We used progressive AF demagnetization of archive-half sections, one whole-core section, one working-half section, and discrete samples to characterize the paleomagnetic signal and resolve the magnetization components recorded in the recovered core. An unambiguous magnetostratigraphy could not be obtained from the only undisturbed core (Core 200-1224C-1H) that was recovered in the sedimentary section; the other sediment cores were extremely disturbed by drilling. In addition, we had time for only a cursory interpretation of the magnetization of the basaltic units, although fairly detailed demagnetization experiments were conducted on split cores and discrete samples.
Given that a number of basalt units were recovered (see "Igneous Petrology" in "Lithology") (Table T5), the magnetization of the basalts should provide a valuable paleolatitude estimate for the Pacific plate at ~45 Ma. This age corresponds to the Pacific plate's abrupt change in motion relative to the hotspots as marked by the kink in the Hawaiian-Emperor hotspot track. A cusp in the Pacific plate APWP may also occur at this age, marking a change in the motion of the Pacific plate relative to the spin axis. The Pacific APWP and hotspot tracks together provide key constraints on estimates of the size of motions between hotspots, ultimately extending our understanding of mantle dynamics (Acton and Gordon, 1994). Additionally, the age also lies within the period (39-57 Ma) when the Hawaiian hotspot has been shown to have moved rapidly southward relative to the spin axis (Petronotis et al., 1994). If geomagnetic secular variation has been averaged by the basalt units and if secondary overprints caused by alteration do not mask the primary magnetization, then we should be able to obtain an accurate paleolatitude. Finally, rock magnetic studies of the basalts should help refine our understanding of the magnetization of the upper oceanic crust and its role in generating lineated marine magnetic anomalies.
We made 88,384 remanent magnetization measurements along the split-core sections and along a whole-core section from Site 1224. Measurements were made every 1 cm before and after AF demagnetization. All sections were progressively demagnetized in steps of 1-5 mT up to peak fields of between 35 and 40 mT, with a few sections measured at higher fields. Sedimentary core sections that were disturbed by coring, short basalt sections (<30 cm), or basalt sections with only multiple small pieces (<10 cm) were not measured. We also measured remanence, before and after AF demagnetization up to 70-80 mT, on 72 discrete samples. Magnetic susceptibility was measured on whole-core sections every 1 cm (Fig. F56). Both remanence and susceptibility data are available from the ODP Janus database.
Unlike at Site 1223, we have not attempted to remove those measurements made near the ends of the core, the ends of basalt pieces, or in gaps as this would have required a substantial amount of time that was not available during the leg. Gaps between basalt pieces are common, so the raw data should be used with care.
All split-core and discrete samples have a drilling overprint. Unlike Site 1223, the steep component was not just directed downward but in some cases pointed upward, as was the case for the XCB and MDCB cores from Hole 1224A. The radial-horizontal component that points toward the center of the core was also present as is evident in the bias in the declinations of the split-cores toward 0° prior to AF demagnetization (Fig. F56). Initial natural remanent magnetization measurements are thus characterized by inclinations greater than +40° and declinations of ~0°. The drilling overprint was much less severe in the basalts at Site 1224 than in the vitric tuffs at Site 1223, which may be related to different acquisition mechanisms, magnetic grain sizes, and, less likely, magnetic mineralogy. In general, ~10-mT AF demagnetization removed the drilling overprint for the basalts, whereas >30 mT was needed for the tuffs.
Results from the sediments were perplexing. Only one core was recovered without a high level of drilling disturbance (Core 200-1224C-1H). We measured the remanence before and after AF demagnetization up to 30 mT on the whole-round section (200-1224C-1H-3). We then compared the result with the archive and working halves. After repeating the 30-mT demagnetization, both halves were measured twice to ensure that any viscous magnetization components that might have been acquired during splitting and handling were removed. Surprisingly, there were large differences in the inclinations of the whole round, archive half, and working half (Fig. F57). To add to the mystery, discrete samples gave inclinations that disagree with the split-core measurements over the 6.5-m interval recovered in Core 200-1224C-1H. We are unsure why such complexity is observed, although deformation caused by splitting or sampling, weak magnetization, or drilling overprints may play a role. We plan to investigate this further postcruise.
The basalt samples display both simple and complex demagnetization paths on orthogonal demagnetization plots. After removing the drilling overprint by ~10 mT, many samples gave linear demagnetization paths that decay to the origin of the plot (Fig. F58). After removing the drilling overprint, other samples gave linear demagnetization paths, but they did not decay directly to the origin. In several cases, multiple components were recorded in a single sample, with the components sometimes being nearly antipodal (Fig. F59). Some level of secondary overprinting is not unexpected given that we recovered altered basalt as well as fresh.