Ppg MB

Typical conditions that might exist are B = 0.05 mT, (p-p0) = 4 x 103 kg m"3 and M= 103 Am"1 for PSD grains of diameter 10 (im. This gives h0 ~ 20 (am and then ((«0.1 s, so that complete alignment is almost instantaneous. Both the height and the time will be even less for smaller grains. Shive (1985) has demonstrated that perfect alignment is achieved in fields of 0.03-0.1 mT as the theory above suggests.

When magnetized grains settle on the bottom there will be a mechanical torque exerted on the grain by the surface on which it settles (it will roll or fall into the position of least potential energy). This torque will misalign M with B and, for grains larger than 10 |im, these mechanical torques become stronger than the magnetic aligning force (Dunlop and Özdemir, 1997). For increasing grain sizes the magnetic alignment achieved during fall will be completely destroyed on settling. Models of these mechanical torques suggest that grain rotations essentially have a random effect on the observed declination D, but they result in a systematic decrease in the inclination I, known as the inclination error. Laboratory studies of these effects show that the sediment inclination /s will invariably be less than the applied field inclination /B, where tan /s = / tan /B (2.3.37)

and/is normally about 0.4 (King, 1955; Griffiths et ai, 1960). Because of the randomizing effects on the declination D, the DRM intensity will also be much weakened.

Although such inclination errors have been observed in some sediments, studies of deep-sea sediments suggest that they do record the field direction without significant inclination error (Fig. 2.19). This is almost certainly due to the effectiveness of PDRM as described below. The effect of compaction may also cause a shallowing of inclination because grains tend to be rotated into the bedding plane (Blow and Hamilton, 1978; Anson and Kodama, 1987) and inclination errors, where observed, are more likely to be caused by this effect than those due to the mechanical torques that are applied when the grains settle.

Post-Depositional Remanent Magnetization

When magnetic grains settle on the water-sediment interface and then acquire the possible inclination errors described above, they generally fall into water-filled voids or interstitial holes where they are still free to rotate (Irving and Major, 1964). As the general theory of DRM shows, the magnetic alignment of the magnetic particles is virtually instantaneous, so that the suspended grains tend to become realigned again in the direction of the applied field B. PDRM is most efficient if the magnetic grains are significantly finer than the silicate grains and they can rotate readily in pore spaces. The density differential during sediment transport seems to ensure that this situation generally holds.

Tucker (1980) has examined the time dependence of PDRM following reorientation of the applied field in an artificial magnetic slurry and Hamano (1980) calculated theoretical time constants for the realignment of PDRM in

10 30 50 70 90

Field inclination (°)

Fig. 2.19. Experimental determination of PDRM inclination in laboratory redeposited deep-sea sediments plotted as a function of the applied field inclination. Vertical bars indicate the spread of replicate measurements. From Kent (1973), reproduced with permission from Nature.

10 30 50 70 90

Field inclination (°)

Fig. 2.19. Experimental determination of PDRM inclination in laboratory redeposited deep-sea sediments plotted as a function of the applied field inclination. Vertical bars indicate the spread of replicate measurements. From Kent (1973), reproduced with permission from Nature.

sediments with various void ratios and axial ratios of magnetic particles. These studies all confirm that PDRM is a viable and natural process and realignment readily occurs within a very short time.

The main factor in the PDRM process is the water content. When water content drops below some critical value the remagnetization process ceases and the particle rotations are blocked. Slow deposition, such as occurs in deep-sea sediments, and fine sediment particle size favor PDRM because they promote high water content and delay compaction. PDRM experiments on redeposited deep-sea sediments (Fig. 2.19) confirm that they record the field direction without significant inclination error (Kent, 1973). Opdyke and Henry (1969) and Schneider and Kent (1990) have shown that the inclinations observed in deep-sea sediments worldwide follow the expected latitude dependence for a geocentric axial dipole field (§1.2.3) with only second-order variations over the past several hundred thousand years.

2.3.8 Viscous and Thermoviscous Remanent Magnetization

After acquiring their primary remanent magnetization on formation, rocks are continually exposed to the Earth's magnetic field throughout their history. Because of the effect of magnetic viscosity (§2.3.3), those grains that have shorter relaxation times x given by (2.3.14) or (2.3.15) can acquire a secondary magnetization long after the formation of the rock. Such a secondary magnetization is termed viscous remanent magnetization (VRM) and involves the realignment of the magnetic moments of those grains having values of x less than the age of the rock. This can usually be erased by using suitable demagnetization techniques discussed in §3.4. The VRM, Mvrm, at a given temperature is acquired according to the relation where t is the time (in seconds) over which the VRM is acquired and S is known as the viscosity coefficient. Because of the logarithmic growth of VRM with time, VRM is usually dominated by that acquired in the most recent field to which the rock has been exposed. Rocks with large VRM components generally have their NRM aligned in the direction of the present geomagnetic field.

Since their time of formation rocks may be subjected to heating either from deep burial and subsequent uplift or from the effects of metamorphism. In §2.3.3 it was shown from the approximate relationship given by (2.3.16) that the effect of maintaining a rock for a short time at a higher temperature is equivalent to maintaining it for a much longer time at a lower temperature. Rocks that have been heated to temperatures below the Curie temperature of their magnetic minerals for a (geological) short time during their history, and then subsequently cooled again in the prevailing magnetic field, will acquire a thermoviscous remanent magnetization (TVRM), a terminology used by Butler (1992), although Chamalaun (1964) and Briden (1965) originally referred to it as a viscous partial thermoremanent magnetization (viscous PTRM).

Pullaiah et al. (1975) developed a blocking temperature diagram approach to the acquisition of TVRM for magnetite and hematite bearing rocks that makes use of the more precise form of (2.3.15) in the following way. Because both B'c and Ms are functions of temperature, (2.3.15) can be rewritten by replacing B'Q with B'C[T] and Ms with Ms [7] to indicate their values at the temperature T so that (2.3.16) can be replaced more precisely with

For SD magnetite the microscopic coercivity is dominated by shape anisotropy, so that B'c is then given by (2.3.6) and B'C[T\ oc MS[T\. For SD hematite the microscopic coercivity is caused by magnetoelastic effects whose temperature variation is not well known. Pullaiah et al. (1975) suggested the relationship Ml holds so that in these two cases (2.3.39) becomes

Magnetite:

Hematite:

Using the known temperature dependence of Ms for magnetite and hematite (Fig. 2.3), the above relationships are displayed in the blocking temperature (TR) versus relaxation time (t) curves of Fig. 2.20. These plots show the locus of the points in x-TB space that reset the same grains and enable the prediction of time-temperature stabilities. For example, point 1 in Fig. 2.20a corresponds to SD

106yr

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