Core recovery at Site 1189 (Roman Ruins hydrothermal site) was low overall, although it improved in the lower sequence of Hole 1189B (>120 mbsf), yet with numerous gaps between pieces. Samples from <120 mbsf in Hole 1189B were too small for magnetic measurements. Standard measurements were made on both archive-half cores and discrete minicores from Hole 1189A down to 106 mbsf and from Hole 1189B from 117 mbsf to the end of the hole. The data from both holes have been combined for the purposes of profile plots in this chapter. As at Site 1188, we had time to conduct additional rock magnetic analyses to determine the nature of magnetic carriers. The results of our analyses show that Site 1189 as represented in the combined profile is similar to the upper half of Site 1188 in many respects, although the susceptibility and remanence values tend to be lower overall at Site 1189. For instance, both sites are characterized by a relatively high remanence at the top (above 25-35 mbsf) and a large interval of lower remanence and remanent intensity values below. As at Site 1188, there is also a region of relatively high susceptibility below 130 mbsf. One important difference between the two sites is that at Site 1188 the susceptibility increases with depth between 135 and 211 mbsf, whereas the remanent intensity decreases. However, at Site 1189, the opposite is true. We see a decrease in susceptibility and an increase in remanent intensity toward the bottom of the lower sequence in Hole 1189B.
The susceptibility of whole cores was first measured using the magnetic susceptibility meter mounted on the MST. The results of these measurements are described in "Physical Properties". Because of significant gaps in the cores, susceptibility readings were taken from the archive-half cores using point measurements instead of making a continuous measurement along the entire length of the archive half. In this way, we avoided taking measurements in core gaps and undesirable pieces of core sections. The susceptibility probe on the archive multisensor track has a depth range of 2 cm. A total of 197 susceptibility measurements were performed on pieces that were longer and thicker than 2 cm. The downhole profile of these measurements is shown in Figure F130. All susceptibility values in this report are given in SI units. The susceptibility profile shows several peaks. The top 25 m in Hole 1189A exhibits high susceptibility values. Although there are a few very low values, in general, the susceptibility ranges from 0.01 to 0.028 SI, which is similar to the top 35 m of Hole 1188A but with a greater degree of scatter (see Fig. F124 in the "Site 1188" chapter). Below 25 mbsf, the susceptibility is very low, except between 57 and 90 mbsf where there is a moderate increase to ~0.01 SI. The susceptibility then increases from ~120 mbsf in the lower sequence of Hole 1189B and reaches its maximum value (0.031 SI) between 130 and 140 mbsf. Below 170 mbsf, there is another region of high susceptibility (0.021 SI), which peaks at ~175 mbsf. The presence of relatively high susceptibility below 130 mbsf is similar to susceptibility data from Site 1188 (see Fig. F124 in the "Site 1188" chapter). There is also an apparent decrease in the susceptibility values in the lower 75 m at Site 1189, whereas no such pattern was recognized in the data from Site 1188. However, the susceptibility values at Site 1189 are lower (maximum values at Site 1189 are a third of the maximum values at Site 1188).
Archive-half cores that had sufficiently long (>10 cm) unbroken pieces were passed through the cryogenic magnetometer for remanent intensity measurements. These cores were then demagnetized progressively in an alternating field (AF) with peak values of 10, 15, 20, and 30 mT. The remanence was measured after each demagnetization step. Measurements on the cryogenic magnetometer were conducted at 2-cm intervals. Because of the gaps in the sample sequences and irregularities in the volume of the core, the remanent intensity values were not downloaded to the Janus database. We show the downhole profile of these uncorrected natural remanent magnetization (NRM) intensity values only to examine the general trend (Fig. F131). The general NRM intensity trend of the archive-half cores is quite similar to that of the susceptibility. The top of the hole shows a peak intensity of 2.85 A/m very near the seafloor. This is followed by a zone of relatively low remanent intensity (<0.5 A/m) between 25 and 120 mbsf. Below 120 mbsf, the remanent intensity shows a significant increase and a maximum intensity of 3.55 A/m is seen at 169 mbsf. We do not observe the apparent decrease in susceptibility values below 130 mbsf reflected in remanent intensity. These data show a gradual increase from 20 to 180 mbsf, with a sharp decrease in intensity over the lowermost 20 m of the section. Based on these observations, it appears that, as at Site 1188, the most significant source of surface magnetic anomalies at Site 1189 would not be at the seafloor, but at a depth >120 mbsf.
