Site 1191 was drilled to only 20.1 mbsf and, therefore, did not help clarify the deep magnetic structure at Pual Ridge. However, by virtue of reasonable recovery of continuous core pieces, this site provides important information regarding the magnetic properties of the near-seabed cap of relatively unaltered dacite lava. The results of our analyses show that Site 1191 has a relatively high remanent intensity compared to samples from other sites at similar depths. However, the susceptibility appears to be substantially to slightly less than that of other sites. Hole 1191A also provides interesting information on magnetic inclination.
The recovery was low in Hole 1191A, and all of the cores had significant gaps between pieces. In many cases, the recovered material was not cylindrical; thus, the split cores often did not fill the core liners. We took susceptibility and remanence measurements of the archive-half cores at places where rock pieces were long enough. Susceptibility measurements of the archive-half cores were taken by setting the point susceptibility meter on the archive multisensor track in manual mode and placing the probe in contact with the piece of core. Figure F16 shows a downhole profile of the susceptibility measurements of the archive-half cores and the minicores. The susceptibility values range from 0.005 to 0.013 SI units, and results for both sample types are consistent.
Three minicore samples were taken from Hole 1191A. Table T11 summarizes the magnetic properties of these samples. We also measured the anisotropy of magnetic susceptibility (AMS) of the minicores. Table T12 summarizes the magnitudes and principal axes of the AMS that were derived from the susceptibility tensor. Figure F17 is a Flinn-type diagram representing the susceptibility ellipsoid in two-dimensional space. There appears to be very little anisotropy of the magnetic susceptibility of samples from Site 1191. The remanent intensities were measured by performing step-wise alternating-field (AF) demagnetization at peak fields of 10, 15, 20, 25, 30, 40, 50, 60, and 80 mT. The changes in the magnetization intensity, inclination, and declination measurements of the minicore samples are summarized in Table T13.
The natural remanent magnetization (NRM) intensity of the minicore samples ranges from 4.5 to 7.6 A/m, which is substantially higher than that of other sites at similar depths (Fig. F18). In the case of Site 1189, one minicore at 9.8 mbsf has an NRM intensity of 1.55 A/m (Fig. F134 in the "Site 1189" chapter), and the maximum intensity of the archive-half cores is <3 A/m (Fig. F131 in the "Site 1189" chapter). This contrasts with the susceptibility values, which are substantially lower than those of Sites 1188 (Fig. F124 in the "Site 1188" chapter) and 1189 (Figs. F130 and F132, both in the "Site 1189" chapter). Consequently, the entire suite of minicore samples from Hole 1191A exhibit very high Koenigsberger ratios. The minicore samples provide valuable information on the magnetic inclination of the fresh volcanic cap to Pual Ridge at Site 1191 (Table T11). The inclination values are quite consistent between samples. The average value of -16° is slightly steeper than that of the present-day Earth field, but is the same as the average value that we obtained for Site 1188. According to the International Geomagnetic Reference Field for December 2000, this drill site has an inclination of -7.7°.
We conducted a thermal demagnetization experiment on only one sample from Site 1191 because all the minicore samples appear to have similar characteristics. Figure F19 shows the thermal demagnetization curve of Sample 193-1191A-2R-2, 30 cm. The sample was first imparted with a saturated isothermal remanent magnetization at an impulse field of 1.1 T. After each step of heating, the sample was cooled back to room temperature and its intensity measured. The demagnetization curve shows a rise in intensity to ~300ºC followed by a steady drop to 600°C (Fig. F19). Based on this observation, it appears that the dominant magnetic mineral is magnetite, which has a Curie temperature of 585ºC (Table T27 in the "Site 1188" chapter). There may be a slight amount of titanomagnetite since a large drop in intensity starts at ~350°C.