Influence of magnetic interactions on the anisotropy of magnetic susceptibility: the case of single domain particles packed in globule aggregates

The influence of magnetic interactions on the anisotropy of magnetic susceptibility (AMS) have been largely studied by several theoretical models or experiments. Numerical models have shown that when magnetostatic interactions occur, the distributions of particles over the volume rather than their individual orientations control the AMS. We have shown recently from a comprehensive rock magnetic study and from a theoretical 2-dimensional (2-D) model that single domain particles closely packed in globule aggregates could produce strong local random interaction magnetic fields which could influence the magnetic susceptibility and decrease the degree of anisotropy. In this paper, we first present in detail this 2-D theoretical model and then we extend it to the 3-D case. The possible distribution function of the magnetostatic interaction fields comprises two extreme states: it is either isotropic or ordered. The former case corresponds to the thermal-demagnetized state while the second case corresponds to the alternating field (AF) demagnetized state. We show that when easy axes of magnetization are not uniformly distributed, the degree of anisotropy decreases as the interaction field increases in both AF- and thermal-demagnetized states in 2-D and 3-D geometry. Thus we conclude that random magnetic fields generated by a random arrangement of magnetic particles over the sample volume decrease the degree of anisotropy of AMS and may alter the magnetic fabric.     

Anisotropy of magnetic susceptibility of rocks in the Rayleigh Law region: Modelling errors arising from linear fit to non-linear data

The anisotropy of magnetic susceptibility (AMS) within the Rayleigh Law range was investigated theoretically, using mathematical modelling. It was revealed that the orientations of the principal susceptibilities and the shape parameter vary with field so weakly that these variations can be regarded as negligible from the practical point of view. The degree of AMS increases with field according to the degree of anisotropy of the initial susceptibility used and according to the intensity of susceptibility change with field of the mineral considered. The degree of AMS calculated using linear theory is very near to the degree of AMS following from the analysis of AMS within the Rayleigh Law range. If it is desirable to correct the field-dependent degree of AMS, a simple technique is suggested based on measurement of the AMS in two fields. fhrouda@agico.cz     

风成沉积物磁组构与中国黄土区第四纪风向变化

通过对黄土高原几个剖面黄土样品磁化率各向异性的初步研究，发现了风成沉积物中的磁组构特征，且它的形成受沉积作用控制并与黄土高原形成时的古风场有关，即风成沉积物磁化率椭球体主轴方向及各轴比值与磁性颗粒分布排列方式亦即与空气动力条件相关．阐明了风成沉积物磁组构形成机制及其与古风向的关系，提出一种能够定量研究黄土高原形成时古风场的方法，为研究黄土高原形成演化和第四纪以来气候变化提供了基础数据．     

Role of single-domain magnetic particles in creation of inverse magnetic fabrics in volcanic rocks: A mathematical model study

The role of single-domain (SD) magnetic particles in creation of inverse magnetic fabrics is investigated on simple mathematical models using a realistic estimate for SD intrinsic susceptibility. In contrast to the fraction created by multi-domain (MD) particles, in which the anisotropy of magnetic susceptibility (AMS) is controlled by both the grain AMS and intensity of the preferred orientation of the particles, the AMS of the SD fraction is controlled solely by the intensity of the preferred orientation. The degree of AMS of ensemble of SD grains with a preferred orientation is therefore much higher than that of the same ensemble of MD particles implying the existence of frequent inverse magnetic fabrics. However, the occurrence of inverse magnetic fabrics due to SD particles is more the exception than the rule. Consequently, the amounts of SD particles is probably in general low. Nevertheless, the presence of SD particles in amounts insufficient to create inverse magnetic fabrics may diminish the whole rock AMS substantially. This can be one of the reasons for relatively low AMS in volcanic rocks whose magnetic particles may be really small obeying the conditions for the existence of SD particles.     

Anisotropy of magnetic susceptibility variations in Icelandic columnar basalts

The anisotropy of magnetic susceptibility (AMS) has been determined for 81 samples from eleven horizontal columnar segments from two Icelandic near-vertical dikes. Near-vertical orientation of short AMS axes and corresponding long-axis horizontal orientations in most columns represents alignments consistent with compactive effects due to vertically directed forces exerted by overlying columns. These data are inconsistent with primary orientations resulting from thermal contractive stresses, but such stresses may be responsible for the secondary orientations observed.     

