hoczew) in the Krosno Beds, Silesian Unit. For the purpose of sedimentological analysis, 121 m of turbidite successions were documented and 126 directional sedimentary structures were measured. The magnetic anisotropy of sandstones revealed typical sedimentary fabrics, often overprinted by variably intense tectonic deformation. Oblate susceptibility ellipsoids from Nasiczne showed tilt coherent with the palaeoflow direction, whereas the rocks from Dwernik and Hoczew contained triaxial magnetic fabric developed during compressional palaeostress. This paper suggests that medium-grained and coarse-grained sandstones, preferably with high mica content, are the most suitable for palaeotransport reconstructions among the studied lithologies.     

Effects of magnetic interactions in anisotropy of magnetic susceptibility: Models, experiments and implications for igneous rock fabrics quantification

Besides granites of the ilmenite series, in which the anisotropy of magnetic susceptibility (AMS) is mainly controlled by paramagnetic minerals, the AMS of igneous rocks is commonly interpreted as the result of the shape-preferred orientation of unequant ferromagnetic grains. In a few instances, the anisotropy due to the distribution of ferromagnetic grains, irrespective of their shape, has also been proposed as an important AMS source. Former analytical models that consider infinite geometry of identical and uniformly magnetized and coaxial particles confirm that shape fabric may be overcome by dipolar contributions if neighboring grains are close enough to each other to magnetically interact. On these bases we present and experimentally validate a two-grain macroscopic numerical model in which each grain carries its own magnetic anisotropy, volume, orientation and location in space. Compared with analytical predictions and available experiments, our results allow to list and quantify the factors that affect the effects of magnetic interactions. In particular, we discuss the effects of (i) the infinite geometry used in the analytical models, (ii) the intrinsic shape anisotropy of the grains, (iii) the relative orientation in space of the grains, and (iv) the spatial distribution of grains with a particular focus on the inter-grain distance distribution. Using documented case studies, these findings are summarized and discussed in the framework of the generalized total AMS tensor recently introduced by Cañon-Tapia (Cañon-Tapia, E., 2001. Factors affecting the relative importance of shape and distribution anisotropy in rocks: theory and experiments. Tectonophysics, 340, 117–131.). The most important result of our work is that analytical models far overestimate the role of magnetic interaction in rock fabric quantification. Considering natural rocks as an assemblage of interacting and non-interacting grains, and that the effects of interaction are reduced by (i) the finite geometry of the interacting clusters, (ii) the relative orientation between interacting grains, (iii) their heterogeneity in orientation, shape and bulk susceptibility, and (iv) their inter-distance distribution, we reconcile analytical models and experiments with real case studies that minimize the role of magnetic interaction onto the measured AMS. Limitations of our results are discussed and guidelines are provided for the use of AMS in geological interpretation of igneous rock fabrics where magnetic interactions are likely to occur.     

井中与钻孔岩芯磁化率测量效果的对比分析

为了解岩(矿)石磁化率测定法(磁化率仪)在反映钻孔中岩层磁性变化规律方面的效果,对同一钻孔分别运用岩(矿)石磁化率测定法和井中磁化率测量法进行了对比分析研究。结果表明,岩(矿)石磁化率仪测定法具有成本低、速度快、灵敏度高等特点,可以作为井中磁化率测量的一部分,起到井中磁化率测量在准确确定矿层厚度、位置、矿层品位等方面的作用,避免测井设备的笨重所带来的麻烦,特别是能够弥补井中磁化率测量在有套管存在井段的严重数据不足之缺陷。     

Tertiary geodynamics of Sakhalin (NW Pacific) from anisotropy of magnetic susceptibility fabrics and paleomagnetic data

