Iii Il

i—i—i—i—r—i—r—i—n—'i r f" "i 100 200 300 400 500 600

Blocking temperature (°)

Fig. 2.20. Blocking temperature versus relaxation time diagrams for (a) magnetite and (b) hematite. Lines on each diagram represent the locus of points where the magnetization in a given population of grains will be reset. Grains in region A have sharply defined blocking temperatures within 100°C of the Curie temperature. Grains in region B have blocking temperatures on the laboratory time scale at least 100°C below the Curie temperature. From Pullaiah etal. (1975), with permission from Elsevier Science.

grains of magnetite with relaxation time of 10 Myr at 260°C. Such grains can be expected to acquire a substantial TVRM if held at 260°C for 10 Myr and then cooled to 0°C. However, the same grains at point 2 have x = 30 mins at 400°C. Therefore by heating these grains to 400°C in the laboratory for 30 mins in zero magnetic field, the TVRM acquired at 260°C for 10 Myr can be unblocked and reset to zero. Point 3 in Fig. 2.20a also corresponds to SD grains of magnetite with relaxation time of 10 Myr but now at 520°C. The same grains at point 4 have x = 30 mins at 550°C. Therefore the TVRM acquired over 10 Myr at 520°C can be unblocked by heating to a slightly higher temperature of 550°C for 30 mins in zero magnetic field.

Two regions labeled A and B are indicated in each of the diagrams in Fig. 2.20. Region A refers to those grains that have sharply defined blocking temperatures within 100°C of the Curie temperature. Such grains are resistant to resetting their magnetization except by heating to temperatures approaching the Curie temperature. Region B includes those grains that have blocking temperatures at least 100°C below the Curie temperature on laboratory time scales (30 mins). They are capable of acquiring TVRM at moderate temperatures (~300°C) if exposed to those temperatures for geologically reasonable lengths of time (-10 Myr). Thus, grains in region B are not good carriers of the primary magnetization and can likely acquire TVRM or VRM. Figure 2.20 indicates that the primary NRM in rocks can survive heating to the greenschist facies (300-500°C) but not to the amphibolite facies (550-750°C).

2.3.9 Stress Effects and Anisotropy

Most rocks are subjected to stress during their history, either from deep burial or from tectonism. Magnetocrystalline anisotropy is stress dependent so that application of stress can cause a change in the magnetization of a grain, the effect being referred to as magnetostriction (see also §2.3.1). It should be noted that stress alone cannot induce magnetic moments; the application of stress to an initially isotropic material causes stress-induced anisotropy, which may change the state of magnetization.

In the early days of paleomagnetism it was thought that the simple, reversible application of elastic stress to a rock would cause a substantial deflection of its remanent magnetization. Thus, although a rock that cooled under stress would acquire a magnetization in the direction of the field in which it cooled, upon release of the stress before measurement, changes in magnetization would occur that would make the original field direction impossible to determine. However, experimental results (Stott and Stacey, 1960; Kern, 1961) showed clearly that for isotropic rocks the TRM was always acquired parallel to the applied field. It appears that when an intrinsically isotropic rock is subjected to stress while cooling in a magnetic field, it acquires a TRM in a direction deflected away from the field by such an angle that it returns to the field direction precisely when unloaded.

The development of folds in rock formations relieves compressive tectonic stress and involves large internal strains. It is possible to "destrain" the natural remanence in deformed rocks if a detailed knowledge of both the strain mechanism and the response of the magnetic grains and their remanence vectors to the strain can be measured (e.g. Cogné and Perroud, 1987; Borradaile, 1993). This will be discussed in more detail in §3.3.2 with respect to the fold test in paleomagnetism.

If rocks have an intrinsic anisotropy, the TRM may be deflected away from the direction of the applied field towards the direction of easy magnetization (Fig. 2.21). Anisotropy is measured by the variation in values of susceptibility, saturation magnetization, IRM, or TRM in different directions in a rock specimen. Normally, anisotropy of susceptibility is considered, since this is the easiest to measure and the procedure does not generally alter the state of magnetization of the specimen. The degree of anisotropy An is then expressed as the ratio of maximum to minimum susceptibility

A value of An of 1.25 means that the maximum susceptibility exceeds the minimum by 25%. Such a specimen is often referred to as having "25 percent anisotropy".

Axis of maximum susceptibility

Axis of maximum susceptibility

Fig. 2.21. Deflection of TRM from an applied field B due to anisotropy.

The main issue that arises in a paleomagnetic context is the degree of anisotropy that can be tolerated in a specimen before the TRM is deflected through a significant angle. It is simplest to consider the most favorable case for the deflection of TRM, which occurs when the direction of the applied field lies in the plane containing the maximum and minimum susceptibility directions. Suppose the applied field B makes an angle 0 with the direction of maximum susceptibility (Fig. 2.21). Since TRM is proportional to the applied field in low fields (M = cB), the specimen will acquire TRM components in the maximum and minimum directions given by

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