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doi:10.2204/iodp.proc.324.103.2010

Paleomagnetism

The major goals of paleomagnetism studies on the recovered cores were to characterize the paleomagnetic remanence and resolve paleolatitude from the magnetization components recorded in the igneous rocks. We focused only on discrete sample measurements from the working halves of the cores using the Molspin Minispin magnetometer and alternating-field (AF) demagnetizations using the DTech degausser because we found strange behavior in demagnetization results given by the onboard 2G magnetometer. We also thermally demagnetized discrete samples (one per flow). The magnetization of the sediments was too low to be measured on the Molspin Minispin; therefore, we only carried out demagnetizations on the basalts.

Working-half discrete sample measurements

We measured and analyzed 20 discrete basalt samples (7 cm³) from various lithologies downhole (Table T11). Four samples were AF demagnetized up to 100 mT in steps of 5 or 10 mT, and 16 were thermally demagnetized up to 600°C. Magnetic susceptibility was measured after each heating step in order to detect possible mineralogical changes (see "Paleomagnetism" in the "Methods" chapter for more details). Room-temperature susceptibilities range between 7 x 10^–3 and 20 x 10^–3 SI. For most of the samples the natural remanent magnetization measurements and first few demagnetization steps show a steep vertical component typical of drilling overprint (e.g., Wilson, Teagle, Acton, et al., 2003), but this is usually erased at ~10 mT or 200°C (Fig. F51).

Alternating-field demagnetizations

After the removal of the drilling overprint, demagnetization characteristics show a dependence on lithology. In AF demagnetization, the two samples from highly altered cores show very low median destructive field (MDF) around 8 mT. In contrast, the two samples from less altered cores show slightly higher MDF values than that of highly altered material (>10 mT) and exhibit univectoral decay toward the origin as demagnetization steps proceed (e.g., Sample 324-U1346A-7R-2, 39–41 cm). Inclinations and declinations were calculated using principal component analyses (PCA) (Kirshvink, 1980) for the steps between 10 and ~80 mT. Low values of maximum angular deviation show that these directional results are of high quality.

Thermal demagnetizations

Most samples have fairly low unblocking temperatures, around 400°–450°C, which is characteristic of titanomagnetite (-maghemite) Fe_3xTi_xO₄ with an ulvospinel fraction of around 0.3–0.4 (Hunt et al., 1995). Bulk magnetic susceptibilities measured at room temperature after each heating step stay more or less constant for heating steps up to ~450°C. At higher temperatures, susceptibilities increase to 2–3 times the room-temperature values (Fig. F51). This could indicate that the primary magnetic carrier could be titanomaghemite that inverts to strongly magnetic magnetite as a result of heating (Özdemir and O'Reilly, 1982). Directional results also show more erratic behavior for the steps higher than ~450°C. For this reason, inclination and declination of each sample were calculated using PCA (Kirshvink, 1980) between ~300°C (after the removal of the overprint) and ~450°C. In this temperature range, directions are univectorial and toward the origin, again with small maximum angular deviation angles (<8°). Three samples for which it was not possible to isolate a stable component between the overprint and the alteration had to be rejected, but the other 13 demagnetizations were of good quality. They all give negative and shallow inclinations.

Tectonic significance of remanence data

The demagnetization results from Hole U1346A are characterized by shallow negative inclinations (Fig. F52). Calculated average paleoinclination from all the negative inclination data points at Site U1346 is –20.5° with standard deviation (1σ) of ±5.4°. There is no systematic change in the downhole inclinations, supporting the theory that the lava flows observed in Hole U1346A were deposited during a single eruption event (see "Igneous petrology"). The average inclination of this hole indicates that the Shirshov Massif was formed close to the Equator. If the contemporaneous Earth's magnetic field was reversed, the massif was formed in the Northern Hemisphere, and if the magnetic field was normal, it was formed in the Southern Hemisphere. However, because we do not have enough constraint from surface magnetic anomalies and radiometric dating to determine the basement age and corresponding magnetic polarity, the location of Shirshov Massif formation remains uncertain.

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