Overall recovery from Site 1188 (Snowcap hydrothermal site) was <20%, and all of the cores have significant gaps between pieces. Because, in many cases, the recovered material was not cylindrical, the split cores commonly did not fill the core liners. Furthermore, in the case of Hole 1188F, some of the routine measurements on the whole- and archive-half cores could not be performed because the new ADCB has a larger diameter (~95 mm) compared to the standard ODP core barrel (~56 mm). Shipboard analyses of Site 1188 cores, however, reveal a distinctive variation of magnetic properties with depth. This variation defines several zones, which may be an important observation for understanding the fluid flow and alteration process of this hydrothermal system.
In addition to routine ODP measurements, the relatively low core recovery at Site 1188 made it possible for us to conduct a series of rock magnetic measurements on discrete minicores sampled for shipboard analyses. The results of isothermal remanent magnetization (IRM) acquisition and subsequent thermal demagnetization experiments reveal quite distinct patterns among samples or groups of samples and may provide an important insight into the nature of magnetic carriers of the rocks from Site 1188.
Susceptibility measurements were first conducted on whole cores using the magnetic susceptibility meter mounted on the MST. These results are described in "Physical Properties". The whole cores were then split, and the susceptibility was measured on the archive halves. Because there were many significant gaps in the cores, instead of taking continuous measurements, we decided to make measurements of the archive half by setting the point susceptibility meter on the archive multisensor track (AMST) in manual mode and placing the probe in contact with the piece of core where we wanted a measurement. The susceptibility probe on the AMST has a depth range of 2 cm. We took point susceptibility measurements on pieces that were at least several centimeters long and a few centimeters thick. In addition, measurements were taken across intervals where there appeared to be changes in lithology or alteration style. The point susceptibility measurements proved to be quite useful at Site 1188 because most of the recovered rock samples were fragmented. Furthermore, the ADCB has a bigger diameter than ODP standard cores, and therefore, susceptibility measurements could not be performed on whole cores recovered from Hole 1188F. A total of 421 intermittently spaced susceptibility measurements were taken from Cores 193-1188A-2R through 22R and from Cores 193-1188F-1Z through 44Z.
Figure F124 shows the resultant downhole profile of magnetic susceptibility for Holes 1188A and 1188F combined. All susceptibility values in this report are given in SI units. Volume susceptibility measurements range between 10-4 and the upper limit of the susceptibility meter of 0.1 SI. The core recovered from the top 35 m of Hole 1188A that is dominated by relatively fresh dacite and rhyodacite has a moderately high susceptibility of 0.018 SI. The susceptibilities of most paramagnetic minerals are <0.01 SI (Collinson, 1983), and therefore, the susceptibility of this section (0-35 mbsf) cannot be explained without a contribution from ferromagnetic minerals, such as titanomagnetite and magnetite. In the interval between 35 and 135 mbsf, the susceptibility becomes extremely low and in some places it becomes even slightly negative, suggesting the presence of diamagnetic minerals such as quartz. This section of core coincides with intense alteration. There are locations where susceptibility increases sharply (e.g., in Sections 193-1188A-7R-1 [48.27 mbsf] and 10R-2 [79.42 mbsf]). However, in both cases the intervals of high susceptibility do not extend very far downhole. Then there is a large increase in susceptibility below 135 mbsf, down to almost the bottom of Hole 1188A. Peak susceptibility values in this interval (Cores 193-1188A-1R to 22R) range from 0.06 to 0.095 SI. In addition to an increase in peak values, there is also a greater degree of variability in the susceptibility values. This is in contrast with measurements from the higher sections, especially the top 35 mbsf, where there is relatively little scatter. The cause of high susceptibility values between 135 and 211 mbsf is discussed later in this chapter.
