The conventional rules, derived from empirical and theoretical considerations, for the interpretation of anisotropy of magnetic susceptibility (AMS) in terms of microstructure and deformation are subject to numerous exceptions as a result of particular rock magnetic effects. Unusual relationships between structural and magnetic axes (so-called inverse or intermediate magnetic fabrics) can occur because of the presence of certain magnetic minerals, either single domain magnetite or various paramagnetic minerals. When more than one mineral is responsible for magnetic susceptibility, various problems appear, in particular the impossibility of using anisotropy to make quantitative inferences on the intensity of the preferred orientation and consequently on strain. In ferromagnetic grains, AMS may also be influenced by the magnetic memory of the grains (including natural remanence). The effect of alternating field or thermal demagnetization on AMS is briefly discussed. As discussed in this article, various rock magnetic techniques, specific to AMS interpretation, have to be developed for a better assessment of the geological significance of AMS data. These techniques mainly rely on measurements of susceptibility versus magnetic field and temperature, together with anisotropy of remanence. 93 refs., 11 figs., 1 tab.

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Rock magnetism and the interpretation of anisotropy of magnetic susceptibility

CONTINENTAL flood basalts, and their associated swarms of mafic dykes, are thought to result from melting in mantle plumes1–3, but the processes by which the resulting magma makes its way from mantle to crust remain obscure. Here we use magnetic anisotropy measurements to reconstruct the pattern of magma flow in the Proterozoic Mackenzie dyke swarm. Our results indicate that magma was injected vertically within 500 km of the focal point of the dyke swarm, and then travelled horizontally at all further distances out to at least 2,100 km. The sharp transition from vertical to horizontal flow at between 500 and 600 km may correspond to the outer boundary of melt generation in the mantle plume believed to be responsible for the Mackenzie igneous events.

Evidence from magnetic fabric for the flow pattern of magma in the Mackenzie giant radiating dyke swarm

Understanding of AMS in pristine basic igneous rocks in terms of flow-orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains (1) typically are irregularly equant or skeletal rather than distinctly ellipsoidal, and (2) usually crystallize interstitially and late, after magma flow has ceased. However, the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly-oriented (by flow) silicate “template”. This constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth, but in addition, we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of AMS. Experiments involving the casting of magnetite-epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected AMS symmetry, i.e. K3 perpendicular to glass plates or K1 parallel to glass rods. Slicing of near isotropic synthetic blank- and natural rock-cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from ∼0.4% to 2%, and the natural rock from ∼0.9% to ∼2.9%. In both the K3 axis migrates to be perpendicular to the glass slides.

Distribution anisotropy: The cause of AMS in igneous rocks?

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.

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

Is this magnetic fabric normal? A review and case studies in volcanic formations

An anistropy of magnetic susceptibility (AMS) study of the relatively fresh Mealy diabase dyke swarm indicates a correlation between the AMS and the deformational effects in the rocks. Several AMS components were revealed during experimental work on 211 samples that had been heated to 640°C during a paleomagnetic study and 22 fresh samples progressively treated in alternating fields. Seventeen sites with slight or no deformation have a magnetic fabric composed of two major components. The stable component, attributed to a primary shape anisotropy, has minimum and maximum principal susceptibility directions preferentially oriented vertically and along dyke strike (≈N 60°E), respectively. This configuration is consistent with an origin due to a post-emplacement stress field, whereby horizontal extension acted on a vertical sheet of partially crystallized magma. The orientation of this component with respect to the present horizontal suggests that the dykes have not been tilted since their emplacement. The unstable component, removed by fields up to 250 mT, is attributed to induced (domain) anisotropy produced by mild tectonic stress during the Grenville Orogeny. The approximate orientation of its principal components, as found by the method of substitutional ellipsoids, suggests an east-west-directed maximum stress. Four other sites with noticeable deformation have magnetic fabric that could not be reliably analyzed, but which may be largely composed of domain and stress-induced anisotropies developed during the Grenville Orogeny.

Alexandre Desbarats

Papers

Magnetic Fabric and Its Significance in the 1400 Ma Mealy Diabase Dykes of Labrador, Canada