Magnetic Anisotropy as a Source of Paleomagnetic Information
The 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:
- Uniaxial models;
- Biaxial models (with various
ratios of uniaxial component anisotropy constants);
- Planar models;
- Models combining planar and
uniaxial MA components;
- Intrinsically isotropic
models of cubic shape.
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 (
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
See also: Thermal behavior of orientational magnetic anisotrory
of sedimentary rocks
P.V.
Dubrovin, V.A. Shashkanov,
I.N. Petrov
See also our
recent publications