Anisotropy Of Magnetic Remanence Research Articles

Decrypting Magnetic Fabrics (AMS, AARM, AIRM) Through the Analysis of Mineral Shape Fabrics and Distribution Anisotropy

AbstractAnisotropy of magnetic susceptibility (AMS) and anisotropy of magnetic remanence (AARM and AIRM) are efficient and versatile techniques to indirectly determine rock fabrics. Yet, deciphering the source of a magnetic fabric remains a crucial and challenging step, notably in the presence of ferrimagnetic phases. Here we use X‐ray micro‐computed tomography to directly compare mineral shape‐preferred orientation and spatial distribution fabrics to AMS, AARM and AIRM fabrics from five hypabyssal trachyandesite samples. Magnetite grains in the trachyandesite are euhedral with a mean aspect ratio of 1.44 (0.24 s.d., long/short axis), and >50% of the magnetite grains occur in clusters, and they are therefore prone to interact magnetically. Amphibole grains are prolate with magnetite in breakdown rims. We identified three components of the petrofabric that influence the AMS of the analyzed samples: The magnetite and the amphibole shape fabrics and the magnetite distribution anisotropy. Depending on their relative strength, orientation and shape, these three components interfere either constructively or destructively to produce the AMS fabric. If the three components are coaxial, the result is a relatively strongly anisotropic AMS fabric (P' = 1.079). If shape fabrics and/or magnetite distribution anisotropy are non‐coaxial, the resulting AMS is weakly anisotropic (P' = 1.012). This study thus reports quantitative petrofabric data that show the effect of magnetite distribution anisotropy on magnetic fabrics in igneous rocks, which has so far only been predicted by experimental and theoretical models. Our results have first‐order implications for the interpretation of petrofabrics using magnetic methods.

Anisotropy of out-of-phase magnetic susceptibility as a potential tool for distinguishing geologically and physically controlled inverse magnetic fabrics in volcanic dykes

Among the mechanisms of the mass transport within the Earth's crust and the upper mantle the magma movement within the dykes is very important. In some dykes, the magma movement is represented by almost free flow infilling more or less open fractures in the host rocks, while it has character of forceful injection of the host rocks in the other dykes. The magnetic fabric originated by the free flow is characterized by the magnetic foliation and magnetic lineation approximately parallel to the dyke plane (normal fabric) whereas the forceful injection gives rise to the magnetic foliation perpendicular to the dyke plane defining the intermediate or inverse magnetic fabric. It is known, however, that the inverse magnetic fabric can also originate through the free flow of magma in the case when the minerals carrying the AMS are represented by single-domain (SD) grains. In order to distinguish between the two mechanisms controlling the intermediate/inverse fabric, the anisotropy of magnetic remanence (AMR) is often used, because it indicates the shape preferred orientation of ferromagnetic minerals regardless of their domain state. We show that not only AMR, but also the anisotropy of out-of-phase magnetic susceptibility (opAMS) is conformable with particle shape regardless of the magnetic state of the particle. Consequently, the opAMS, which is measured automatically with the standard AMS using the KLY-5 Kappabridge, may substitute the AMR at least in some cases of volcanic dykes.

Relative paleointensity estimates from magnetic anisotropy: Proof of concept

Relative paleointensity data from sedimentary rocks play an important role to decipher the workings of the geodynamo and to correct for atmospheric cosmogenic radionucleide production, so it is important to understand how sediments acquire remanent magnetizations and to better assess the quality of relative paleointensity data. We present experimental results from sediments deposited in controlled magnetic fields to observe the changes in magnetic anisotropy as a function of applied field strength going from near Earth-like values to almost full saturation. Relative paleointensity values followed a very well defined power law through the entire range of applied field intensities. Magnetic remanence fabrics evolved from oblate with maximum anisotropy axes in the sedimentary plane at low field strengths to prolate with maximum anisotropy axes parallel to the applied field direction at high fields. Anisotropy of magnetic susceptibility also evolved with field strength, but in a much less coherent manner than anisotropy of magnetic remanence. The experiments used well-characterized, natural sediments containing single domain magnetite, which made it possible to numerically model the data. The model matches the field dependency of both relative paleointensity and magnetic fabric development using a simple assumption that a large proportion (∼80%) of the remanence carriers in the sediments are unable to align with the magnetic field while a small fraction are free to align. Anisotropy of magnetic remanence thus holds promise to improve and assess relative paleointensity estimates and helps improve theoretical treatment of magnetic recording in sediments.

