Rock Anisotropy Of Magnetic Susceptibility Research Articles

Anisotropy of magnetic susceptibility in salt rocks from the German Zechstein Basin, Sondershausen mine

SUMMARY The anisotropy of magnetic susceptibility (AMS) of rocks reflects the alignment of certain minerals, and therefore it can be used to investigate the deformation history of rocks. However, for salt rocks, very few studies on the AMS of salt rocks and the influence of accessory minerals exist. In this study, we analysed the potential to use the AMS of salt rocks with low impurity content for fabric characterization. Samples of rock salt, sylvinite and carnallitite from a salt mine in Sondershausen (Germany) from the Late Permian (Zechstein 2, Stassfurt series) are investigated. The results of low-field AMS (LF-AMS) measurements show a very weak but significant magnetic anisotropy for sylvinite, carnallitite, and rock salt with an elevated content of accessory minerals. The AMS results are consistent in individual layers of the same rock type. In order to identify the magnetic minerals, which cause the magnetic anisotropy, the high-field AMS (HF-AMS) was measured using a torque magnetometer in order to separate ferrimagnetic and paramagnetic contributions to the AMS. A significant paramagnetic subfabrics exists, which reflects the alignment of phyllosilicates. The magnitude of the LF-AMS is considerably greater than that of the paramagnetic subfabric. This indicates the existence of a ferrimagnetic subfabric due to magnetite, which can have a different orientation than the paramagnetic subfabric. Differences in the orientation of the AMS in samples from two sites suggest a relationship of deformation history and AMS. At a site with dipping layers, the AMS orientation is independent of the bedding and shows large differences between individual lithological layers. In a tight fold, the AMS of all rock types has similar shape and orientation. We conclude that AMS in salt rocks can give meaningful information on the mineral fabric, which could be used in the analysis of the deformation history.

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.

Origin of magnetic fabrics in ultramafic rocks

The magnetic fabric of a rock, defined by the anisotropy of magnetic susceptibility (AMS), is often used as a tectonic indicator. In order to establish a quantitative relationship between AMS and mineral texture, it is important to understand the single crystal intrinsic AMS of each mineral that contributes to the AMS of the rock. The AMS and crystallographic preferred orientation (CPO) of amphiboles, olivine and pyroxenes has been analyzed in a series of amphibolites, peridotites and pyroxenites that do show preferred mineral alignment. The CPO of each mineral phase was determined based on electron backscatter diffraction (EBSD). Whole- rock AMS was computed based on the CPO and single crystal AMS of the respective minerals. A comparison between measured and modelled magnetic anisotropy shows that the directions of the principal susceptibility axes agree well in amphibolite and peridotite. Pyroxenite is a good example for competing AMS fabrics in polyphase rocks.

Magnetic anisotropy in clinopyroxene and orthopyroxene single crystals

AbstractPyroxenes constitute an important component in mafic igneous and metamorphic rocks. They often possess a prismatic habit, and their long axis, the crystallographic c axis, helps define a lineation in a textured rock. Anisotropy of magnetic susceptibility (AMS) serves as a fabric indicator in igneous and metamorphic rocks. If a rock's AMS is carried by pyroxenes, it can be related to their crystallographic preferred orientation and degree of alignment. This requires knowing the intrinsic AMS of pyroxene single crystals. This study provides a comprehensive low‐field and high‐field AMS investigation of chemically diverse orthopyroxene and clinopyroxene crystals in relation to crystal structure, chemical composition, oxidation state of Fe, and the possible presence of ferromagnetic inclusions. The paramagnetic anisotropy, extracted from high‐field data, shows clear relationships to crystallographic directions and Fe concentration both in clinopyroxene and orthopyroxene. In the diopside‐augite series, the intermediate susceptibility is parallel to b, and the maximum is at 45° to the c axis. In aegirine, the intermediate axis remains parallel to b, while the maximum susceptibility is parallel to c. The AMS of spodumene depends on Fe concentration. In enstatite, the maximum susceptibility aligns with c and the minimum with b, and in the case of hypersthene, the maximum susceptibility is normal to the exsolution lamellae. Magnetite inclusions within augite possess a ferromagnetic anisotropy with consistent orientation of the principal susceptibilities, which dominates the low‐field anisotropy. These results provide better understanding of magnetic anisotropy in pyroxenes and form a solid basis for interpretation of magnetic fabrics in pyroxene‐bearing rocks.

