Anisotropy of magnetic susceptibility and rock magnetic applications in the Deccan volcanic province based on some case studies

Anisotropy of Magnetic Susceptibility (AMS) as a tool has been explored here to investigate the nature of petrofabrics in Deccan Volcanic Province (DVP) of west-central Indian region by representative sampling in typical pahoehoe and rubbly pahoehoe lava flows, dykes within flows, shear zone and the impact crater units. The rock magnetic analysis indicate varying degree of concentration of titanomagnetite compositions dominated by multi domain (MD) to pseudo single domain (PSD) grains favoring shape anisotropy of minerals that form primary fabrics. The pahoehoe type lava flows shows planar oblate fabrics without any preferred orientation of principle susceptibility axis (K₁) depicting crystal settling (of magnetic grains) as chief mechanism of fabric development. The rubbly pahoehoe type lava flow exhibit prolate fabrics with well clustered maximum susceptibility axis within horizontal to sub-horizontal planes depicting their response to viscosity shear. The dykes show well clustered K₁ parallel to it’s plane locked during rapid contractional cooling. The sampling at Lonar impact crater was unable to trace any clear fabric due to impact/shock induced deformation and rather preserve the primary fabrics. Further, the shear zone depict random fabrics demanding more detailed and systematic sampling in both the cases. The present investigation infer that the magnetic mineralogy and magnetic fabric variations in the DVP are controlled by the flow mechanism and style of cooling that is characteristic of the given flow unit or dyke and any secondary or superimposed fabric needs to be examined by critical sampling strategy. While more detailed attempts are required to establish the AMS as a tool to record various aspects including the flow dynamics and rate of effusion in the vast terrain of DVP; the present approach is useful to characterize and correlate the lava flow units and dyke occurrences.     

黄河源地区磁组构特征及其地质意义

黄河源区位于青藏高原东北部,区内主要为三叠系沉积地层,发育一系列由北向南的推覆构造带,间有早期近直立的韧性剪切带。笔者对黄河源地区巴颜喀拉山群沉积岩进行了磁组构分析,结果显示岩石磁化率各向异性度P值和磁化率百分率各向异性度H值均不大,反映该地区总体韧性变形较弱,较强韧性变形仅发育于局部地段;岩石磁组构具有磁面理发育、磁线理不发育、磁化率椭球呈压扁形椭球体的特点,反映在挤压应力作用下,岩石发生了压扁变形,主应力方位主要为NNE-SSW(近SN)向,其次为NE-SW向。根据岩石磁组构分析认为黄河源地区存在两条韧性剪切带,韧性剪切带与现今湖泊水体的展布有一定的耦合关系;北部韧性剪切带沿现今黄河河谷分布,控制着扎陵湖、鄂陵湖和玛多"四姐妹湖"的展布;南部韧性剪切带沿岗纳格玛错—野牛沟一线展布,控制着岗纳格玛错和尕拉拉错等残余湖泊的分布。     

The Chinamora batholith, Zimbabwe: structure and emplacement-related magnetic rock fabric

The origin of dome-and-keel structural geometries in Archean granite–greenstone terrains appears to lack any modern analogues and is still poorly understood. The formation of these geometries is investigated using structural and anisotropy of magnetic susceptibility (AMS) data for the Chinamora batholith in Zimbabwe. The roughly circular-shaped batholith is surrounded by ca. 2.72–2.64 Ga greenstones. The batholith granitoid suites have been divided on the basis of their ages and fabric relationships into four distinct units: (i) banded basement gneisses; (ii) granodioritic gneisses; (iii) equigranular granites; and (iv) central porphyritic granites. In the gneissic granites a partial girdle (N–S) of poles to the magnetic foliation is developed that has been folded around a consistent, flat lying magnetic lineation plunging at shallow angles to the E or W. In the equigranular granites, the magnetic lineation generally plunges to the NW. The magnetic foliation has a variable strike, no clear trends can be distinguished. The AMS measurements of the porphyritic granite revealed a NW–SE striking foliation and showed subhorizontal magnetic lineations. The magnetic foliation is subparallel to the macroscopic foliation. Wall rocks are moderately inclined and show radial or concentric lineations, triaxial strain ellipsoids and kinematics that demonstrate off-the-dome sliding and coeval pluton expansion. The results of the observations do not point to a single emplacement process. Neither the observed structural data nor the magnetic fabric support a model envisaging spherically ‘ballooning’. It is argued that pluton diapirism played a major part in the formation of the fabrics in the gneisses, whereas the fabrics in the porphyritic granites reflect emplacement as laccolith-like sheets.     

