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doi:10.2204/iodp.proc.327.103.2011

Paleomagnetism

Hole U1362A was cored with the RCB, resulting in generally small, often unoriented pieces. Measurements were made on 79 discrete pieces that were <10 cm long and on portions of 23 core sections having pieces that were >10 cm long. When measured sections contained several pieces, the data were filtered to exclude 5 cm intervals at the ends of pieces. In addition, the volume of the archive-half pieces was variable (70–210 cm3 for individual pieces). Although irregular pieces are not often used in paleomagnetic studies, we considered these measurements worth pursuing because recovery was low and the working half was reserved for petrology, geochemistry, and physical property sampling.

Samples were demagnetized at 5 or 10 mT steps from 0 to 50 mT using the cryogenic magnetometer’s in-line AF coils. The remanent magnetization direction of selected samples was determined by plotting orthogonal vector plots of demagnetization steps and using principal component analysis (Kirschvink, 1980). Intensity values range from 0.18 to 14.27 A/m (Table T10). Demagnetization results show that some samples have a steep, downward-directed overprint that is usually attributed to the isothermal remanent magnetization imparted during coring (Acton et al., 2002). This remagnetization probably affects samples with low-coercivity magnetic grains and is usually removed in the first few demagnetization steps.

Overall, our results are consistent with previous paleomagnetic investigations conducted during Expedition 301 (Expedition 301 Scientists, 2005). Most inclinations are positive (Table T10), indicating that the ocean crust formed during a normal polarity interval, consistent with the 3.5 m.y. age of the Juan de Fuca plate at this location (see Fig. F11 in the “Methods” chapter). Most samples display simple magnetization behavior, with downward-directed inclinations and univectorial decay to the origin, indicating that this vector represents the characteristic remanent magnetization (ChRM) (Fig. F37). Some samples have steep downward-directed inclinations possibly influenced by an overprint imparted by the drill string (Fig. F37B). Very few samples have upward-directed inclinations (Fig. F37D). Negative inclinations cannot be entirely explained by accidentally flipping discrete samples because negative inclinations are seen for both discrete and continuous section measurements.

A more likely explanation is that these negative inclinations may represent a remagnetization of the ocean crust that occurred at a later time. Remagnetization can occur when heating or chemical changes alter the magnetic minerals in crustal basalt. Typically, the new magnetization has the same direction as the old as long as the remagnetization occurs within the same magnetic chron. Reversed magnetization could be imparted, however, if the magnetic mineral replacement occurred during a later period of time with the opposite magnetic polarity. During hydrothermal alteration, magnetic minerals such as magnetite are converted into iron sulfide minerals (Thompson and Oldfield, 1986). This is consistent with petrological and geochemical investigations in Hole U1362A that show abundant evidence of hydrothermal alteration (see “Petrology, hard rock geochemistry, and structural geology”), including the presence of iron sulfide minerals. A plot of intensity and inclination values versus depth shows a distinct zone of scattered inclinations around 460–470 mbsf in Unit 6 (Fig. F38). This interval corresponds to a zone with a higher frequency of veins (Fig. F30). The Hole U1301B interval that corresponds to the same depth has similarly scattered inclinations (Expedition 301 Scientists, 2005).