Sakhalin has been affected by several phases of Cretaceous and Tertiary deformation due to the complex interaction of plates in the northwest Pacific region. A detailed understanding of the strain is important because it will provide constraints on plate-scale processes that control the formation and deformation of marginal sedimentary basins. Anisotropy of magnetic susceptibility (AMS) data were obtained from fine-grained mudstones and siltstones from 22 localities in Sakhalin in order to provide information concerning tectonic strain. AMS data reliably record ancient strain tensor orientations before significant deformation of the sediments occurred. Paleomagnetically determined vertical-axis rotations of crustal rocks allow rotation of the fabrics back to their original orientation. Results from southwest Sakhalin indicate a N035°E-directed net tectonic transport from the mid-Paleocene to the early Miocene, which is consistent with the present-day relative motion between the Okhotsk Sea and Eurasian plates. Reconstruction of early–late Miocene AMS fabrics in east Sakhalin indicates a tectonic transport direction of N040°E. In west Sakhalin, the transport direction appears to have remained relatively consistent from the Oligocene to the late Miocene, but it has a different attitude of N080°E. This suggests local deflection of the stress and strain fields, which was probably associated with opening of the northern Tatar Strait. A northward-directed tectonic transport is observed in Miocene sediments in southeast Sakhalin, mid-Eocene sediments in east Sakhalin, and in Late Cretaceous rocks of west and northern Sakhalin, which may be associated with northwestward motion and subduction of the Pacific Plate in the Tertiary period. The boundaries of the separate regions defined by the AMS data are consistent with present-day plate models and, therefore, provide meaningful constraints on the tectonic evolution of Sakhalin.     

无侧限压缩条件下黏性土磁各向异性研究

近年来基坑、隧洞等开挖工程活动中的卸载导致黏性土体发生过大侧向变形,引起的工程事故越来越多。目前多采用测斜仪对基坑周围土体变形进行水平位移监测,然而由于软土的特殊工程性质,在监测中软土无法带动测斜仪一起变形,往往监测效果不尽人意,不能准确给出实际软土的真实变形。因此,需要加强开挖卸荷工程施工中土体的侧向变形监测技术研究工作。本文基于磁性矿物受荷载变形具有定向性特征,运用磁组构试验对无侧限压缩后的黏性土试样进行磁各向异性研究,同时与有侧限压缩试验变形测试结果进行比较,分析试验轴向应力、土样含水率对无侧限压缩条件下磁各向异性的影响规律,以及主磁化率值与应变的数学关系,探讨了由黏性土磁各向异性推求其受力变形规律的可行性。研究成果表明,受力作用的黏性土变形与其磁性矿物定向性具有较好的一致性。这一认识对于有效监测软土地区卸荷工程周围软土变形情况具有重要的理论意义和实际应用价值。     

Regional scale strain variations in Banded Iron Formations of Eastern India: results from anisotropy of magnetic susceptibility studies

Anisotropy of magnetic susceptibility (AMS) data are used as a tool to determine strain variations in different parts of the Banded Iron Formations (BIFs) of the Bonai Synclinorium, eastern India. AMS data of 88 cylindrical cores drilled from 29 samples collected from the limb and hinge parts of mesoscopic scale folds as well as different parts of the entire synclinorium are presented. It is found that the samples from limbs of small-scale folds and also from limbs of the regional scale synclinorium have higher degrees of anisotropy than the hinges. This is inferred to indicate that the limbs accommodated higher strain than the hinges. AMS orientation data are analysed in conjunction with field data. It is concluded that the magnetic fabric developed in the limbs as well as hinges of the BIFs of the study area is related to deformation and is not a manifestation of sedimentary fabric.     

磁化率各向异性与剪切带

岩石组构记录了地壳形成与演化的关键信息,提取这些信息对分析和恢复地球动力学过程具有重要意义.磁化率各向异性(AMS)是一种重要的岩石组构方法,可以有效地揭示岩石的应变特征,分析其地球动力学过程,是研究构造变形性质以及应力作用方式的有效手段.本文在梳理AMS的研究历史、主要成果和最新进展的基础上,系统阐述了AMS的基本原...     

末次间冰期以来黄土-古土壤序列的磁组构特征及其指示的古风向

黄土高原粉尘物质的搬运、沉积过程与来自中高纬度地区的季风密切相关。研究冬季风演化历史将有助于深入理解全球变化背景下,东亚地区气候变化的动力机制。通过对黄土高原的磁组构研究发现:1)各向异性度(P)与磁面理度(F)相关性较高,因此各向异性主要由磁面理引起,磁化率椭球体为压扁状;2)磁面理大致水平,K3近垂直于水平面;3)黄土平均磁面理为1.007,古土壤为1.004,古土壤层的磁面理与各向异性度均要低于其下伏的母质黄土层。成土作用在某种程度上破坏了黄土的原生磁组构;4)P,F和粉尘粒度间具有一定内在联系,一般而言冬季风越强,搬运的粉尘颗粒越粗,从而导致在沉积过程中形成较高的磁面理;5)K1方向的等面积赤平投影图和玫瑰花图解表明,末次间冰期以来该地区的主导风向为NW向冬季风。     

Magnetic Characterization of Amphibolites from the Fomopéa Pluton (West Cameroon): Their Implication in the Pan-African Deformation of the Central African Fold Belt

The Fomopea granitic pluton is emplaced in gnessic and amphibolitic basement.These gneissic and amphibolitic basement rocks are represented in the pluton's body as sub-rounded,elongated or stretched xe...     