Magnetic fabrics of the Bloodgood Canyon and Shelley Peak Tuffs,southwestern New Mexico: implications for emplacement and alteration processes

Anisotropy of magnetic susceptibility (AMS) of the middle Tertiary Bloodgood Canyon and Shelley Peak Tuffs of the Mogollon-Datil volcanic field has been used to (1) evaluate the ability of AMS to constrain flow lineations in low-susceptibility ash-flow tuffs; (2) establish a correlation between magnetic fabric, magnetic mineralogy, tuff facies, and characteristics of the depositional setting; and (3) constrain source locations of the tuffs. The tuffs are associated with the overlapping Bursum caldera and Gila Cliff Dwellings basin. The high-silica Bloodgood Canyon Tuff fills the Gila Cliff Dwellings basin and occurs as thin outcrops outside of the basin. The older Shelley Peak Tuff occurs as thin outcrops both along the boundary between the two structures, and outside of the complex. AMS data were collected from 16 sites of Bloodgood Canyon Tuff basin fill, 19 sites of Bloodgood Canyon Tuff outflow, and 11 sites of Shelley Peak Tuff. Sites were classified on the basis of within-site clustering of orientations of principal susceptibility axes, based on the categories of Knight et al. (1986). Most microscopically visible oxide minerals in the Bloodgood Canyon Tuff outflow and basin fill, and in the Shelley Peak Tuff are members of the hematite-ilmenite solid solution series. However, IRM acquisition data indicate that Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff have magnetic mineralogy dominated by single- or pseudo-single-domain magnetite, and that the magnetic mineralogy of the Bloodgood Canyon Tuff outflow is dominated by hematite. Hematite in Bloodgood Canyon Tuff outflow is likely to be the result of deuteric and/or low-temperature alteration of magnetite and iron silicate minerals. Bulk magnetic susceptibility is higher in magnetite-dominated ash-flow tuff (Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff) than it is in hematite-dominated ash-flow tuff (Bloodgood Canyon Tuff outflow). Bloodgood Canyon Tuff outflow has the highest total anisotropy (H) of the three units, followed by Shelley Peak Tuff and Bloodgood Canyon Tuff basin fill. All three ash-flow tuffs are genearlly characterized by oblate susceptibility ellipsoids, with those of the Bloodgood Canyon Tuff basin fill nearest to spherical. At high values of total anisotropy, Shelley Peak Tuff susceptibility ellipsoids attain a prolate shape; those of Bloodgood Canyon Tuff outflow attain an increasingly oblate shape. Three factors may influence differences in total anisotropy and susceptibility ellipsoid shape: (1) ash which travelled the greatest distance before deposition may show the best development of magnetic fabric, particularly of magnetic lineation; (2) deposition of ash in a closed basin may inhibit laminar flow throughout the sheet and the resulting development of flow textures; and (3) replacement of magnetite and iron silicates preferentially oriented within the foliation plane by hematite with strong crystalline anisotropy may enhance the magnetic susceptibility within that plane. Scatter in AMS axis orientation within sites may result from: (1) greater orientation inaccuracy in block-sampled than in fielddrilled samples; (2) rheomorphism; and (3) low accuracy of AMS measurement in low-susceptibility ashflow tuffs. Evaluation of flow lineation based on AMS of sites with well-clustered K ₁ axes indicates that (1) Bloodgood Canyon Tuff basin fill flowed along a generally northwest-southeast azimuth; (2) Shelley Peak Tuff located on the boundary of the Bursum caldera and the Gila Cliff Dwellings basin flowed along a nearly east-west azimuth; and (3) Bloodgood Canyon Tuff outflow sites have K ₁ susceptibility axes generally radial to the Bursum-Gila Cliff Dwellings complex, but within-site scatter of K ₁ orientations is generally too large to draw conclusions about flow lineation orientation. Limited petrographic work on pilot thin sections adds flow direction information to AMS-derived flow lineation information.     