The susceptibility measurements of archive-half cores from Hole 1188F show two zones with different patterns. The susceptibility between 220 and 270 mbsf is almost two orders of magnitude lower than that between 280 and 375 mbsf, where the peak susceptibility ranges from 0.06 to 0.08 SI. The susceptibility shows a greater variability between 280 and 375 mbsf than above 270 mbsf. Although there is an ~10-m gap in the core section where no rock samples were recovered, the transition from the low- to high-susceptibility region appears to be quite sharp. The relationship of this transition between susceptibility and alteration style in Hole 1188F is again discussed later in this chapter.
At Site 1188 we were only able to perform remanent intensity measurements and alternating-field (AF) demagnetization on cores recovered from Hole 1188A. Most of the samples recovered from Hole 1188F were too fragmented and/or the diameter of the core was too large to pass through the magnetometer. The information regarding the remanent intensity of Hole 1188F came solely from the measurements that we conducted on minicore samples.
The natural remanent magnetization (NRM) of most of the archive halves from Hole 1188A was measured using the pass-through cryogenic magnetometer. The cores were then progressively AF demagnetized at peak values of 10, 15, 20, and 30 mT. The remanent intensity was measured after each step. Both the NRM and remanent intensity were measured at 2-cm intervals. Again, because of the poor recovery and significant gaps in the sequence of rock samples, the remanent intensity data were not downloaded to the Janus database directly. The purpose of these measurements was to look at the overall trend in the NRM of archive halves. The cryogenic magnetometer has response curves whose half-power points are ~7-10 cm wide. Therefore, data from core pieces <10 cm long should be excluded from processing. Several postprocessing steps were taken to remove the outliers and bad measurement points from the data. Despite this effort, however, many erroneous data points remain. In general, they are lower than the true values because they correspond to places where the rock sample did not fill the core liner or to measurements that were taken near the edge of the rock samples. Figure F125 shows the postprocessed downhole profile of the NRM taken from the archive halves for Holes 1188A and 1188F combined. Again, we note that this profile includes uncorrected measurements because the exact volume of rock samples that passed through the cryogenic magnetometer is not known; the purpose of showing this plot is only to look at the general trend of values. Some of the core sections, especially those from the upper part of the hole, were not measured at all because they contained insufficiently long rock samples. Overall, the intensity measurements of the half cores show a similar pattern to that of susceptibility (Fig. F124). A zone of high intensity appears below 135 mbsf (Fig. F125). However, in the case of the magnetization intensity, the increase is more abrupt. Also, there appears to be some decay toward the bottom of the hole, which is not seen in the susceptibility profile of Hole 1188A.
A total of 40 discrete minicore samples were taken from Holes 1188A and 1188F. Table T24 summarizes the location, dimensions, and brief descriptions of the color, texture, and hardness of these minicores. For Hole 1188A, all of the minicores were from oriented pieces of the core and cut perpendicular to the core axis except for the first two samples from the very top of the core (Samples 193-1188A-3R-1, 14 cm, and 5R-1, 45 cm) where no oriented pieces were recovered. In the case of Hole 1188F, many of the minicore samples from below 300 mbsf were from unoriented fragments of core. Before any demagnetizing steps were taken, we measured the anisotropy of magnetic susceptibility (AMS) at 15 different positions using the Kappabridge, and the data were processed with the program ANI20 supplied by Geofyzika Brno. The susceptibility of some samples was too low to be accurately measured on board the ship (Table T24). For instance, we were not able to measure the susceptibility of Samples 193-1188A-7R-1, 74 cm, and 9R-1, 25 cm, and we only report the average susceptibility values for a couple of positions for Samples 193-1188A-7R-2, 78 cm; 10R-1, 118 cm; and 14R-1, 103 cm; and 193-1188F-43Z-1, 4 cm. Figure F126 shows a downhole profile (combining Holes 1188A and 1188F) of the mean susceptibility of all the discrete samples. The magnetic susceptibility of minicores varies by several orders of magnitude. Minicores taken above 135 mbsf generally have magnetic susceptibility values <0.020 SI. Between 135 and 200 mbsf, the average susceptibility is much higher, ~0.060 SI. A maximum susceptibility of 0.115 SI was measured in Section 193-1188A-21R-1 (~194 mbsf). The susceptibility drops to <0.001 SI in the upper half of Hole 1188F. Sections 193-1188F-1Z-1 through 22Z-1 represent the low-susceptibility region of Site 1188. The susceptibility begins to increase below 295 mbsf, and there are rock samples with susceptibility values almost as high as those at the bottom of Hole 1188A. In general, the pattern is quite similar to that of point measurements taken on archive halves (Fig. F124). As in the archive halves, the high susceptibility values are found near the bottom of both Holes 1188A and 1188F.
