We carried out a series of rock magnetic investigations on small chips of sediment samples to characterize the carriers of magnetization. Thirteen samples were selected from Hole 1138A and seven from Hole 1140A. Differences in lithology and in AF demagnetization characteristics were used as a selection criterion.
The selected samples were used in IRM experiments in which the samples were placed in progressively stronger magnetic fields up to a maximum of 230 mT, and their magnetization was measured. The results of these IRM experiments (Fig. F5A, F5B) show that samples display two types of behavior.
Samples from Site 1138 (Fig. F5A) and most of Site 1140 (Fig. F5B) show a sharp increase in magnetization in fields <150 mT followed by a slow increase through 230 mT. The rapid increase of the magnetization in low fields is typical for magnetite or Fe-Ti spinel, which, because of its high spontaneous magnetization, tends to dominate the IRM even when it is less abundant than other minerals. The second type (e.g., Sample 183-1140A-21R-4, 73 cm) displays a more continuous increase up to 230 mT, and it might be caused by the presence of a high-coercivity mineral.
Hysteresis parameters were obtained from hysteresis loops for a determination of the magnetic domain state. Of interest were the coercive force Hc and the ratio of saturation remanence Irs over saturation magnetization Is. The coercivity of remanence Hcr was determined by measuring the backfield curves of the 16 samples that contained sufficient ferrimagnetic material for a determination of hysteresis parameters.
Hysteresis loops of samples from Site 1138 show four different characteristic behaviors. Very broad hysteresis loops with high Hc could be caused by single-domain particles of magnetite (Fig. F6A).
Some samples clearly show a diamagnetic behavior at high fields and a ferrimagnetic component that dominates at smaller fields (Fig. F6B). Very weakly magnetic samples with a high content of calcite also show a strong diamagnetic component with a superimposed ferrimagnetic component (Fig. F6C). Hysteresis loops with strong paramagnetic components have been corrected for high field slope before interpretation (Fig. F6D).
Hysteresis loops of samples from Site 1140 all show the same behavior with a paramagnetic component that differs from sample to sample (Fig. F6E, F6F).
Following the method of Day et al. (1977), we determined the domain status by plotting the Irs/Is ratio vs. Hcr/Hc (Fig. F7). The results from almost all samples are characteristic for pseudo-single domain (PSD) particles. Two of the selected samples show characteristics of single-domain particles.
For thermomagnetic analyses, some selected samples were heated between room temperature and 700°C and cooled to room temperature in the presence of a magnetic field (100 mT). For Site 1138, two examples are shown in Figure F8A and F8B.
Sample 183-1138A-12R-2, 120 cm (Fig. F8A), shows reversible behavior during the heating-cooling cycle, indicating that no secondary mineral phase has been produced during heating. The constant intensity between 20° and 400°C is followed by a major, steep decline in intensity between 400° and 600°C that is consistent with the removal of a mineral whose blocking temperature lies within this range (magnetite or a low Ti magnetite). Irreversible behavior during the treatment cycle is shown by Sample 183-1138A-68R-3, 104 cm (Fig. F8B). The heating curve of this sample shows a sudden decrease in intensity between 20° and 80°C, which might be caused by the presence of goethite. This initial decrease is followed by a constant intensity up to 180°C and another decrease between 180° and 400°C, which is ascribed to the removal of a low blocking temperature mineral. The sudden high peak after 400°C is due to the formation of secondary magnetite.
For Site 1140, Sample 183-1140A-21R-4, 73 cm, is shown in Figure F8C. Its irreversible behavior, consistent with the conversion of a low-magnetization component to a higher magnetization component, is somehow similar to the one for Sample 183-1138A-68R-3, 104 cm. After a decrease between 20° and 100°C and a following constant magnetization up to 300°C, a sudden decrease from 300° to 350°C is observed and is ascribed to the removal of a low blocking temperature mineral. From 350° to 600°C, a somewhat continuous decrease is displayed.