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。     

Magnetic forces in an oriented matrix of the high-gradient magnetic separator

A new expression has been derived for calculation of the magnetic force acting in the space of an oriented matrix of the high-gradient magnetic separator. The matrix geometry and the interaction of the individual matrix elements is taken into consideration. The new method for making visible the elevation of the paramagnetic fluid in the neighbourhood of the matrix rod element, which depends on the magnetic field intensity, allows of verifying theoretical calculations of the magnetic force and determining the direction of its highest effect.     

Small-scale mélange fabric between serpentinite block and matrix: Magnetic evidence from the Mitsuishi ultramafic rock body, Hokkaido, Japan

The Mitsuishi ultramafic rock body in Hokkaido, Japan, consists mainly of serpentinized peridotites that originated from a depleted mantle. This study aims to show new evidence of small-scale mélange fabric of serpentinite matrix in the rock body. Each serpentinite block in the serpentine matrix shows large and stable intensities of natural remanent magnetization (NRM). However, the directions of serpentinite blocks' NRM in the matrix are randomly scattered. A Curie temperature (Tc) of 580 °C corresponding to pure magnetite was also observed. Additionally, there is no evidence of heating over 580 °C after serpentinization. The blocks in the matrix must have obtained crystallization remanent magnetization (CRM) during serpentinization. The directions of the blocks' characteristic remanent magnetization (ChRM) are also scattered. It shows that serpentinite blocks were magnetized prior to uplifting. The results of the study indicate that the magnetic carrier of the serpentinite blocks in the matrix is mainly composed of magnetite, and it can keep original magnetization before uplifting. The results also imply that the scattering directions of NRM indicate the presence of small-scale mélange fabric of serpentinite matrix.     

阿尔泰南缘东段变形岩石磁组构分析

磁组构是指岩石磁化率的各向异性,磁组构方法已经被广泛应用于构造变形分析。阿尔泰南缘东段地区岩石磁组构特征是磁各向异性度P值大,反映本区总体韧性剪切变形强烈。萨尔布拉克—科克萨依脆性劈理化带和玛因鄂博韧性剪压构造带的E>0占优势,磁面理发育,部分样品磁线理发育,反映变形以压扁变形为主,主压应力方向为NE(NEE)向,伴随弱的左行剪切;达拉维孜—阿热勒托别韧性流变构造带和锡泊渡—富蕴深层次变晶糜棱岩带E>0和E<0均存在,磁面理和磁线理均发育,反映以剪切变形为主;其中达拉维孜—阿热勒托别构造带主压应力方向为NE(NEE)向,为左行剪切,而锡泊渡—富蕴构造带主压应力方向为SN向,为右行剪切。在达拉维孜—阿热勒托别构造带中的哈腊苏铜矿和卡拉先格尔—老山口一带一些叠加蚀变矿化的强变形岩石的P值明显减小,说明在韧性变形之后发生过矿化热液作用导致磁化率各向异性发生了均一化。结合区域构造分析,可以认为萨尔布拉克—科克萨依构造带、达拉维孜—阿热勒托别构造带和玛因鄂博构造带构成一个完整的板块碰撞聚合带,而锡泊渡—富蕴构造带可能为在古生代期间被强烈改造的具有前震旦纪结晶基底的微大陆的残留(或断片)。     

Magnetic and plagioclase linear fabric discrepancy in dykes: a new way to define the flow vector using magnetic foliation