The isotropy of magnetic susceptibility (AMS) of a rock sample is caused by a number of different factors (Tarling and Hrouda, 1993). However, in general, the mineral composition (Borradaile et al., 1987), grain shape (Uyeda et al., 1963), and mineral-preferred orientation are thought to be some of the important factors that affect the anisotropy. If the sample is dominated by magnetite, AMS may provide information on the grain shape, or if not, its cystallographically preferred orientation. Table T25 summarizes the magnitudes and principal axes of the AMS that were derived from the susceptibility tensor. There appears to be only a slight degree of anisotropy of magnetic susceptibility (P = kmax/kmin) in the samples from Site 1188. The maximum anisotropy determined is 1.16 from Sample 193-1188A-17R-2, 30 cm (~147 mbsf). Figure F127 is a Flinn-type diagram representing the susceptibility ellipsoid in two-dimensional space. Except for Sample 193-1188A-17R-2, 30 cm, the shape of the ellipsoids is close to spheroidal, which means that there is no evidence for a strongly preferred orientation of susceptibility.
The NRM of minicore samples (Fig. F128) was measured using the cryogenic magnetometer. Again, the NRM profile of Site 1188 shows a similar pattern to that of susceptibility (Figs. F124, F126). For Hole 1188A, where we also have the NRM intensity (Fig. F126) of the archive-half cores, the profiles match quite well. The minicores from the top 35 m of the hole exhibit high intensities, whereas those taken between 35 and 135 mbsf show very low intensities. The intensity of NRM increases dramatically between 135 and 211 mbsf. A very high intensity value of 11 A/m is present at 146 mbsf (Sample 193-1188A-17R-1, 109 cm). Unlike susceptibility, however, the NRM intensity values tend to decrease toward the bottom of Hole 1188A. A corresponding decrease is shown by the archive-half NRM measurements (Fig. F126). Therefore, it is most likely a true trend and not an artifact caused by insufficient sample points. The remanent intensities were measured by performing progressive AF demagnetization at peak fields of 10, 15, 20, 25, 30, 40, 50, 60, and 80 mT. Table T24 shows the NRM intensity and stable inclination and declination of the minicore samples. The changes in magnetization intensity, inclination, and declination of the minicore samples with progressive AF demagnetization are summarized in Table T26. Figure F129 is an inclination plot of the stable magnetization. Directions are reported using the conventional ODP core orientation. Inclination generally ranges between -30° and 5° in the upper 200 m of the hole. In Hole 1188F, because of the fragmented nature of the rock samples, we were only able to conduct reliable magnetization direction measurements on 5 samples out of 23. However, the inclination values of those few measurements fall within the range measured in cores from Hole 1188A. The low negative inclination values in the upper 200 m of the hole are consistent with the fact that our drill site lies just south of the magnetic equator. In Hole 1188F, the sample taken near the bottom of the hole (Sample 193-1188F 43Z-1, 56-89 cm) has an inclination of -16°, which is reasonably consistent with the present-day Earth field. According to the International Geomagnetic Reference Field (IGRF), our drill site has declination and inclination values of 5.5° and -7.7°, respectively.