Magmatic lineations inferred from anisotropy of magnetic susceptibility fabrics in Units 8, 9, and 10 of the Rum Eastern Layered Series, NW Scotland

The Eastern Layered Series of the Rum Layered Suite, NW Scotland, comprises a sequence of sixteen (30–150 m thick) cyclic units. The upper troctolite–olivine gabbro parts of each of these units exhibit small-scale modal layering and a pervasive, layer-parallel mineral lamination that is often associated with ‘soft-sediment’ deformation structures. A sporadic, macroscopic magmatic lineation measurable on mineral lamination surfaces is also observed in places. Anisotropy of magnetic susceptibility (AMS) fabrics were studied in three of these cyclic units, (8, 9, and 10) in the northern part of the Eastern Layered Series. Magnetic fabrics measured in the troctolites and gabbros yield one dominant trend in which magnetic foliations parallel magmatic layering and magnetic lineations trend NW–SE and plunge gently. Magnetic fabrics measured for two detailed traverses through Unit 10 on the northern side of Hallival also yield one dominant trend, similar to that measured elsewhere in the Eastern Layered Series. However, toward the centre of Unit 10 in each traverse, magnetic lineations sometimes plunge approximately downdip (SW) on the magnetic foliation planes. The implications of these results are discussed with reference to previous textural and fabric observations on Rum. A model is suggested in which weak linear arrangements of cumulus olivine and plagioclase crystals are developed due to slumping and soft-sediment deformation of unconsolidated crystal mushes during central sagging of the Rum Layered Suite.     

New insights into the deformation of a Middle Pleistocene glaciotectonised sequence in Norfolk,England through magnetic and structural analysis

North Norfolk is a classic area for the study of glacial sediments with a complex glaciotectonic deformational history, but the processes leading to the formation of some structures can be ambiguous. Anisotropy of magnetic susceptibility (AMS) analyses, providing quantitative fabric data, have been combined with the analysis of visible structures and applied to the Bacton Green Till Member, exposed at Bacton, Norfolk. Thermomagnetic curves, low temperature susceptibility and acquisition of isothermal remanent magnetism (IRM) reveal that the magnetic mineralogy is dominated by paramagnetic phases. The magnetic foliation is parallel to fold axial planes and weakly inclined to bedding, whilst the magnetic lineation is orientated parallel to stretching, indicated by the presence of stretching lineations and the trend of sheath folds. Variations in the orientation of the magnetic lineation suggest that the Bacton section has been subject to polyphase deformation. After subaqueous deposition, the sequence was overridden by ice and glaciotectonically deformed which involved stretching initially north–south, then east–west. These results show that AMS can be used to detect strain in three dimensions through a glaciotectonite where paramagnetic mineralogy is dominant. This approach therefore provides further support to the use of AMS as a fast, objective and accurate method of examining strain within deformed glacial sediments.     

Deformation microstructures of olivine and chlorite in chlorite peridotites from Almklovdalen in the Western Gneiss Region,southwest Norway,and implications for seismic anisotropy

Chlorite peridotites from Almklovdalen in southwest Norway were studied to understand the deformation processes and seismic anisotropy in the upper mantle. The lattice preferred orientation (LPO) of olivine and chlorite was determined using electron backscattered diffraction (EBSD)/scanning electron microscopy. A sample with abundant garnet showed [100] axes of olivine aligned sub-parallel to lineation, and [010] axes aligned subnormal to foliation: A-type LPO. Samples rich in chlorite showed different olivine LPOs. Two samples showed [001] axes aligned sub-parallel to lineation, and [010] axes aligned subnormal to foliation: B-type LPO. Two other samples showed [100] axes aligned sub-parallel to lineation, and [001] axes aligned subnormal to foliation: E-type LPO. Chlorite showed a strong LPO characterized by [001] axes aligned subnormal to foliation with a weak girdle subnormal to lineation. Fourier transform infrared (FTIR) spectroscopy of the specimens revealed that the olivines with A-type LPO contain a small amount (170 ppm H/Si) of water. In contrast, the olivines with B-type LPOs contain a large amount (340 ppm H/Si) of water.