Anisotropy of magnetic remanence: A brief review of mineralogical sources,physical origins,and geological applications,and comparison with susceptibility anisotropy

The magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low-field magnetic susceptibility (AMS). However, alternative methods based on remanent magnetization (measured in the absence of a magnetic field) rather than induced magnetization (measured in the applied field) have distinct advantages for certain geological applications. This is particularly true for; (1) adjunct studies in paleomagnetism, in order to assess the fidelity with which a natural remanence records the paleofield orientation; (2) studies of weakly magnetic or weakly deformed rocks, for which susceptibility anisotropy is very difficult to measure precisely; and (3) quantitative applications such as strain estimation. The fundamental differences between susceptibility and remanence (and their respective anisotropies) are due to several factors: (1) susceptibility arises from all of the minerals present in a sample, whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals; (2) ferromagnetic minerals are generally more anisotropic than para- and diamagnetic minerals; (3) for ferromagnetic minerals, remanence is inevitably more anisotropic than susceptibility; and (4) a number of common minerals, including single-domain magnetites, possess an inverse anisotropy of susceptibility, i.e., they tend to have minimum susceptibility parallel to the long axis of an individual particle; remanence is immune to this phenomenon. As a consequence of all these factors, remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample.Contribution number 9102 of the Institute for Rock Magnetism, University of Minnesota.     

Flow and emplacement direction determined for selected basaltic bodies using magnetic susceptibility anisotropy measurements

Low-field anisotropy of magnetic susceptibility (AMS) has been determined for a total of 248 basaltic specimens taken from cross sections between the cooling interfaces of 6 subaerial lavas, 6 deep-sea lavas, and 6 intrusives (5 dikes and 1 sill). Statistically significant AMS clusters are exhibited by all the dikes examined and, based upon these clusters, derivation of emplacement direction becomes possible. Two lavas are observed to have statistically significant AMS clusters which can be used for flow direction determinations. The methods of emplacement and flow direction analysis are discussed as well as the statistics used. It is concluded that most of the dikes examined have low angle emplacement directions. A classification scheme for AMS data distributions is presented.The AMS analysis shows that intrusives and deep-sea lavas can be distinguished from subaerial lavas approximately 80% of the time by the random AMS ellipsoid orientations exhibited in subaerial lavas. Contrasts in the fluid properties, degassing, wall effects with subsequent distortion of the fluid, and grain interaction during the extrusion of subaerial lavas can be expected to distort magnetic grain alignment. Further effects such as convection and secondary processes contribute to yield the random distributions observed for most of these bodies.     

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.     

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.     

The internal structure of lava flows—insights from AMS measurements II: Hawaiian pahoehoe, toothpaste lava and 'a'

We studied the anisotropy of magnetic susceptibility (AMS) of 22 basaltic flow units, including S-type pahoehoe, P-type pahoehoe, toothpaste lava and 'a' emplaced over different slopes in two Hawaiian islands. Systematic differences occur in several aspects of AMS (mean susceptibility, degree of anisotropy, magnetic fabric and orientation of the principal susceptibilities) among the morphological types that can be related to different modes of lava emplacement. AMS also detects systematic changes in the rate of shear with position in a unit, allowing us to infer local flow direction and some other aspects of the velocity field of each unit. 'A' flows are subject to stronger deformation than pahoehoe, and also their internal parts behave more like a unit. According to AMS, the central part of pahoehoe commonly reveals a different deformation history than the upper and lower extremes, probably resulting from endogenous growth.     

Magnetic anisotropy study of a columnar basalt from San Anton,Morelos, Mexico

Anisotrophy of magnetic susceptibility (AMS) results from 27 specimens drilled from the top and two sides of a single columnar basalt segment are presented. The magnetic foliation plane is nearly horizontal for all parts of the column, which is consistent with a primary magma flow pattern, without evidence of local convection or differentiative processes. The shape of AMS ellipsoids is however predominantly prolate, which may be indicative of increased magnetic grain elongation due to crystal growth or grain realignment normal to a vertical stress field (due to thermal contraction). Apparent systematic variations related to column shape are found in bulk susceptibility, anisotropy degree and degree of lineation and foliation; some of the variation may also be related to weathering effects. The results are consistent with a primary AMS pattern resulting from thermal contractive stresses during column formation. Comparison of results from previous studies of columnar basalts reveals that there is a relatively large variation in AMS properties. There appears to exist a number of factors which may locally control the magnetic anisotropy of columns and very likely some of their other characteristics.     