We also computed the Koenigsberger ratio (Q) of the minicore samples, which compares the relative contribution of the remanent magnetization against the magnetization induced in the cores by Earth's magnetic field. The Koenigsberger ratios range from 5 to as high as 184 (Table T24), which indicates that the in situ magnetization of these rocks is dominated by a remanent magnetization rather than magnetization induced by Earth's magnetic field.
Overall, the AF demagnetization appears to be quite effective in removing the secondary magnetization. Figure F130 includes two examples from Hole 1188A (Samples 193-1188A-10R-1, 118 cm, and 17R-2, 30 cm) showing the decay in intensity and convergence toward the origin in the vector end-point diagrams with progressive demagnetization. Figure F131 shows an example of a strong secondary magnetization (Sample 193-1188A-16R-2, 46 cm). In this case, the initial declination and inclination values of 308° and -28° changed to 198° and -20°, respectively, after AF demagnetization (Table T26). This sample derives from a strongly altered, layered unit, where the presence of a secondary magnetization might be expected.
Besides standard shipboard analyses, we conducted a few additional experiments on the minicore samples to examine the magnetic properties of Site 1188. IRM and backfield isothermal remanent magnetization (BIRM) data are often used to discriminate between high- and low-coercivity magnetic phases. The field was applied at increasing steps of 50, 100, 150, 200, 250, 300, 400, 500, 800, and 1100 mT. In general, low-coercivity minerals include magnetite and titanomagnetite, whereas minerals such as hematite fall in the high-coercivity group. However, the grain size and oxidation and domain states also affect the coercivity.
All 40 minicore samples from Site 1188 were imparted with an IRM using impulse fields. For Hole 1188A, the experiment was performed on three groups of five to six samples each. Most of the samples exhibit a steep gradient of IRM acquisition up to the saturation level (defined as 95% of the maximum IRM intensity), after which no significant increase in intensity occurs with increasing applied field (Fig. F132). The maximum remanence that is acquired is referred to as the saturated isothermal remanent magnetization (SIRM). The sample from near the top of the core (Sample 193-1188A-3R-1, 14 cm) became saturated at a low field (<100 mT), which probably suggests that the magnetic carrier is predominantly magnetite or titanomagnetite. The saturation of Sample 193-1188A-5R-1, 46 cm, occurred at a much higher field (300-400 mT). Similarly, Samples 193-1188A-10R-1, 118 cm; 12R-2, 44 cm; and 16R-2, 46 cm, exhibited relatively high coercivity (500 mT). The high coercivity may be in part caused by a subtle change in the style of alteration or alteration minerals. The presence of hematite was noted in Sample 193-1188A-16R-2, 46 cm (see "Hydrothermal Alteration"). In general, there appears to be a quite close relationship between the NRM and SIRM values. Samples with high NRM tend to have high SIRM. For example, all the minicore samples between 135 and 211 mbsf exhibit relatively high SIRM intensities.
Another notable feature is that some samples exhibit high variability of IRM intensity values as a function of applied field. The fluctuation is especially noticeable for Samples 193-1188A-17R-2, 30 cm, through 21R-1, 83 cm (Fig. F132). The high variability in IRM is an instrumental effect. The 2G Enterprises cryogenic magnetometer used in this study is an extremely sensitive instrument, but the measurements tend to become unstable at high magnetization values.
We measured IRM intensities of all 23 samples collected from Hole 1188F. Some of these samples were also measured for BIRM intensities. To examine the behavior of magnetic minerals at low fields (<100 mT), we added 10, 25, and 70 mT to the impulse field steps. Figure F133 shows the results of IRM measurements conducted for Hole 1188F samples. The experiment was performed in five groups, using four to five samples per group. As in Hole 1188A, we see correlation between NRM and SIRM values, with samples with relatively high NRM values having high SIRM values (Fig. F133). Samples 193-1188F-1Z-1, 85 cm, through 19Z-1, 14 cm, which roughly correspond to the depth interval 218-270 mbsf, exhibit a monotonous rise in IRM intensity with increase in applied field. Below this depth, from 270 to ~325 mbsf (Samples 193-1188F-22Z-1, 123 cm, through 31Z-1, 36 cm), there is a small deviation from the monotonous curve observed above. A significant change occurs below 325 mbsf. The rock samples start to show an immense fluctuation in the IRM intensity, similar to what we saw near the bottom of Hole 1188A.
