This is Pavel Dubrovin, the man who leads the projectMagnetic Anisotropy as a Source of Paleomagnetic Information

Magnetic anisotropy energy 3D surface in the case of planar anisotropyThe main purpose of this project is to assess a possibility of paleomagnetic determinations using magnetic anisotropy (MA) characteristics. Some components of magnetic anisotropy (so called "paleoinformative" components) can arise form the effect of geomagnetic field during the rock formation, and, therefore, can be used to derive the paleomagnetic field direction and intensity.
An excellent example is the orientational anisotropy of sediments. MA acquisition in sediments (precipitated in still water, i.e. in the absence of water flows and any other disturbing effects) is mainly controlled by two factors: (i) gravitational compaction of a sediment and (ii) effect of the geomagnetic field aligning magnetic particles during sedimentational and post-sedimentational processes. These processes lead respectively to the formation of two MA components: a planar (vertical) component, easy plane of which coincides with the bedding plane of sediment, and an orientational component, which is of a minor importance in comparison with a usually relatively strong planar anisotropy. Since the latter is only due to geomagnetic field, it can be used for the determination of geomagnetic field direction during sediment formation, i.e. for palaeomagnetic determination.
An important advantage of using the anisotropy characteristics for paleomagnetic determinations is that the magnetic anisotropy is much more stable than NRM both in time and with respect to various physical processes altering NRM. Therefore, a paleomagnetic technique based on the analysis of the magnetic anisotropy would be very helpful in the case when common methods fail. So, the ultimate goal of our project is to develop a new paleomagnetic technique which would make possible to obtain paleomagnetic data from the magnetic anisotropy characteristics.
 

A brief description of the first steps on this way is given below.

Physical modelling of magnetic anisotropy

The first goal of the project was to develop an interpretation technique, which would allow to obtain an objective information about MA properties of the sample, to separate palaeoinformative MA components, and to determine the spatial orientation of their axes in the paleomagnetic coordinate system. In this line, the set of models imitating  the most common types of  magnetic anisotropy has been produced:

The magnetic anisotropy of these models has been studied by the high-field magnetic torque method.
Interpretation of torque curves  was carried out by two independent techniques: in the uniaxial approximation - using phases of second harmonics of the torque curves measured in three orthogonal planes of the sample (phase method), and using the spherical harmonic analysis of magnetic anisotropy energy.
Comparison of these techniques has shown that the phase method is only applicable in the case of uniaxial (one-component) anisotropy, while the spherical analysis is applicable to any anisotropy type, as it does not use any a priori information concerning the magnetic anisotropy. Using amplitude and phase analysis of the torque curves in the case of uniaxial anisotropy, the criteria of "uniaxiality" of MA (the criteria of phase method applicability) have been established.
The application of spherical analysis to the multiaxial model samples showed that it allows to resolve the magnetic anisotropy into different physical components and to determine orientations of their axes both for biaxial models and models combining uniaxial and planar anisotropies. Using mathematical analysis of the spherical harmonic series coefficients of anisotropy energy, the procedure of resolving MA into physical components (uniaxial and planar) has been developed.
The experiments have been carried out with the isotropic models to assess the extent in which cubic shape of a sample (one of traditional in paleomagnetism) affects its total MA. It has been established that even for magnetite bearing rocks the MA component due to cubic shape of a sample can be neglected in the most practical cases (when the magnetite content is less than 10%).
 

Determination of sedimentation field direction using magnetic anisotropy of artificial sediments

To assess a possibility of determination of sedimentation field direction using an orientational anisotropy component, the large-square artificial sediment produced in  a laboratory geomagnetic field (Ho =0.54 Oe, D = 0, I = 73 in sample coordinate  system) was studied. As a deposition material, the homogeneous mixture of non-magnetic blue Proterozoic clay, chalk powder and magnetite particles (0.5 - 30 micrometers in  size) were used; magnetite content was 0.5% by weight. The cubic samples have been cut  from the prepared sediment. Magnetic anisotropy was studied with the high-field magnetic torque method for 27 samples of the sediment. To resolve an observed MA into planar and orientational components, the technique based on spherical harmonic analysis of magnetic anisotropy energy was developed. Most of the samples did not show a distinct orientational anisotropy, but for the six samples this component has been revealed. The orientation of easy axis appeared very close to the DRM direction for all six samples. The Fisher mean direction of DRM (D = 2, I = 60,a95 = 6) and that of easy axes (D = -6, I = 59, a95 = 18) coincided within a95.
On the other hand, the averaging of the torque curves over these samples and MA energy spherical analysis using averaged curves yielded an easy axis direction (D = -17, I = 71) fairly close to that of the sedimentation field and considerably different from the DRM mean direction. Now it is not clear, which way of averaging is preferable for the natural sediments: averaging of individual easy axis orientations or averaging of the torque curves and subsequent separation of the "mean" orientational MA component. Further experiments will probably help answer this question.

See also:  Thermal behavior of orientational magnetic anisotrory of sedimentary rocks


P.V. Dubrovin, V.A. Shashkanov, I.N. Petrov

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