Diverse response of paramagnetic and ferromagnetic minerals to deformation from Intra-Carpathian Palaeogene sedimentary rocks: Comparison of magnetic susceptibility and magnetic remanence anisotropies

All the minerals in a rock contribute to its anisotropy of magnetic susceptibility (AMS). In weakly magnetic sedimentary rocks, the strongest contributors are the paramagnetic minerals, predominantly represented by clay minerals, together with the ferrimagnetic minerals that are typically magnetite and/or maghemite. In contrast, ferrimagnetic minerals solely control the anisotropy of magnetic remanence (AMR). An investigation of both AMS and AMR can therefore help distinguish among the deposition, diagenetic and deformation histories of the paramagnetic and ferrimagnetic minerals in the same rock specimen. We present a test case from relatively weakly deformed Intra-Carpathian Palaeogene sandstones (Skorušina Hills, Slovak Republic) where AMR and AMS fabrics generally coaxial yet also exhibited notable differences. In particular, AMR foliations were more readily deflected away from the bedding plane, perpendicular to the maximum stress axis, than the AMS foliations. These factors are interpreted in terms of higher sensitivity of the ferrimagnetic minerals to ductile deformation than that of the paramagnetic clay minerals.

Magnetic fabrics in garnet peridotites–pyroxenites and host felsic granulites in the South Bohemian Granulites (Czech Republic): Implications for distinguishing between primary and metamorphism induced fabrics

In the high-grade Moldanubian Zone of the European Variscides, numerous bodies of ultramafic rocks occur embedded in granulite. The anisotropy of magnetic susceptibility and its low-field variation as well as the anisotropy of magnetic remanence were used to investigate magnetic fabrics of some ultramafic bodies and host granulite. In granulite, the magnetic foliation is roughly parallel to the metamorphic foliation and the magnetic lineation is near the mineral alignment lineation. In ultramafite, the magnetic foliation is relatively scattered spatially, but mostly oriented in a different way than that in granulite. The magnetic lineation is also scattered, but still relatively well defined spatially. Again, its orientation is mostly different than that of granulite. The magnetic fabric in ultramafic rocks is therefore different from that in the host granulite even though both rock types underwent at least partially common structural history. The componental movements forming the granulite fabric, mostly during amphibolite facies retrograde metamorphism, were evidently not intensive enough to strongly overprint the magnetic fabric of ultramafite. The ultramafite is therefore strong enough to maintain its pre-metamorphism fabric even at such high temperatures and pressures that are characteristic of high amphibolite facies retrograde metamorphism.

Magnetic fidelity of lunar samples and implications for an ancient core dynamo

Some lunar rocks have stable magnetizations that provide compelling evidence for an ancient lunar core dynamo. However, a longstanding problem has been interpreting the unstable alternating field (AF) demagnetization behavior observed in the remaining majority of the Apollo lunar sample suite. Similar unstable behavior has also been observed for many meteorites. It is unclear whether the behavior of these samples indicates that they formed in the absence of an ancient magnetizing field or whether they simply have poor magnetic recording properties. It is necessary to distinguish between these two possibilities in order to determine whether paleomagnetic fields were present on a sample's parent body. To address this issue, we analyzed five samples whose rock magnetic properties span the full suite of observed demagnetization behaviors: mare basalts 15556, 15016, 12017, 10020, and troctolite 76535. We demonstrate that the effects of spurious anhysteretic remanent magnetization (ARM) during AF demagnetization, in combination with multidomain magnetic carriers and magnetic remanence anisotropy, are likely responsible for the apparently poor magnetic behavior of many lunar samples. Therefore, the unstable AF demagnetization behavior observed for many lunar samples is not alone sufficient for ruling out the presence of an ancient lunar core dynamo. Nevertheless, spurious ARM may explain the observation of surprisingly high (≥1μT) Apollo-era paleointensity measurements for samples thought to have formed after the cessation of a lunar core dynamo (<1.5Ga).

