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doi:10.2204/iodp.proc.324.107.2010

Paleomagnetism

We measured and analyzed 42 discrete basalt samples (7 cm³) from various lithologies downhole (Table T7). Their bulk magnetic susceptibilities are on the order of 10^–2 SI. Eighteen samples were alternating-field demagnetized up to 140 mT in steps of 2, 5, or 10 mT up to fields between 80 and 150 mT, and twenty-four were thermally demagnetized in steps of 50° or 25°C up to temperatures between 550° and 600°C. Magnetic susceptibility was measured after each heating step in order to detect possible mineralogical changes.

Most natural remanent magnetization measurements and first demagnetization steps show a steep vertical component, typical of drilling overprint (e.g., Audunsson and Levi, 1989; Wilson, Teagle, Acton, et al., 2003). This drilling overprint is usually erased at ~10 mT or 100°C (Fig. F46).

Working-half discrete sample measurements

Alternating-field demagnetization

The demagnetization behavior of the samples can be classified into two types. The first type, observed in samples from Sections 324-U1350A-7R-1 through 24R-3, is characterized by low coercivity, suggested by median destructive fields <10 mT. These samples also have a magnetization that decays exponentially (e.g., Sample 324-U1350A-21R-2, 67–69 cm) (Fig. F46A), indicating the presence of multidomain grains as magnetization carriers (Argyle et al., 1994). The second type of demagnetization behavior is observed in samples from Sections 25R-2 through 26R-1 (corresponding to stratigraphic Unit IV plagioclase-phyric pillow succession; see "Igneous petrology"). This second type of behavior is characterized by an initial plateau on the demagnetization spectra, which could indicate that the magnetization carriers are single-domain grains (Argyle et al., 1994) (e.g., Sample 324-U1350A-26R-1, 78–80 cm) (Fig. F46B). Inclinations of these samples were calculated using principal component analyses (Kirschvink, 1980). All the samples show a very stable univectorial decay toward the origin of orthogonal vector plots after the removal of overprints at 7–10 mT (Fig. F46), with maximum angular deviation values <5° for all samples and <2° for 16 out of 18 samples. The resulting inclinations are dominantly shallow but quite scattered (between –12° and 14°), with one anomalously high value (27°) in Sample 324-U1350A-12R-1, 50–52 cm.

Thermal demagnetization

Thermal demagnetization spectra show that magnetization decreases gradually with temperature. This suggests a large distribution of unblocking temperatures, which indicates a large distribution of Ti content in the titanomagnetite (-maghemite), the most likely carrier of the magnetization (Hunt et al., 1994). In some cases, the magnetization decrease starts from the very first demagnetization step and most of the sample is demagnetized by 400°C (e.g., Sample 324-U1350A-18R-1, 97–99 cm) (Fig. F46C). In the majority of cases, however, the main magnetization decrease occurs over two steps: one broad step at ~400°C and another sharper step at 500°C (e.g., Sample 324-U1350A-21R-1, 35–37 cm) (Fig. F46D). This two-step decrease indicates that several phases are present: a Ti-poor phase (close to pure magnetite) and a Ti-rich phase with a distribution of Ti content. A partial self-reversal of magnetization at ~300°C might be occurring in several samples from Sections 324-U1350A-16R-1 through 21R-1 (e.g., Sample 324-U1350A-16R-1, 35–37 cm) (Fig. F46E). In these samples, we observe an increase of the magnetization during the demagnetization procedure at ~300°C, and the direction corresponding to this temperature step is antipodal to the direction of the primary remanent magnetization. Such behavior is not uncommon in submarine basalts (e.g., Dubrovine and Tarduno, 2004). Only five thermal demagnetizations did not allow the definition of a primary magnetization component pointing toward the origin of the orthogonal vector plot. The other 19 samples gave scattered inclination values between –31° and 31° and very small maximum angular deviation values (<4°) in the majority of cases.

The variation of bulk magnetic susceptibility with temperature displays two main behaviors. For Sections 324-U1350A-7R-1 through 23R-2, the susceptibility increases at ~350°C to twice its room-temperature value, then decreases, and returns to room-temperature value by 500°–600°C. This could indicate the inversion of the primary titanomaghemite into magnetite (Özdemir and O'Reilly, 1982), which alters even more with further heating.

For Sections 24R-1 through 26R-8, magnetic susceptibility starts decreasing at ~400°C and then increases sharply at ~550°C, suggesting that the magnetic mineralogy in this part of the core is different from that of the previous part.

Directional study

Igneous lithology observed in Hole U1350A is composed of piles of thin massive inflated lavas and alternating layers of pillow lavas and baked interpillow sediments (limestone). The igneous basement in Hole U1350A was divided into five igneous units (see "Igneous petrology"). Average inclinations, calculated for samples with maximum angular deviation <8°, are: 6.2° ± 15.6°, 15.9° ± 9.3°, 6.6° ± 10.7°, and –1.1° ± 6.3° for Subunits IIa, IIb, and IIc and Unit IV, respectively. The large error bars (1σ) show that the calculated downhole inclinations are quite scattered within each igneous lithology (Fig. F47). This scatter is larger than in other holes. Moreover, the average inclinations for the four units are not statistically different from one another. More inclination measurements are needed in order to interpret the directional results of Hole U1350A.

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