In basaltic dykes the magnetic lineation K1 (maximum magnetic susceptibility axis) is generally taken to indicate the flow direction during solidification of the magma. This assumption was tested in Tertiary basaltic dykes from Greenland displaying independent evidence of subhorizontal flow. The digital processing of microphotographs from thin sections cut in (K1, K2) planes yields the preferred linear orientation of plagioclase, which apparently marks the magma flow lineation. In up to 60% of cases, the angular separation between K1 and the assumed flow direction is greater than 45°. This suggests that the uncorroborated use of magnetic lineations in dykes is risky. A simple geometrical method is proposed to infer the flow vector from AMS in dykes based solely on magnetic foliations.     

Magnetic susceptibility studies of the upper Albian-Danian sediments of Cauvery basin,south India

The Cauvery basin of the Indian Peninsular shield, formed during the fragmentation of the Gondwana Supercontinent, continued to evolve until the end of Neogene through rift, pull-apart, shelf sag and tilt processes. The basin witnessed many cycles of transgression, regression, erosion and deposition. A more or less complete succession of upper Cretaceous–Paleocene sediments is exposed in the Ariyalur–Pondicherry depression of the basin. Anisotropy of magnetic susceptibility (AMS) technique is applied in an attempt to examine the depositional and eustatic conditions prevailed during the formation of these sedimentary sequences. AMS results point to the sedimentation history dominated by marked sea level changes with several phases of transgression and regression. The sedimentation occurred mainly in a shallow epicontinental sea which has been punctuated with terrigenous supply more often. Greater utility of magnetic fabrics is suggested as a tool to trace fluvial responses to tectonic and climatic changes.     

Error analysis of frequency-dependent magnetic susceptibility measurements: magnetic viscosity studies with the Bartington MS2 system

Magnetic viscosity of rocks associated with magnetic relaxation of ultrafine ferrimagnetic mineral grains (superparamagnetism) is employed in magnetic grain size measurements. Magnetic viscosity is most often estimated from dual frequency measurements of magnetic susceptibility. The measured susceptibility values bear uncertainty that comprises two components: an instrument error and a drift. The instrument error refers to the accuracy of the measurement system and shows how precise the data are in ideal operation conditions. This error affects especially the low susceptibilities of weak samples, which thus should be measured on a high sensitivity range. Drift is due to external factors, such as changes in the temperature of sensors and/or samples, as well as in the orientation of the samples relative to the sensor, vibration, electromagnetic noise, etc. Drift, more critical for measurements on strong samples, is manageable by the operator. To reduce drift, every effort should be made to maintain suitable ‘quiet’ operation conditions.     

Magnetic properties of the clinopyroxenes aegirine and hedenbergite: a magnetic susceptibility study on single crystals

Two natural clinopyroxene single crystals were investigated, an aegirine-augite (AEG) and a magnesian hedenbergite (HED). Both samples were carefully characterized by electron microprobe, X-ray diffraction, and Mössbauer spectroscopy. Magnetic susceptibility measurements of powdered samples reveal low temperature antiferromagnetic coupling and Curie-Weiss behaviour with T _N =7.5(5)?K, Θ _P =?19(1)?K for AEG, and T _N =31(1)?K, Θ _P =+21(1)?K for HED, respectively. Low temperature Mössbauer spectra exhibit relaxation phenomena. Magnetic susceptibility measurements of the single crystals show the direction of the magnetic moments to be lying within the a/c plane for both samples: 50(±2)° from a and 57(±2)° from c in AEG, and 45(±2)° from a and 60(±2)° from c in HED, respectively. The antiferromagnetic interchain interaction competes with the ferromagnetic intrachain interaction in both pyroxenes. In the magnesian hedenbergite a field induced magnetic transition is found. Its dependence on temperature, magnetic field and crystallographic direction is investigated and described.     

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.     