Detachment and rotation of a metamorphic core complex during extensional deformation: Palaeomagnetic study of the Catalina–Rincon Core Complex, Basin and Range Province, Arizona

The Catalina–Rincon complex exposes a metamorphic core of mid-crustal rocks exhumed by extensional tectonism along a major detachment exposed along the southeast side of the Catalina–Rincon Mountains near Tucson, Arizona. The juxtaposition of core complexes against high level crust segmented into rotated and tilted fault blocks is an important component to models for crustal extension, and key questions surround the way(s) that cores are exhumed. We report palaeomagnetic results from rocks beneath the detachment in the Catalina–Rincon core and additional data from lavas and tuffs in adjoining hanging wall rocks immediately above the detachment. Magnetic remanence in the metamorphic core is carried predominantly by magnetite with a dominant WNW–ESE dual polarity direction (A1: D/ I = 288/+ 50°, N = 59 components, R = 55.60, α 95 = 4.6°). This remanence is interpreted to have been acquired through a broadened blocking temperature spectrum during protracted exhumation and cooling in Oligocene times. Since the magnetic inclination accords with the mid-Cenozoic field it is difficult to explain this magnetization in terms of large later tilting and it appears to be the record of near-vertical axis CCW rotation of ∼ 63 ± 5° relative to cratonic North America. Although magnetic inclination identifies no significant horizontal axis rotation, this could be fortuitous and hide the influence of magnetic anisotropy and resultant inclination-shallowing. To assess this contribution both anisotropy of magnetic remanence (AMR) and anisotropy of magnetic susceptibility (AMS) have been investigated. AMS resolves only weak fabrics comparable with the top-NE mylonite deformational fabric near the detachment which probably dates from a mid-Eocene high temperature event. AMR shows that deflections of magnetic remanence of up to several tens of degrees into the foliation may be anticipated although weak alignment of the magnetite is likely to reduce this effect. The full importance of horizontal axis tilting remains unresolved although a possible relict component in the core (A2: D/ I = 269/− 9°, N = 24, components, R = 22.91, α 95 = 6.6°) is rotated ∼ 20° more CCW than A1 and also of presumed mid-Cenozoic cooling-related origin; it suggests that segments of middle crust comprising the exposed core might have been rotated by large amounts into their present position. Highly faulted Early Oligocene Pantano Formation volcanic rocks in the hanging wall immediately above the detachment zone appear to have rotated by a much smaller amount CCW on the south west side ( D/ I = 161/− 45°, 6 sites), and by larger, variable amounts on the northeast side. Movements of the upper plate immediately above the detachment are difficult to resolve with precision due to complex fault block rotations, although the underlying core complex has clearly undergone large differential slip relative to these cover rocks. The magnitude of inferred vertical axis rotation of the Catalina–Rincon core implies that it is not rooted into adjoining crust as proposed in classical simple shear models. Instead the core exposed below the detachment may be part of a fault carapace that has decoupled and rotated over a ductile lower crust.

Plane-confined magnetic lineations in mingled mafic and felsic magmas, the Sázava pluton, Bohemian Massif

Multiple magnetic lineations were revealed using the anisotropy of magnetic susceptibility (AMS) and anisotropy of magnetic remanence (AMR) methods in a domain of mafic–felsic magma mingling at the northwestern margin of the Sázava pluton, Central Bohemian Plutonic Complex, Bohemian Massif. Gabbrodioritic enclave swarms and sheets are steeply oriented and exhibit magnetic lineations plunging at steep to moderate angles (mostly 40–70°) whereas in their tonalitic host lineations plunge from vertical to horizontal angles, but mostly less than 40°. The magnetic lineations in both rock types spread along a ~ NNE-SSW steep plane that could be simplified as representing an “average” margin-parallel magmatic foliation in the pluton concordant with an “average” regional cleavage in the wall-rock. The plane-confined lineations are interpreted as having recorded the heterogeneous superposition of two processes: (1) vertical stretching during emplacement and magma mingling which left behind the steep lineations; and (2) regional tectonic stretching, which progressively rotated the mineral grains in rheologically weaker domains (chiefly in the host tonalite) to form the sub-horizontal lineation. The average foliation bearing the multiple lineations is interpreted as a composite foliation that recorded both margin-perpendicular shortening during emplacement, overprinted by coaxial regional tectonic shortening. This example reaffirms that (1) magmatic fabrics in crystallizing magmas can record accumulated strain resulting from both emplacement and regional tectonic deformation; and that (2) separating magmatic (and also magnetic) fabrics related exclusively to the internal chamber processes from fabrics caused by regional tectonic deformation is problematic or even impossible in cases where composite fabrics are recognized.