We analyzed 14 discrete minicore samples from Hole 1189A and the lower sequence (>120 mbsf) of Hole 1189B. Table T22 summarizes the location, dimensions, and brief descriptions of these minicores. The anisotropy of magnetic susceptibility of minicore samples was measured at 15 different positions. The results are provided in Table T23. Magnetic susceptibility varies by several orders of magnitude. Figure F132 shows a plot of average susceptibility (k) vs. depth. Overall, the susceptibility of minicores is consistent with the downhole profile of the archive-half cores (Fig. F130). The susceptibility values of the minicores tend to be considerably lower at Site 1189 than at Site 1188, as is found in the values of the archive-half cores at Site 1189. Susceptibility values >0.01 are too high to be explained without the presence of a ferromagnetic mineral, because most paramagnetic minerals have lower magnetic susceptibilities (Collinson, 1983). As at Site 1188, there appears to be only a minor amount of anisotropy among the minicore samples from Site 1189. The maximum anisotropy (1.0877) was determined for Sample 193-1189B-11R-1, 64 cm, which is the minicore sample with the highest average susceptibility (0.028). Figure F133 is a Flinn-type diagram showing the lineation and foliation of the susceptibility ellipsoid. Sample 193-1189B-12R-1, 35 cm, which has a high lineation (0.081), also has a relatively high average susceptibility (0.022).
Figure F134 shows the NRM intensity measurements taken from the minicore samples at Site 1189. The only fresh dacite minicore from the upper 25 m of the profile is Sample 193-1189A-2R-1, 10 cm, which exhibits a remanent intensity of 1.55 A/m. This value is 45% lower than that of fresh dacite at Site 1188. There is a significant increase in the NRM intensity between 120 and 190 mbsf. This region of high intensity is separated by a zone of low intensity at 160 mbsf. The presence of a high NRM intensity region below 130 mbsf is similar to Site 1188. However, the intensity values are much lower than those of Site 1188. For example, Samples 193-1188A-17R-1, 109 cm (146.2 mbsf), and 17R-2, 30 cm (146.9 mbsf), have NRM intensities of 11.6 and 4.7 A/m, respectively, which are three to four times higher than the highest intensity samples measured from Site 1189. In addition, the peak intensity values at Site 1189 occur at greater depths (>160 mbsf) as compared to >140 at Site 1188.
Figure F135 shows a plot of inclination. In general, the inclination values of minicore samples are consistent with the present-day International Geomagnetic Reference Field Earth model, which predicts an inclination of -7.7° at Site 1189. However, there are several exceptions. Sample 193-1189A-10R-1, 76 cm, exhibits a high positive inclination of 87°. This sample piece is quite long (~25 cm) and it is highly unlikely that it was rotated during recovery or curation. Furthermore, a 180° rotation of the sample piece still would not explain its steep angle. Examination of this sample shows that it consists of brecciated pieces, and therefore, based on the observed inclination, it appears that this particular sample and possibly the whole unit has suffered some degree of deformation or rotation. Another notable exception is Sample 193-1189B-18R-2, 66 cm, from the very bottom of Hole 1189B. This sample shows a positive inclination of 27°. An examination of its demagnetization curve shows an irregular behavior during AF demagnetization (Fig. F136), which suggests that this sample is an amalgamation of materials that have different magnetic orientations. Sample 193-1189B-11R-2, 88 cm, is another exception. It has a steep negative inclination of -40°. This is again a very long piece (>22 cm), and it is unlikely that it was misoriented during curation.
Because of low recovery at Site 1189, we had time to conduct a few additional experiments that are not part of the standard ODP shipboard analyses. Monitoring the acquisition of isothermal remanent magnetization (IRM) is often used to distinguish between high-coercivity minerals such as hematite and low-coercivity minerals such as magnetite and titanomagnetite. Figures F137 and F138 show plots of IRM intensity vs. applied impulse field for Holes 1189A and Hole 1189B, respectively. The field was applied at increasing steps of 50, 100, 150, 200, 250, 300, 400, 500, 800, and 1100 mT. As in Hole 1188A, the magnitude of saturation isothermal remanent magnetization (SIRM) correlates with that of the NRM.