Modelling accuracy limits for frequency-dependent anisotropy of magnetic susceptibility of rocks and soils

The frequency-dependent anisotropy of magnetic susceptibility (AMS) can be most easily determined through subtracting directional susceptibilities measured at two operating frequencies along each of the measuring directions and the differences subsequently processed using standard methods for AMS computation. The effect of the measurement precision on the accuracy of the determination of the frequency-dependent AMS is investigated through mathematical simulation of the measurement process using the statistical theory of measuring the AMS of rocks. The accuracy of the AMS determination is presented in terms of the errors in determining the principal susceptibilities and principal directions. By modelling the measuring errors, the limits are discovered for the investigation of the frequency-dependent AMS. The modelling shows extremely high requirements for measurement accuracy met by the most sensitive instruments, only.

Modelling relationship between bulk susceptibility and AMS in rocks consisting of two magnetic fractions represented by ferromagnetic and paramagnetic minerals — Implications for understanding magnetic fabrics in deformed rocks

Measurement of Anisotropy of Magnetic Susceptibility (AMS) has become an important tool for Structural Geological analysis in the past few decades. In the past, AMS data have been used for petrofabric analysis of deformed rocks as well as for gauging strain. However, the AMS of some rocks can be carried by both ferromagnetic and paramagnetic minerals. Separating effects of these mineral groups on the rock’s AMS is difficult because of expensive and commercially less available instrumentation. On the other hand, instrumentation is available in most rock magnetic and palaeomagnetic laboratories for resolving bulk susceptibility into ferromagnetic and paramagnetic components. Mathematical modelling was made of the relationship between bulk susceptibility and AMS. If the contribution of the ferromagnetic or the paramagnetic fraction to the rock susceptibility is dominant (let us say higher than 80%), the resultant AMS is relatively near to the AMS of the dominating fraction in all aspects, the degree of AMS, shape parameter and orientation of principal susceptibilities. In the interpretation of the AMS of rocks with dominating one fraction, the resolution of the AMS into paramagnetic and ferromagnetic components is not necessary, the resolution of bulk susceptibility into components is sufficient that can be made using the instrumentation available in most rock magnetic and palaeomagnetic laboratories.

Alternating field-impressed AMS in rocks

We studied the evolution of anisotropy of magnetic susceptibility (AMS) during stepwise alternating fields (AF) demagnetization on various kinds of rock samples (loess and palaeosols, diorite, granite, gneiss) with very different degree of magnetic anisotropy. The variation of the magnetic fabric appears to be related to both the magnetic fabric before AF demagnetization and to the direction of AF application. The anisotropy change is mainly controlled by the initial magnetic fabric. The more anisotropic is the initial magnetic fabric, the less is the effect of the direction of field application. This can be clearly shown by determination of the difference of the susceptibility ellipsoids after and before AF application. Even for rocks with weak magnetic anisotropy, the effect of the initial AMS is significant. The difference ellipsoids allow in particular cases to point out that the initial magnetic fabric is composite.

MARTÍN-HERNANDEZ, F., LÜNEBERG, C. M., AUBOURG, C. & 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 …

How deformed are weakly deformed mudrocks? Insights from magnetic anisotropy

Abstract For over thirty years the anisotropy of magnetic susceptibility (AMS) has been exploited in rock fabric studies. Our knowledge of the mechanisms leading to magnetic fabric development in deformed rocks has grown substantially, even though some details are still under debate. This paper reviews recent developments in AMS studies applied to the characterization of early deformation stages in mudrocks. From the current data set of AMS in rocks deformed at low temperature and low pressure it is possible to revisit the concept of weakly deformed rocks. We contend that weak deformation in mudrocks and the appearance of magnetic cryptofabric are concatenated. Furthermore, magnetic anisotropy studies in weakly deformed rocks suggest that cleavage fabric, the most common planar fabric in deformed mudrocks, builds up gradually from the earliest stages of deformation rather than suddenly at some strain threshold.