MARTÍN-HERNANDEZ, F., LÜNEBERG, C. M., AUBOURG, C. &amp; JACKSON, M. (eds) 2004. Magnetic Fabric. Methods and Applications. Geological Society Special Publication no. 238. vi + 551 pp. London, Bath: Geological Society of London. Price £135.00, US $243.00; GSL members' price £67.50, US $122.00; AAPG/SEPM/GSA/RAS/EFG/PESGB members' price £81.00, US $146.00 (hard covers). ISBN 1 86239 170 X

As a student in the 1960s I recall the admiration we felt for our mentors who had not only mastered use of the universal stage, but were prepared to spend long hours resolving mineral orientations grain by grain. The end results were very satisfying but must have left them yearning for a more rapid method for resolving rock fabrics. Nowadays of course, we have such a method in the determination of anisotropy of magnetic susceptibility (AMS) and anisotropy of magnetic remanence (AMR). AMS remains the dominant application because measurements are fast, non-destructive and very precise. This Geological Society Special Publication brings together 26 research papers originally presented at European and American Geophysical Union meetings in 2003. By appearing exactly half a century since John Graham published a seminal paper proposing that AMS in rocks could provide a sensitive and readily-determined tool for resolving petrofabrics, it represents a Jubilee landmark. During these 50 …

Distribution anisotropy: the influence of magnetic interactions on the anisotropy of magnetic remanence

Abstract The anisotropy of magnetic remanence (AMR) is often used as a tool for examining magnetic anisotropy of rocks. However, the influence of magnetostatic interactions on AMR has not been previously rigorously addressed either theoretically or experimentally, though it is widely thought to be highly significant. Using a three-dimensional micromagnetic algorithm, we have conducted a systematic numerical study of the role of magnetostatic interactions on AMR. We have considered both lineation and foliation, by modelling assemblages of ideal single domain grains and magnetically non-uniform magnetite-like cubic grains. We show that magnetostatic interactions strongly affect the measured AMR signal. It is found that depending on the orientation of the single-grain anisotropy and grain spacing it is possible for the AMR signal from a chain or grid of grains to be either oblate or prolate. For non-uniform grains, the degree of anisotropy generally increases with increasing interactions. In the modelling of AMR anisotropy, saturation isothermal remanence was chosen for numerical tractability. The influence of interactions on other types of more commonly measured AMR, are considered in light of the results in this paper.

The use of the anisotropy of magnetic remanence in the resolution of the anisotropy of magnetic susceptibility into its ferromagnetic and paramagnetic components

The anisotropy of magnetic susceptibility (AMS) is often controlled by both ferromagnetic (sensu lato) and paramagnetic minerals. The anisotropy of magnetic remanence (AMR) is solely controlled by ferromagnetic minerals. Jelı́nek (Trav. Geophys. 37 (1993)) introduced a tensor derived from the isothermal AMR whose normalized form equals the normalized susceptibility tensor provided that the ferromagnetic fraction is represented by multi-domain magnetite. The present paper shows the close correlation between these tensors for a collection of strongly magnetic specimens containing multi-domain magnetite. In addition, acceptable correlation between the tensors was also found for a collection of specimens containing single-domain magnetite. A new method is developed for the AMS resolution into ferromagnetic and paramagnetic components using the AMR. Some examples are presented of this resolution in mafic microgranular enclaves in granodiorite and in gneisses of the KTB borehole.

Limitations of tensor subtraction in isolating diamagnetic fabrics by magnetic anisotropy

The anisotropy of low field susceptibility (AMS) represents the orientation distribution of all minerals in a rock, whereas the anisotropy of magnetic remanence (AMR, preferably anhysteretic) isolates that of the accessory remanence-bearing minerals. The subtraction of normalized AMR from AMS, in theory and under limited practical circumstances, may isolate the paramagnetic+diamagnetic anisotropy contribution and thus the orientation distribution of the matrix minerals ( Borradaile et al., 1999. Geol. Soc. Lond., Sp. Publ. 151, 139–145). Limitations include the great sensitivity of the subtraction process to the precision of the definition of the respective (AMS, AMR) tensors, and a requirement that single-domain and superparamagnetic grains are absent. The latter is particularly important for superparamagnetic minerals because iron oxides may be part of the orientation distribution of the main group of remanence-bearing minerals, although they would be excluded from the AMR fabric. Low ratios of saturation isothermal remanence to induced susceptibility characterize those rare rocks in which superparamagnetic behavior is a significant contribution.