The seismic anisotropy of the olivine and chlorite was calculated. Olivine showed Vp anisotropy of up to 3.8% and a maximum Vs anisotropy of up to 2.7%. However, the chlorite showed a much stronger Vp anisotropy, up to 21.1%, and a maximum Vs anisotropy of up to 31.7%. A sample with a mixture of 25% of olivine and 75% of chlorite can produce a Vp anisotropy of 14.2% and a maximum Vs anisotropy of 22.9%. Because chlorite has a wide stability field at high pressure and high temperature in the subduction zone, the strong LPO of chlorite can be a source of the observed trench-normal or trench-parallel seismic anisotropy in the mantle wedge as well as in subducting slabs depending on the dipping angle of slab in a subduction zone where chlorite is stable.     

藏南泽当地区大反向逆冲断层变形研究——以磁组构与EBSD组构分析为例

仁布-泽当逆冲断层是喜马拉雅大反向逆冲断层（GCT）在藏南地区的重要延伸部分,也是喜马拉雅造山带北部边界新生代最为活动的构造单元之一。新生代以来特提斯喜马拉雅的构造变形组构特征的研究对于深入理解碰撞造山带演化与高原隆升具有重要构造意义。本文综合GCT泽当-琼结段断层的宏观与微观变形特征,对断裂带石英脉、围岩中石英和云母矿物的电子背散射（EBSD）组构及断层两侧岩石磁组构（AMS）特征进行对比分析。结果表明对AMS主要贡献来自顺磁性云母、绿泥石等,磁化率各向异性椭球体以压扁状为主,磁面理与构造面理（劈理、断层面）基本重合,显示较强的构造变形磁组构特征;磁线理优选方向近南北向,且与观测北向逆冲断层方向一致,揭示剪切作用在变形过程中的持续作用。研究发现泽当地区GCT附近石英微观结构从围岩至断层区,石英至少呈现3种不同类型的微观变形机制：围岩区溶解蠕变、断裂带石英以膨凸重结晶和亚颗粒旋转重结晶作用为主。断裂带石英的c轴EBSD组构指示变形为低温（300~400℃）环境,其中黑云母的结晶学优选（CPO）与磁组构主轴优选方向存在高度的一致性,进一步证实了顺磁性矿物黑云母对AMS的主要贡献。综合研究表明泽当地区GCT的韧性变形是断层处在中上地壳韧性带的活动阶段变形的结果,也代表了特提斯喜马拉雅在碰撞、高原隆升期的变形主要特征。

     

磁化率各向异性的原理及应用实例

磁化率各向异性(AMS)在地质领域中的应用极为广泛,可以用来研究古流向造成的磁性矿物的定向排列,以及构造应力作用引起的岩石内磁性矿物的定向重结晶、定向排列及韧性变形。本文介绍了AMS的基本原理和参数,并并介绍了前人及作者应用AMS详细分析研究了二个实例:(1)以假多畴(MD)高钛磁铁矿为主要载磁矿物的玄武岩样品的AMS变化及其对构造运动的响应;(2)以MD磁铁矿为主要载磁矿物的湖泊沉积物样品在沉积过程中AMS变化。AMS可以灵敏地检测样品中磁性矿物的定向排列,因此在在地质领域中具有很好的应用前景。     

内蒙古中部地区岩矿石磁化率特征

为了解内蒙古中部的锡林浩特、巴林左旗等地区岩矿石磁性特征,更好地对该地区的航磁异常进行综合解释,使用航遥中心研制的ZH-1磁化率仪,于2009~2011年间先后四次在该地区进行了野外岩矿石磁化率实测。并在统计、分析实测磁化率数据的基础上,研究了该地区岩矿石磁化率分布情况,分别叙述了区内沉积岩、侵入岩、火山岩、变质岩、矿石及围岩等各种类型岩矿石的磁化率特征,对岩矿石磁化率与航磁、矿产间的联系进行探讨,为磁法勘查解释、地质找矿提供了重要依据。     