In addition to variability in IRM intensities, the samples from Hole 1188F exhibit a range of coercivity fields. In general, coercivity decreases with increasing depth. Samples 193-1188F-1Z-1, 85 cm, through 3Z-2, 122 cm, show relatively high coercivity (>400 mT). The fact that there is no significant increase in IRM intensity beyond 500 mT suggests that the high coercivity in these samples is not caused by the presence of hematite but rather by other factors that affect coercivity such as grain size and oxidation and domain states. The coercivity decreases to 200-250 mT for Samples 193-1188F-13Z-1, 32 cm, through 19Z-1, 14 cm. It diminishes further to ~100 mT for Samples 193-1188F-22Z-1, 123 cm, through 26Z-1, 24 cm, and becomes extremely low in the samples that exhibit high IRM fluctuations.
Figure F134 shows the BIRM measurements that were performed on some of the groups of samples. BIRM intensity measurements were conducted to make sure that some of the features that we observed in the IRM experiment appear in the BIRM experiment as well. In general, the characteristics that we observed in the IRM experiment can be seen in the BIRM results. Very high coercivity minerals such as hematite do not seem to be dominant in any of the samples from Hole 1188F.
Another additional experiment that was conducted on the shipboard minicore samples was to monitor the magnetization intensity with increasing temperature. By examining the thermal demagnetization curves, one may be able to determine the magnetic minerals in the minicore samples based on their Curie temperature. Table T27 summarizes the Curie temperatures of some of the more common magnetic minerals. We chose four minicore samples from Hole 1188A (193-1188A-5R-1, 46 cm; 7R-1, 74 cm; 17R-1, 109 cm; and 21R-1, 28 cm) that were already saturated with IRM magnetization. The samples were progressively heated up to 700°C, with smaller temperature intervals beyond 500°C. Figure F135 shows the variation in remanent intensity as a function of temperature. A sharp drop in intensity was observed for Sample 193-1188A-5R-1, 46 cm, at 350-400°C, which suggests that the remanence is carried by a single magnetic phase. This phase is most likely titanomagnetite. In general, the Curie temperature falls with increasing titanium content as well as other impurities such as Mg, Ca, Al, Cu, V, and Si (Thompson and Oldfield, 1986). However, if the magnetic carrier were a single-phase titanomagnetite, the Curie temperature of 400°C would correspond to a composition of roughly 35% Fe3O4 and 65% FeTiO4. Sample 193-1188A-7R-1, 74 cm, shows a feature similar to Sample 5R-1, 46 cm. However, it has a much more subdued intensity. This probably suggests a greater degree of low-temperature alteration in which titanomagnetite became oxidized to titanohematite. The Curie temperatures of titanohematite and titanomagnetite are almost identical within the range of titanium concentrations that we have observed.
Samples 193-1188A-17R-1, 109 cm, and 21R-1, 28 cm, exhibit somewhat complex thermal demagnetization curves. Sample 193-1188A-17R-1, 109 cm, shows an increase in the remanent intensity between 200° and 350°C and a small drop at 560°C followed by a larger drop between 600° and 680°C. The first decrease of intensity at 560°C corresponds to the Curie temperature of magnetite, whereas the second decrease at 680°C corresponds to that of hematite. However, hematite has a much lower value of saturation magnetization. For instance, at room temperature (20°C) it is only ~5% and 9% of that of magnetite and maghemite, respectively. Therefore, a dominance of hematite does not explain the high remanent intensity of Sample 193-1188A-17R-1, 109 cm. One possible explanation may be that maghemite converted to hematite upon heating. The conversion to hematite at 300°C is one of the characteristic properties of maghemite. If so, this conversion may explain the drop of intensity of Sample 193-1188A-17R-1, 109 cm, above 300°C (Fig. F135). In this case, the dominant magnetic carriers in the preheated original sample would have been magnetite and maghemite.