Magnetic fabric in folds of the easternmost Rheno-Hercynian Zone

The magnetic fabric in folds was investigated in the easternmost Rheno-Hercynian Zone, the Nízký Jeseník Mts. In their eastern areas, the rocks show signs of only weak achimetamorphism and very gentle ductile deformation; SE vergent buckle folds of long wavelength are developed whose magnetic fabric can be easily unfolded geometrically. In the central areas, spaced cleavage and NW vergent buckle folds can be found; the folds can be unfolded mostly only partially. In the western areas, NW vergent cleavage folds and very well developed slaty cleavage occur. The magnetic fabric in the folds is homogeneous, the folds cannot be unfolded at all. The cleavage is transformed into metamorphic schistosity at the western border of the area.     

Magnetic susceptibility of natural diamonds

Doklady Earth Sciences -     

Anisotropy of magnetic susceptibility of eclogites: Mineralogical origin and correlation with the tectonic fabric (Cabo Ortegal,Spain)

The fabric and the anisotropy of magnetic susceptibility of the Cabo Ortegal eclogite (NW Spain) are studied. These mafic rocks were metamorphosed and deformed under high pressures and temperatures between 390 and 370 Ma in a subduction/collision tectonic setting. Massive eclogite slices and deformed eclogite in shear zones have bulk magnetic susceptibilities of 31 to 82 · 10⁻⁵ S.I. and 28 to 75 · 10⁻⁵ S.I., respectively. The paramagnetic mineral fraction is the principal magnetic susceptibility carrier. This fraction includes notably garnet and clinopyroxene as matrix minerals, and ilmenite and rutile as accessory constituents. Though magnetic anisotropy degree varies between 3.1 % and 6.6%, variations of this parameter in each rock type are marked. In the deformed eclogite, magnetic lineation (K_max) and the pole to the magnetic foliation (K_min) are coaxial and coincident with macroscopic petrofabric elements (foliation and lineation). In the massive eclogite, the magnetic fabric is dispersed along the principal structural planes and inversions are associated with samples with small degrees of anisotropy. The anisotropy of magnetic susceptibility is interpreted as being due to the crystallographic preferred orientation and spatial organisation of the polymineralic aggregate. Relating the evolution of the symmetry of magnetic fabric to the symmetry of petrofabric or deformation is rather precluded since susceptibility has multiple origins and bulk magnetic fabric is due to minerals of different symmetry.     

An anisotropy of magnetic susceptibility (AMS) investigation of the till fabric of drumlins: support for an accretionary origin

This paper describes the results of a spatially dense anisotropy of magnetic susceptibility (AMS) till fabric study of a single drumlin in the Weedsport Drumlin Field, New York State, USA. AMS till fabrics provide a robust, quantitative and unbiased approach to assess subglacial till kinematics and infer ice‐flow dynamics. The drumlin selected for this detailed investigation was systematically sampled at 18 locations to evaluate the patterns of ice flow and associated till kinematics within a drumlin and to test erosional vs. depositional models for its formation. AMS till fabric analysis yielded strong fabrics that increase in strength towards the drumlin crest, indicating that bed deformation occurred during till deposition and that deformation within the drumlin was greater than that in the interdrumlin low. Fabric orientations reveal drumlin convergent, divergent and parallel ice‐flow paths that illustrate a complex interaction between ice flow and the drumlin form; fabric strength and shape reveal systematic differences in bed deformation between the interdrumlin and drumlin regions. These observations are inconsistent with purely erosional models of drumlin genesis; instead, these observations are more consistent with syndepositional streamlining of till transported, probably locally as a deforming bed, from the interdrumlin low towards the drumlin locality.     