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...     

嵊泗岛辉绿岩墙群的侵位方式:来自磁组构的证据

岩墙磁组构能反映岩浆的侵位方式.中国东部嵊泗岛广泛发育了晚白垩世辉绿岩岩墙群.我们对其中8条不同走向岩墙进行了采样,沿岩墙两边部及横跨岩墙剖面获得共273个独立定向岩芯样品.岩石磁学分析表明辉绿岩的主要携磁矿物为多畴贫钛磁铁矿,可能含少量磁赤铁矿.各条岩墙的磁组构均具有低的各向异性度Pj＜1.2,且主轴的空间方位各不相...     

Magnetic fabric, welding texture and strain fabric in the Nuraxi Tuff, Sardinia, Italy

Anisotropy of magnetic susceptibility (AMS) has been used to interpret flow directions in ignimbrites, but no study has demonstrated that the AMS fabric corresponds to the flow fabric. In this paper, we show that the AMS and strain fabric coincide in a high-grade ignimbrite, the Nuraxi Tuff, a Miocene rhyolitic ignimbrite displaying a wide variability of rheomorphic features and a well-defined magnetic fabric. Natural remanent magnetization (NRM) data indicate that the magnetization of the tuff is homogeneous and was acquired at high temperatures by Ti-magnetite crystals. Comparison between the magnetic fabric and the deformation features along a representative section shows that AMS and anisotropy of isothermal remanent magnetization (AIRM) fabric are coaxial with and reproduce the shape of the strain ellipsoid. Magnetic tests and scanning electron microscopy observations indicate that the fabric is due to trails of micrometer-size, pseudo-single domain, magnetically interacting magnetite crystals. Microlites formed along discontinuities such as shard rims and vesicle walls mimicking the petrofabric of the tuff. The fabric was thus acquired after deposition, before late rheomorphic processes, and accurately mimics homogeneous deformation features of the shards during welding processes and mass flow.     

巴基斯坦Bahawalpur黄土岩石磁学特征及磁化率变化机制研究

风成黄土是陆地上分布最广泛的沉积物之一,记载了各种古气候演化信息.目前巴基斯坦的黄土研究甚少,磁化率与气候对应的变化机制研究尚未开展.本文对位于巴基斯坦印度河平原Bahawalpur地区新发现的黄土-古土壤剖面进行系统的岩石磁学研究,结合粒度和漫反射光谱(DRS)数据,讨论巴基斯坦黄土的磁化率变化机制.实验结果显示:Bahawalpur(BH)剖面黄土层主要的载磁矿物为磁铁矿,同时含有少量磁赤铁矿和针铁矿,磁性颗粒以原生的MD和PSD颗粒为主.相对于黄土层,古土壤层则是以针铁矿为主,含有顺磁性矿物和少量磁铁矿.BH剖面磁化率与成土作用关系和中国黄土高原典型剖面相反,磁化率的变化可能存在一个阈值12.8×10^-8 m³·kg^-1,在阈值之上,强磁性矿物(磁铁矿、磁赤铁矿)占主导;阈值之下,以弱磁性矿物(主要是针铁矿)为主,这种磁性矿物的转变可能导致磁化率降低.本文可为今后利用磁化率解读该地区地层蕴含的古气候信息提供新线索.     

Correlation of strain with anisotropy of magnetic susceptibility (AMS)

Existing correlations between strain and anisotropy of low-field magnetic susceptibility (AMS) have been re-assessed using a single parameter to express both anisotropies. TheP parameter (Hrouda, 1982) shows potential as a powerful single expression of the intensity of strain and of AMS. Previous correlations are improved by use of this parameter. Cautious optimism is justified for correlations between strain and susceptibility in a certain strain window between a lower limit (excluding the incomplete overprint of predeformation anisotropy) and an upper limit (excluding the effects of saturation anisotropy). For successful correlations the influence of stress-controlled recrystallisation should be minimal and the mineralogical sources of susceptibility must predate deformation.     