The thin-section analysis shows no original hematite in Sample 193-1188A-17R-1, 109 cm. However, IRM measurements taken on the same sample after it was thermally demagnetized show the possible presence of hematite. Figure F136 shows the results of IRM intensity measurements taken on samples after they were thermally demagnetized to over 700°C. Sample 193-1188A-17R-1, 109 cm, is characterized by a sharp rise in IRM below 100 mT and a gradual increase beyond 500 mT. The sharp increase may be caused by a small amount of magnetite or maghemite that was not converted to hematite, whereas the latter increase may be caused by converted hematite. Another notable feature is the large drop in the IRM intensity before and after the heating. Considering that Sample 193-1188A-17R-1, 109 cm, has one of the highest NRM intensities of Hole 1188A and thus a very high IRM intensity, the drop in IRM intensity after thermal demagnetization is significant.
Sample 193-1188A-21R-1, 28 cm, does not show a single drop in intensity but instead a series of decays (Fig. F135). Therefore, it appears to be a mixture of different magnetic minerals. On the basis of the thermal demagnetization curve, it appears that this sample may have as many as four different phases of magnetic minerals. Still, the predominant magnetic carriers appear to be titanomagnetite and magnetite. Beyond 640°C, the sample loses all its remanent intensity, so it is unclear whether or not it contains hematite. However, thin-section study of the original sample shows a small amount of hematite.
The IRM measurement taken on Sample 193-1188A-21R-1, 28 cm, after it was thermally demagnetized shows three small steps of increase in IRM intensity in a relatively low field (<350 mT), followed by a gradual increase in IRM in the higher field (>400 mT) (Fig. F136). Such features are not seen in the IRM measurements of the preheated sample. One possible explanation of this difference in the IRM behavior of samples before and after heating is that the extremely low coercivity mineral in Sample 193-1188A-21R-1, 28 cm, may be affecting the initial IRM measurements. Upon heating to 700°C and then cooling, the extremely low coercivity mineral has disappeared or been reduced to some other form. The exact Curie temperature of this magnetic carrier is uncertain.
Both the archive-half susceptibility and the discrete remanent intensity and susceptibility measurements of Site 1188 show consistent patterns (Figs. F124, F128). Based on magnetic properties, Hole 1188A may be divided into three intervals. The top 35 m of the cored material is characterized by high magnetic susceptibility (0.018 SI) and remanent intensity (0.82-2.76 A/m). This depth roughly corresponds to the relatively fresh dacite-rhyodacite section at the top of the hole. The degree of alteration of the recovered rhyodacite ranges from none to slight. The susceptibility of 0.018 SI is consistent with the presence of ~1% magnetite (Hrouda and Kahan, 1991). The magnetic measurement values show a considerable drop from 35 to ~135 mbsf. This section represents a highly altered interval in Hole 1188A. There are small intervals within this section where magnetization and susceptibility are elevated. These intervals are coincident with more intense silicification, and magnetite was commonly reported from silica-rich intervals (see "Hydrothermal Alteration"). A marked increase in both the susceptibility and remanent intensity occurs below 135 mbsf. In this interval, NRM intensities are as high as 11 A/m and susceptibility increases as much as three orders of magnitude compared to the interval above. Magnetite-bearing veins are found throughout this interval (see "Structural Geology"), which would account for the increased values. However, the remanent intensity and susceptibility vary in different manners. Magnetic susceptibility shows higher values toward the bottom of Hole 1188A, whereas remanent intensity actually decreases. This feature is unlikely to be a sampling artifact because it is apparent in the archive-half as well as minicore sample measurements. The discrepancy between the magnetic susceptibility and remanent intensity trends is reflected in the relatively low Koenigsberger ratio (Q = 5) of Sample 193-1188A-21R-1, 83 cm. The cause of this difference is unclear. Whereas the presence of paramagnetic minerals such as pyrite can increase the magnetic susceptibility, and Sample 193-1188A-21R-1, 83 cm, is from an interval that includes pyrite veins, there is no notable increase in pyrite abundance in the core descriptions from this interval.