Factors affecting the relative importance of shape and distribution anisotropy in rocks: theory and experiments

The effect of magnetic interactions has been commonly neglected in the interpretation of the anisotropy of magnetic susceptibility (AMS) of igneous rocks, despite the likely occurrence of magnetic interactions due to their relatively high content of ferromagnetic minerals. Current models of magnetic interactions apply to highly simplified systems and are not directly applicable to natural rocks. On this work, a model that includes variables previously neglected is used to assess the relative importance of magnetically interactive and non-interactive fractions in a rock. Results of the model show that a variable proportion of magnetically interactive grains, the relative values of the bulk susceptibility, degree of anisotropy and orientation of the anisotropies characteristic for each fraction all play an important role in controlling the rock AMS. Experimental evidence illustrating the importance of the distribution of the ferromagnetic grains within a specimen is also reported, and supports the model predictions. Both theory and experiments indicate that if a rock has unequant magnetic grains and clusters of magnetic grains, the contribution to the AMS of the rock will depend on the relative values of all the model variables and not only on the occurrence of magnetic interactions as previously presumed.

Successive Granitic Magma Batches During Pluton Emplacement: the Case of Cabeza de Araya (Spain)

We present a dynamic scenario of granite emplacement in the upper crust for the pluton of Cabeza de Araya (Extramadura, Spain), based on interpretations of gravity data, internal structures and geochemical variations. The three-dimensional shape of the pluton is derived from the inversion of gravity data, and the magma flow structures are obtained by mapping the rock anisotropy of magnetic susceptibility. The deepest zones of the pluton, which also correspond to vertical lineations, are interpreted as feeder zones from which magma upwelled. These feeders form small, distinct areas connected to the pluton's floor 6 km below the present surface level. We propose that the two main feeders of the pluton represent tension gashes that formed at the base of the brittle crust and served as conduits for the magma that was ready to be tapped. In the southern half of the pluton, granite type A is cross-cut by type B in the magmatic state. These magmas show a continuum in their chemistries attributed to progressive differentiation. A fine-grained leucocratic granite, more evolved chemically cross-cuts the surrounding facies and crops out above the southern feeder zone. It is interpreted as a late magma batch, coming from a different source that intruded into the incompletely crystallized surrounding facies. The normal petrographic zoning of the massif of Cabeza de Araya is therefore viewed as a succession of continuous and discontinuous infillings, sequentially permissive and forceful, of increasingly evolved magma batches into a reservoir that opened at varying rates within the brittle crust.

Control of magnetic rock fabrics by mica preferred orientation: a quantitative approach

Magnetic anisotropy analysis represents a well-established and frequently used method in structural geology. Several attempts were made to relate the anisotropy of magnetic susceptibility (AMS) to rock fabrics and to discover the sources of the AMS. In gneissic rocks, paramagnetic phyllosilicates with lattice-dependent magnetic properties are assumed to control the whole rock AMS i.e. it is dominated by mica lattice preferred orientation (texture). In this study, AMS was modelled on the basis of the mica texture, the rock composition and compared to the experimentally determined AMS. The orientations of the modelled and experimental tensors agree quite well, whereas all the other characteristic parameters (shape, anisotropy, mean susceptibility) display large differences. It has been concluded that accessory high susceptible phases, as well as the diamagnetic rock constituents, may have significant influence on AMS. Consequently, mica pole figure measurements cannot be replaced by AMS measurements without control of the sources of AMS. It was also concluded, that the March model to evaluate strain from mica-preferred orientations is not valid for highly strained rocks. Beside the fact that the obtained strains are too low, the deformation is generally inhomogeneous, which leads to an overprint of the mica texture. Since the microstructural fabric elements which originate in different strain regimes (prolate or oblate) may create similar modifications of the mica orientation patterns, it is obvious that the relationship between AMS and deformation is rather complex.