Frequency-dependent susceptibility and other magnetic properties of Celtic and Mediaeval graphitic pottery from Bohemia: an introductory study

Frequency-dependent magnetic susceptibility, its anisotropy (AMS), its temperature variation, natural remanent magnetization and time-dependent isothermal remanent magnetization as well as M?ssbauer spectroscopy of a small collection of Celtic and Mediaeval graphitic pottery from Southern Bohemia were investigated. The mineral composition of the pottery is dominated by fragments of quartz, accompanied mainly by various silicates from granitoids and paragneisses, or by calcite, within the plastic component being probably illite but also graphite. No ferrimagnetic minerals were found in optical microscope, among Fe-oxides only limonite was observed, even though the bulk susceptibility of the pottery varies in the orders of 10^?4 to 10^?2 [SI]. This may indicate presence of ferromagnetic particles in the ultrafine (superparamagnetic, SP) state, which is confirmed by frequency-dependent susceptibility ranging from 3% to almost 16%. The low temperature susceptibility vs. temperature curves are only moderately sloped, showing the Verwey transition only in one case. The high temperature curves mostly show presence of two magnetic phases, maghemite and magnetite. Cooling curves show distinctly lower susceptibilities than the heating curves indicating instability of the assemblage of ferrimagnetic minerals, particularly in temperatures slightly under 700 °C. M?ssbauer spectroscopy confirmed the results of the frequency-dependent susceptibility, showing the increase of ferrimagnetic sextets in the spectra measured at 4.2K, likely indicating maghemite as the distinct ferrimagnetic phase. The frequency-dependent AMS indicates preferred orientation of SP1,16 particles, coaxiality between SP_1,16 grain AMS and whole specimen AMS indicate that all grains, ultrafine and coarser ones, were oriented by the same process, i.e. copying the pottery structure created during wheel-turning.     

Microstructure and magnetic fabric in the Shuanghe pluton: A synkinematic granite in Eastern Dabie Mountains,China

Strain analyses for the Shuanghe pluton show that the main strain planes suffered distinct deformation. The main strain value (XZ) is up to 1.59-2.18, and the value of Flinn index (K) ranges from 0.11 to 0.82. Anisotropy of magnetic susceptibility (AMS) measurements reveal that the orientations of the magnetic foliation and lineation gently dip SE, consistent with the macroscopic foliation of the pluton. The value of anisotropy degree (P) ranges from 1.109 to 1.639, and the shape parameter (7) from 0.079 to 0.534. These studies prove that the pluton was deformed under strong compression. Quartz c-axis textures, defined by monoclinic or triclinic asymmetry, usually developed the high maxima paralleling the b-axis, which is defined by the developed in the high-ultrahigh pressure rocks (UHP) which were captured in the pluton or country rocks. It is concluded that the Shuanghe pluton emplaced under regional compression slightly after the formation of UHP, and it is characterized by synkinematic granitic deformation.     

Anisotropy of magnetic susceptibility in welded tuffs: application to a welded-tuff dyke in the tertiary Trans-Pecos Texas volcanic province,USA

Consideration of published anisotropy of magnetic susceptibility (AMS) studies on welded ignimbrites suggests that AMS fabrics are controlled by groundmass microlites distributed within the existing tuff fabric, the sum result of directional fabrics imposed by primary flow lineation, welding, and (if relevant) rheomorphism. AMS is a more sensitive indicator of fabric elements within welded tuffs than conventional methods, and usually yields primary flow azimuth estimates. Detailed study of a single densely welded tuff sample demonstrates that the overall AMS fabric is insensitive to the relative abundances of fiamme, matrix and lithics within individual drilled cores. AMS determinations on a welded-tuff dyke occurring in a choked vent in the Trans-Pecos Texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke, while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width. The dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit, followed by residual squeezing between the failed block and in situ wallrock. Irrespective of the precise mechanism, widespread occurrence of both welded-tuff dykes and point-welded, aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process.