Anisotropy of magnetic susceptibility of eclogites: mineralogical origin and correlation with the tectonic fabric (Cabo Ortegal,Spain)

Abstract

The fabric and the anisotropy of magnetic susceptibility of the Cabo Ortegal eclogite (NW Spain) are studied. These mafic rocks were metamorphosed and deformed under high pressures and temperatures between 390 and 370 Ma in a subduction/collision tectonic setting. Massive eclogite slices and deformed eclogite in shear zones have bulk magnetic susceptibilities of 31 to 82·10^?5 S.I. and 28 to 75·10^?5 S.I., respectively. The paramagnetic mineral fraction is the principal magnetic susceptibility carrier. This fraction includes notably garnet and clinopyroxene as matrix minerals, and ilmenite and rutile as accessory constituents. Though magnetic anisotropy degree varies between 3.1 % and 6.6 %, variations of this parameter in each rock type are marked. In the deformed eclogite, magnetic lineation (K_max) and the pole to the magnetic foliation (K_min) are coaxial and coincident with macroscopic petrofabric elements (foliation and lineation). In the massive eclogite, the magnetic fabric is dispersed along the principal structural planes and inversions are associated with samples with small degrees of anisotropy. The anisotropy of magnetic susceptibility is interpreted as being due to the crystallographic preferred orientation and spatial organisation of the polymineralic aggregate. Relating the evolution of the symmetry of magnetic fabric to the symmetry of petrofabric or deformation is rather precluded since susceptibility has multiple origins and bulk magnetic fabric is due to minerals of different symmetry. © Elsevier, Paris     

Structural and magnetic fabric studies of recess structures in the western Himalaya: Implications for 1905 Kangra earthquake

Kinematic information from deformation structures and magnetic fabrics are used to infer recent tectonics around Kangra and Dehradun recesses, western Himalaya. Three types of magnetic fabrics (Type I & II, III, and IV) are identified based on the angle between K3 axis and bedding pole. It was observed that Kangra recess shows more mature fabric type IV as compared to Dehradun recess, where orientation of K3 axes and tectonic fabric reveals dominance of superposed deformation. In the vicinity of Dehradun recess, normal faults occur in un-indurated Quaternary fan deposits confirming their recent formation. The observations are in conformity with earlier studies that during the 1905 Kangra earthquake, the Main Boundary Thrust (MBT) (or one of its subsidiary thrusts) near Kangra showed thrusting whereas the MBT near Dehradun underwent either normal faulting or post seismic adjustments characteristic of normal faulting. The thrusting could be a result of subsurface processes whereas the normal faulting was a result of prevailing surface strains.     

EPR and magnetic susceptibility studies in well samples from some Venezuelan oil fields

Electron Paramagnetic Resonance (EPR) and Magnetic Susceptibility (MS) measurements were carried out in near-surface samples of 6 wells (producers and non-producers) from some Venezuelan oil fields. EPR measurements were performed to determine the organic matter free radical concentration (OMFRC) in the samples. MS anomalous levels, where the restrictive presence of microscopic framboidal magnetic minerals has been recognized, were found only at the producer wells. Moreover for these same wells, anomalies of OMFRC were observed at depths close to MS anomalous levels. These preliminary results seem to suggest a relationship between the presence of OMFRC and MS anomalies, possibly associated with the underlying reservoir.     

The magnetic fabric of some Irish Granites

Magnetic anisotropy measurements have been made on fifty-four oriented samples from the Carna and Screeb areas of the Galway Granite, and from the Ardara pluton and the Main Granite (including the Trawenagh Bay Granite) in Donegal. The origin of magnetic anisotropy in plutonic magnetic rocks is discussed, and it is concluded that it is probably due to grain alignment of inequant magnetite, which may occur either as free grains or as secondary inclusions in ferromagnesian minerals. The grain alignment may occur either whilst the magma is essentially fluid, giving a magnetic lineation transverse to the flow, or at a much later stage of consolidation during plastic deformation, in which case the magnetic lineation parallels the direction of greatest elongation. The results of the measurements are consistent with macroscopic foliations and lineations where these have been observed, but also indicate the existence of fabrics too weak to observe by conventional methods. This is because the magnetic method assesses the alignment of a very large number of grains, and is therefore much more sensitive than conventional fabric analysis. The weak fabrics can be qualitatively explained in terms consistent with other evidence relating to the emplacement of the granites, and experimental work now in progress may make possible a more quantitative interpretation.