In the case of Hole 1189A, all samples exhibit a steep rise in IRM acquisition within the first 200 mT (Fig. F137), which suggests that the dominant magnetic carriers are low-coercivity minerals. Sample 193-1189A-2R-1, 10 cm, shows a rather large fluctuation in IRM intensity. The high variability is caused by the shipboard magnetometer's inability to accurately measure intensity at high magnetization values.
The IRM curves from Hole 1189B are much more complex (Fig. F138). Samples from Sections 193-1189B-11R-1 through 12R-1, which have relatively high susceptibility and remanent intensity, all show very low coercivity. This complexity clearly contrasts with samples taken from cores in the upper part (Sections 193-1189A-2R-1 through 10R-1) of Hole 1189A. Core descriptions report the presence of magnetite in trace amounts in Cores 193-1189B-11R and 12R, consistent with the low-coercivity behavior. Sample 193-1189B-13R-1, 55 cm (148 mbsf), which has a much lower SIRM shows some stability in the IRM intensity values. It also shows a slightly higher coercivity (150-200 mT) than the samples above it. Core descriptions note a distinct lithology change between Cores 193-1189B-12R and 13R, the lower case being much more anhydrite rich and barren of Fe-Ti oxides. Samples 193-1189B-14R-1, 101 cm, and 15R-1, 87 cm, also have very low SIRM intensity and no magnetite reported in core descriptions. The alteration log for Sections 193-1189B-15R-2 and 16R-2 report the presence of trace amounts of magnetite, reflected in the low coercivity of the samples. Sample 192-1189B-18R-2, 66 cm, has a low SIRM value and similar characteristics as Samples 193-1189B-14R-1, 101 cm, and 15R-1, 87 cm; however, the description of the interval encompassing this sample reports the presence of magnetite. The discrete minicore sample likely intercepted a thin interval devoid of magnetite within the broader magnetite-bearing interval.
Only two minicore samples from Hole 1189A were treated with thermal demagnetization and monitored for their magnetization intensity with increasing temperature. The two samples (Samples 193-1189A-7R-1, 82 cm, and 10R-1, 76 cm) that were already saturated from the previous IRM experiment were progressively heated up to 700°C, with smaller temperature intervals above 500°C. Figure F139 shows the variation in the intensity as a function of temperature. Sample 193-1189A-7R-1, 82 cm, shows a gradual decay in intensity between 100° and 500°C. This decay is followed by a sharper drop between 540° and 620°C. On the basis of the demagnetization curve, it appears that the dominant magnetic carrier is magnetite with a small contribution from titanomagnetite. In the case of Sample 193-1189A-10R-1, 76 cm, most of the magnetization intensity is lost gradually between 100° and 400°C. From 400° to 600°C, there is another gradual decay where the remaining intensity is lost. Our interpretation based on this demagnetization trend is that again magnetite and titanomagnetite are the two most probable magnetic carriers; however, the latter appears to be more dominant. Furthermore, the smooth decay in intensity of Sample 193-1189A-10R-1, 76 cm, suggests that the titanomagnetite may have been altered, in which case it may have been replaced by titanohematite. It is also possible that the complexity in the demagnetization curve is caused by impurities such as Mg, Al, Cu, V, and Si within the titanomagnetite (Thompson and Oldfield, 1986).
Site 1189 has similar features as Site 1188. The uppermost sections of profiles at both sites are characterized by a zone of high susceptibility and high remanent intensity. The magnetic carriers in this zone consist primarily of magnetite and titanomagnetite. A second zone of high susceptibility and high remanent intensity occurs deeper in the section (below 130 mbsf). The low recovery at both sites makes it difficult to make a thorough comparison between the two zones. However, based on our analyses of the recovered cores, it appears that the deeper zone (below 130 mbsf) may contribute a significant component to the magnetic anomalies measured at sea level.
There are several notable differences between Sites 1188 and 1189. For instance, the susceptibility and remanent intensity are generally lower at Site 1189. Also, the peak magnetization intensity is deeper at Site 1189 than at Site 1188.
By examining the magnetic orientation of the minicore samples we were able to identify some sample pieces that may be have been deformed or rotated. In general, except for those pieces, the inclination value at Site 1189 is consistent with that predicted by the present-day Earth field but has a steeper angle.