Abstract
The orientation of stable single domain (SSD) ferrimagnetic particles in an igneous rock sample was determined by a sensitive technique utilizing gyroremanent magnetization (GRM). Components of GRM were measured in the sample upon exposure to an alternating field (AF) at various orientations in 3 orthogonal planes. The major components of GRM exhibited a sin(2θ) dependence on AF orientation in the respective perpendicular planes. This was in accordance with theory [1] and contrary to some previously reported experimental results on magnetic recording tape, which produced a distorted sin(2θ) dependence of the GRM [1]. The explanation is likely due to the SSD ferrimagnetic particles in the rock sample being more dispersed (less interacting) compared to the highly interacting SSD particles in the magnetic tape sample of the previous study. The GRM results were consistent with another remanence anisotropy method, anisotropy of isothermal remanent magnetization (AIRM). This method again measures the anisotropy of the remanence carrying ferrimagnetic particles, but the IRM is also acquired by larger multidomain (MD) particles as well as by the SSD particles. The results were also consistent with the visible rock anisotropy (petrofabric), the anisotropy of magnetic susceptibility (AMS), and the shear wave velocity anisotropy. A comparison of all the methods demonstrated that the fine SSD particles, which make up only a small proportion of the rock, were aligned in quite a similar orientation to that of the main rock forming minerals that constituted the bulk of the sample.
Highlights
Gyroremanent magnetization (GRM) is produced by exposing a sample containing an anisotropic distribution of fine ferro- or ferrimagnetic particles to a single alternating field (AF) application [1], or by exposing a sample containing an isotropic distribution of such particles to two successive applications of an AF [2]
The results demonstrate that the orientations of the fine stable single domain (SSD) and MD ferrimagnetic particles are quite similar to those of the main rock forming minerals plagioclase and quartz
1) The magnitudes of the primary gyroremanent magnetization (GRM) components for the natural rock sample studied here exhibited a sin(2θ) dependence on AF axis orientation in respective perpendicular planes. This was consistent with theory [1] and previous experiments on man-made samples [5] for non-interacting SSD particles, and contrary to the distorted sin(2θ) experimental GRM curve previously reported for interacting SSD particles on magnetic recording tape [1]
Summary
Gyroremanent magnetization (GRM) is produced by exposing a sample containing an anisotropic distribution of fine ferro- or ferrimagnetic particles to a single alternating field (AF) application [1], or by exposing a sample containing an isotropic distribution of such particles to two successive applications of an AF [2]. GRM is theoretically only produced by stable single domain (SSD) remanence carrying particles [1] These are the very particles that are most useful for both man-made recording processes, and for palaeomagnetic purposes for recording the magnitude and direction of the ancient geomagnetic field in rocks over geological time periods. The method is the most sensitive remanence anisotropy method, and allows one to exclusively determine the anisotropic distribution of the SSD particles [3] This means that GRM measurements represent a unique magnetic means of isolating the anisotropic distribution of SSD particles. This is important since an anisotropic distribution of these particles can deflect the natural remanent magnetization (NRM) in a rock sample away from the ancient field direction. The MD components are generally less stable over geological timescales
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