The investigation of minicore samples shows that remanence dominates the magnetization induced by Earth's field. The inclinations of samples with high Koenigsberger ratios (Q > 20) range between -22° and -7°, which is slightly steeper than that predicted by the present-day Earth field reference model for zero-age material. Acquisition of IRM suggests that minicore samples from the top of Hole 1188A (Sample 193-1188A-3R-1, 14 cm) and from below 149 mbsf (samples taken from Sections 193-1188A-17R-2 through 21R-1) have relatively low coercivity (<200 mT) (Fig. F132). A number of minicore samples in between (20-149 mbsf) show much higher coercivity (300-500 mT), which may be caused by alteration or the presence of high-coercivity minerals such as hematite. This could not routinely be checked on shipboard samples because of a limit on the number of thin sections that could be prepared. However, for one sample (193-1188A-12R-2, 44 cm) that has a very high coercitivity (400-500 mT), hematite was identified in thin section. The SIRM values among the minicore samples are similar to those of the NRM.
Thermal demagnetization performed on four representative minicore samples suggests that at least in the top 50 m of the hole the dominant magnetic carrier is titanomagnetite with variable degrees of alteration (Fig. F135). At 146 mbsf (Sample 193-1188A-17R-1, 109 cm), magnetite and possibly maghemite may be the magnetic carriers. Sample 193-1188A-21R-1, 28 cm (193 mbsf), shows a complex demagnetization with a gradual decrease between 300° and 640°C. Our interpretation is that this sample contains a mixture of magnetic minerals, including magnetite and titanomagnetite, probably in various phase and domain states.
In Hole 1188F, the diameter of the recovered ADCB samples was larger than the standard ODP core, preventing long-core remanence measurements on the archive-half cores. In addition, most of the samples that were recovered were either rubble or rocks broken into short pieces. As a result, we were not able to obtain reliable estimates of the magnetization direction from many of the minicore samples. The few samples that were long enough for orientation measurements have inclination values similar to those of Hole 1188A.
One of the most notable features of Hole 1188F is a sharp rise in the susceptibility values below 275 mbsf (Fig. F124). The zone between 211 and 275 mbsf is uniformly low in magnetic susceptibility (<0.002 SI). Below 275 mbsf, the peak susceptibility increases by almost two orders of magnitude. The change in magnetic susceptibility corresponds to a major change in the alteration style in Hole 1188F (see "Hydrothermal Alteration"). The NRM shows a similar pattern to susceptibility, although the most significant rise in the remanence occurs at a greater depth (>330 mbsf) (Fig. F128). The maximum NRM value of 12.66 A/m occurs at 336 mbsf in Hole 1188F. The high magnetization intensity at the bottom of the hole is consistent with the presence of magnetite, which was identified by XRD and optical microscopy (see "Hydrothermal Alteration"). As in Hole 1188A, there is a large contrast in IRM behavior between the upper and lower parts of Hole 1188F. A dramatic increase in the variability of SIRM values occurs below ~330 mbsf (Fig. F133). We attribute this behavior to extremely low coercivity magnetic minerals present in the rock samples. Because we have not done any thermal demagnetization experiments on these samples, we are not certain of the nature of this low-coercivity magnetic mineral. One characteristic of Hole 1188F, which distinguishes it from Hole 1188A, is that there is a fairly steady decrease in the coercivity from top to bottom.