Anisotropy of Anhysteretic Remanent Magnetization and its Application to Study of Collisional Zone

Rock anisotropy of magnetic remanence refers to anisotropy of anhysteretic remanence and anisotropy of isothermal remanence, which are recently developed techniques for rock fabric study. They are more clear and definite to show the deformation and dislocation of magnetic carriers in rocks than conventional anisotropy of magnetic susceptibility. The concept of anhysteretic remanence and the method to measure its anisotropy are introduced. One case study was cited to show the application in Jiangshan-Shaoxing collisional zone.

Anisotropy of magnetic susceptibility in a Palaeozoic flysch basin: the Windermere Supergroup, northern England

The Windermere Basin of northern England was initiated in Late Ordovician times and developed into a foreland basin by collision of the Laurentian and Eastern Avalonian Plates during Silurian times. It was largely filled by axial turbidite input in mid to Late Silurian times. Rock magnetic studies suggest that paramagnetic minerals, probably mostly chlorite, dominate the magnetic susceptibility. Studies of anisotropy of susceptibility (AMS) identify a bedding-related fabric which has survived later Acadian deformation in all but the oldest (U. Ordovician) units. The magnitude of the fabric becomes amplified in the higher more-argillaceous parts of individual beds but the direction of maximum susceptibility (k1) remains aligned with palaeoflow direction throughout the massive and parallel-laminated divisions. AMS is best defined in those units with well developed planar and cross bedding. The mechanism responsible for rippling the upper parts of the beds involves rolling of detrital grains along the bed during traction and alignment of the long axis (the grain's rotation axis) normal to the current; consequently the intermediate axis of AMS (k2) is in the direction of the secondary current responsible for rippling. Some beds which are rippled throughout show the same feature. Hemipelagic facies comprising laminated silty mudstones have highly uniform fabrics to which detrital hematite is an important contributor. The k1 directions in the bedding planes show cyclic swings in declination which may be related to some long-period (perhaps orbitally-forced) secular oceanographic variation. Since these sediments were deposited in optimum conditions for preserving grain alignment with the geomagnetic field, AMS and AMR (Anisotropy of Magnetic Remanence) study of such facies may provide methods for recovering secular variation in the remote geological past and for estimating rates of deposition.

Regional kinematics inferred from magnetic subfabrics in Archean rocks of Northern Ontario, Canada

Strain analysis, observations of L-S fabrics and studies of the anisotropy of magnetic susceptibility (AMS) and of anisotropy of isothermal remanence (AIRM) reveal a sequence of progressive fabric development during regional deformation. L-S fabrics of quartz and feldspar grains are most transposed in regional transpression whereas the AMS subfabrics, controlled by the properties of late metamorphic silicates, record a later orientation of the ambient stress field. Anisotropy of magnetic remanence is caused by the still younger minerals, magnetite and pyrrhotite. Its anisotropy is less transposed than the AMS subfabric. Consequently the relative orientations of the three types of fabric yield a kinematic sequence of subfabrics developed during transpression. The L-S fabrics are most transposed, the AMS less transposed and the AIRM fabrics least transposed toward the plane of regional flattening. The results indicate that the deformation recorded by the three subfabrics was non-coaxial in the most general sense, with all three principal directions spinning with respect to the rocks during progressive straining.

Deformation and magnetization of the Hudson Valley, eastern New York: Results of a study of calcite twinning and anisotropy of magnetic remanence in the Onondaga Limestone

Results of a study of calcite-twin analysis and anisotropy of anhysteretic remanent magnetization (AARM) for the Onondaga Limestone in the Hudson Valley Fold and Thrust belt (HVB), raise questions about the deformation history of this area and the nature of AARM. Two bedding parallel shortening directions are inferred from calcite twinning: one normal to HVB fold axes, NW-SE to ENE-WSW, and the other at high angles to this direction, NNE-SSW to N-S. Two hypotheses are put forward to explain the inconsistent shortening direction: (1) even during the initial stages of folding and thrusting in the HVB, deformation was three-dimensional in character; (2) the HVB developed synchronous with Main phase Alleghanian deformation. AARM results are consistent with the magnetic anisotropy being in part caused by the deformation associated with folding and thrusting. This is a surprising result because the magnetization is presumably of diagenetic origin, and it was acquired after folding.

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.