Fig. 3.23. Mrs/Ms versus BCJ/BC plot for (a) the young Vulcano Island basalts from the Aeolian Islands, Sicily (from Laj et al., 1997, with permission from Elsevier Science) and (b) the Jurassic Lesotho basalts of southern Africa (Kosterov et al., 1997). The magnetic grain size boundaries are those listed in Table 3.3.
given in Fig. 3.23. Most grain sizes fall within the PSD grain size range as might be expected. For sediments these plots have been found useful in distinguishing between remagnetized and unremagnetized limestones (§2.3.6). Channel] and McCabe (1994) found that the unremagnetized Maiolica limestones from Italy followed the typical SD-PSD trends on these plots (Fig. 3.24). However, the remagnetized limestones from both North America and Europe followed an unusual trend with high SD-PSD values of MJMS but PSD-MD values of BJBC from about 3 to 10. These high values of BJBC could result from a mixture of SP and stable SD grains as the scanning electron microscope images of Fig. 2.18 suggest since high values of BJBC are a characteristic of SP as well as MD grains. The intercept of the trend of the remagnetized limestones at BJBC - 1 is MJMS «0.87 implying that magnetocrystalline anisotropy dominates, as would be expected for spherical grains. If this is so, then the observed MJMS values have been considerably reduced by the addition of SP grains.
The classical hysteresis loops from SD, PSD, and MD grains are frequently found in rocks. However, rocks rarely contain just one domain state. A sample containing a mixture of grain sizes of a single magnetic mineral or a mixture of different magnetic minerals can sometimes give rise to hysteresis loops that are similar to those of a population of grains of uniform composition and identical size. Further complication can occur because the loops are frequently distorted, having constricted middle sections (wasp-waisted loops) or spreading middle sections and slouching shoulders (potbellied loops). It seems that the requirement for such distortions is the presence of at least two magnetic components with strongly contrasting coercivities. Numerical simulation of hysteresis loops shows that both wasp-waisted and potbellied loops can easily be
Was this article helpful?