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doi:10.2204/iodp.proc.301.204.2009

Results

NRM and directional measurements

Sample natural remanent magnetization (NRM) and directional measurements are given in Table T1. NRM values range from 1.3 to 73.0 A/m with a median value of 6.9 A/m. Most samples have NRM values <10 A/m, and only seven have NRM values >50 A/m (Fig. F1). These values are typical of submarine basalts, and the skewed distribution with a small number of large NRM values is also commonly observed (Johnson and Pariso, 1993; Johnson et al., 1996; Zhao et al., 2006). High NRM values are scattered more or less evenly throughout the section (Fig. F1).

During demagnetization, Hole U1301B samples show a remarkable variety of behavior and ChRM directions (Figs. F2, F3). Samples gave better results with thermal demagnetization, which is why we used that technique for most of the demagnetization experiments. Some AF-demagnetized samples displayed magnetizations that veered from the origin at high field values (Fig. F2D), implying some sort of spurious magnetization imparted by the demagnetization equipment (e.g., a rotational remanent magnetization or gyroremanent magnetization). Most samples displayed a large, steeply downward NRM direction that is probably indicative of the drill-string overprint that is common among paleomagnetic samples cored onboard the JOIDES Resolution (see Fig. F2A, F2C, F2E) (Acton et al., 2002; Fuller et al., 2006). In some low-coercivity samples, this overprint dominated magnetizations to the point that it was difficult to determine the ChRM. In samples from the upper part of the igneous section, both AF and thermal demagnetization revealed a ChRM with a downward-directed (positive) inclination between ~30° and 90° (Figs. F2A, F2C, F2F, F3). This magnetization direction indicates normal polarity in the Northern Hemisphere. Some samples give a reversed polarity inclination (i.e., negative or upward). Typically, this ChRM is found at high-temperature steps and is masked by a drill string overprint or a lower temperature normal component similar in dip to the normal ChRM samples (Fig. F2E). A few samples, such as 301-U1301B-6R-2, 35 cm (Fig. F2B), are reversed with no evidence of a downward overprint, and such samples may have been inverted during handling.

To examine ChRM trends, we arbitrarily classify ChRM directions into five classes:

• Type 1 has low inclination, reversed ChRM.

• Types 2 and 3 have normal ChRM inclinations, with those of type 2 having values below ~40° and those of Type 3 having higher values.

• Type 4 is characterized by a dominant drill string overprint (typically low-coercivity samples).

• Type 5 is has two components, a high-temperature reversed inclination and a lower temperature normal direction.

Most samples are Types 2 or 3 (Table T1), especially in the upper part of the section. In the lower part of the section, below 470 mbsf, Types 2 and 3 are still common, but all of the different types are represented with little apparent correlation between adjacent or nearby samples. This part of the section also displays ChRM inclinations with highly scattered values, including negative (apparently reversed) inclinations (Fig. F3).

Because Site U1301 is on a normal polarity magnetic anomaly, we have interpreted the normal inclinations as “normal” and the reversed inclinations as spurious (see the “Site U1301” chapter). Even in the apparently normal-behaving upper section, normal polarity inclinations do not precisely match the expected inclination. The average inclination for the section above 470 mbsf is 53.5°, which is less than the geocentric axial dipole inclination for the site of 66.5°. Even if this value is corrected for the slight shallowing caused by averaging azimuthally unoriented samples (corrected expected inclination = 64.2° using Cox and Gordon, 1984) the difference is >10°.

Rock magnetic measurements

IRM acquisition curves all saturate quickly in low (<100 mT) applied fields (Fig. F4; Table T2). This low-field IRM saturation is characteristic of titanomagnetite grains. The flat high-field sections of the curves indicate the absence of high-coercivity magnetic minerals, such as hematite. Observed differences between samples from alteration halos and nearby samples from slightly altered core are small but systematic. Samples from alteration halos consistently require slightly greater applied fields to reach saturation (Fig. F4), implying that the alteration slightly raises the coercivity of the bulk assemblage of magnetic grains.

Thermomagnetic curves for Hole U1301B basalt samples are usually nonreversible, although some samples do give reversible curves (Fig. F5). Nonreversibility implies changes in the magnetic mineral assemblage caused by heating. In these samples, we are probably seeing the conversion of maghemite into other magnetic minerals. The thermomagnetic curves give Curie temperatures for Hole U1301B samples ranging from 128° to 370°C (Tables T3, T4) with a median value of 339°C. Most samples give values in the 240°–320°C range. These values are typical for altered ocean basalts with titanomagnetite magnetic grains.

Magnetic hysteresis data (Tables T3, T4) show that all samples have saturation remanence/saturation magnetization (M_r/M_s) ratios <0.5 and remanent coercive force/ordinary coercive force (H_cr/H_c) ratios of <5. Most samples plot near the single domain (SD) field on the Day plot (Fig. F6), although some samples have hysteresis parameter ratios that fall in the pseudosingle domain (PSD) region. Of the paired samples from alteration halos and nearby slightly altered rock, it appears that the more altered samples plot preferentially toward the SD region of the plot, suggesting that the alteration process makes magnetic grains behave more like SD grains. Most samples plot near model lines (Dunlop, 2002) for mixtures of SD and multidomain (MD) grains. Given microscopic observations (see below) that indicate both large and small magnetic grains, these samples may indeed be displaying the behavior of a SD/MD grain mixture.

On a Day plot, samples with different demagnetization behaviors show no indication of clustering (Fig. F7). Samples with each different demagnetization behavior are found scattered throughout the hysteresis ratio range of the entire sample population. This implies that different types of demagnetization behavior are not a function of magnetic grain size.

SEM photomicrographs (Fig. F8) show that large titanomagnetite grains are found in sample thin sections. These grains are often rectangular or deltoid in appearance, which is typical of titanomagnetite. Many of these grains are tens of micrometers in diameter, which is far larger than the normal size for SD or PSD grains. Indeed, grains of this size should behave as multidomain grains. An explanation for the difference is that many of the grains show trellis, dendritic, skeletal, and cruciform patterns within the grains, indicating that the grains are broken up internally into smaller pieces. Furthermore, the surfaces of these grains are often invaded by cracks, implying further divisions. In sum, the naturally occurring fractures and crystal structures tend to be much smaller than the whole grain. In addition, electron dispersive scattering (EDS) tests imply that titanomagnetites are also found within the groundmass, so there is likely to be a very small size grain component also existing within these samples.

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