黄土沉积物原生组分磁化率与风力的物质分选

本研究选取我国西北地区典型荒漠表土和黄土-古土壤序列样品为研究对象,探讨黄土沉积物及其潜在物源区表土的原生磁性矿物浓度在不同粒级间的变化规律、黄土剖面中粒度与磁化率的关系,以验证中国黄土沉积物中磁性组分在搬运过程中是否存在分选作用及其变化规律。来自西北地区表土的地质证据表明,磁性组分主要集中于>63 μm和31~63 μm粒径组分,并以后者的含量最高;与之相对应,黄土沉积中原生磁性组分主要集中于31~63 μm粒径组分,但>63 μm粒径组分不再是主要载磁组分,其磁性显著低于31~63 μm粒径组分。同时,多个剖面的证据表明黄土沉积的原生磁化率与31~63 μm粒径组分的含量是同相位变化的,进一步支持沉积物中原生组分的磁化率受到风动力的分选作用影响。需要强调的是,沉积物原生磁化率的主要载磁粒径组分为31~63 μm的颗粒物,而并非最粗粒组分。本研究结果暗示,相较于沉积物的全样粒度,干旱半干旱地区风成沉积的磁化率在指示古风场强度方面具有潜力。

     

Use of magnetic susceptibility, density, and modal mineral data as a guide to the composition of granitic plutons

Magnetic susceptibility, density, modal mineral, and whole rock geochemical data have been collected from two granitic plutons in Quebec. Each of the first three variables is correlated with SiO₂ to some degree. Multiple regression using all three variables simultaneously as independent variables and SiO₂ as a dependent variable yielded pluton dependent equations which were then used to calculate SiO₂ values for each analysis. Calculated values of SiO₂ typically differ from the analysed values by 1.5% for the first pluton and 1.0% for the second. The polynomials thus can be used to estimate the SiO₂ value of unanalyzed samples with some confidence. Plots of calculated SiO₂ reveal details of chemical variation with the pluton which were not observed when using chemical data alone due to the small number of analyzed rocks. The method also provides an independent evaluation of possible analytical errors.     

The heat capacity of two natural chlorite group minerals derived from differential scanning calorimetry

The heat capacity of natural chamosite (X_Fe=0.889) and clinochlore (X_Fe=0.116) were measured by differential scanning calorimetry (DSC). The samples were characterised by X-ray diffraction, microprobe analysis and Mössbauer spectroscopy. DSC measurements between 143 and 623?K were made following the procedure of Bosenick et?al. (1996). The fitted data for natural chamosite (CA) in J?mol^?1?K^?1 give: C _p,CA = 1224.3–10.685?×?10³?×?T ^??0.5???6.4389?× 10⁶?×T ^??2?+?8.0279?×?10⁸?×?T ^??3 and for the natural clinochlore (CE): C _p,CE = 1200.5–10.908?×?10³?×T ^??0.5?? 5.6941?×?10⁶?×?T ^??2?+?7.1166?×?10⁸?×?T ^??3. The corrected C _p-polynomial for pure end-member chamosite (Fe₅Al)[Si₃AlO₁₀](OH)₈ is C _p,CAcor = 1248.3–11.116?× 10³?×?T ^??0.5???5.1623?×?10⁶?×?T ^??2?+?7.1867?×?10⁸×T ^??3 and the corrected C _p-polynomial for pure end-member clinochlore (Mg₅Al)[Si₃AlO₁₀](OH)₈ is C _p,CEcor = 1191.3–10.665?×?10³?×?T ^??0.5???6.5136?×?10⁶?×?T ^??2?+ 7.7206?×?10⁸?×?T ^??3. The corrected C _p-polynomial for clinochlore is in excellent agreement with that in the internally consistent data sets of Berman (1988) and Holland and Powell (1998). The derived C _p-polynomial for chamosite (C _p,CAcor) leads to a 4.4% higher heat capacity, at 300?K, compared to that estimated by Holland and Powell (1998) based on a summation method. The corrected C _p-polynomial (C _p,CAcor) is, however, in excellent agreement with the computed C _p-polynomial given by Saccocia and Seyfried (1993), thus supporting the reliability of Berman and Brown's (1985) estimation method of heat capacities.     

磁组构形成机理与动力学意义

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