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doi:10.2204/iodp.proc.301.204.2009

Methods

Shipboard measurements

During Expedition 301, the igneous section was extensively sampled for paleomagnetic study, with measurements made from a total of 330 discrete samples (Tables T1, T2). All measurements were made on discrete samples because the igneous core was too discontinuous to allow for reliable measurements from whole core sections. A total of 172 discrete ~15 cm³ paleomagnetic samples were taken at an average spacing of ~2/m of recovered core. These samples were acquired using a two-blade saw to produce cubes ~25 mm on a side. In order to obtain a greater number of paleomagnetic data points, three other types of samples were also measured. One was ~15 cm³ discrete cube samples, similar to the paleomagnetic samples except that they were taken for physical property measurements (23 samples). Another was irregular-sized intact oriented pieces (~50–200 cm³) of the archive-half core (127 samples). In addition, eight pairs of small irregular-sided pieces, usually <5 cm³ in volume, were collected to investigate the differences in magnetic properties inside and outside of halo alteration zones.

All but the small irregular samples were measured on the shipboard 2G Enterprises model 760R pass-through cryogenic superconducting quantum interference device (SQUID) magnetometer (see the “Methods” chapter). Measurements were made using the usual ODP core reference frame with the x-axis in the upcore direction (see the “Methods” chapter). The characteristic remanent magnetization (ChRM) was isolated using either stepwise alternating-field (AF) or thermal demagnetization. All AF demagnetization steps were run on the 2G Enterprises model 2G600 inline demagnetizer that is a part of the pass-through magnetometer system. Thermal heating was accomplished with a Schoenstedt model TSD-1 magnetically shielded oven.

Because the measurement of physical property samples and archive-half core pieces had to be nondestructive, AF demagnetization was used on these samples. Typically, AF demagnetization was done in 5 mT steps from 10 to 40 mT and in 10 mT steps for higher fields. In contrast, most of the samples acquired for paleomagnetic study were thermally demagnetized (see Table T1). Most thermal demagnetization analyses used 50°C steps from 150° to 550°C; however, other temperatures were used in some instances to explore magnetization properties.

AF and thermal demagnetization data were plotted for each sample on an orthogonal vector diagram (Zijderfeld, 1967) to aid in interpreting magnetization components and directions. ChRM is assumed to be shown by a section of high-temperature or high-AF field univectorial decay observed on the orthogonal vector plot. Magnetization directions were calculated using principal component analysis (Kirschvink, 1980) from that section of univectorial decay. With only a few exceptions, all of the magnetization directions were determined from calculations that were not anchored to the orthogonal vector plot origin. Typically four to six measurements were used for the principal component analysis. For two samples, the principal component analysis was constrained by only two measurements, so these calculations were anchored to the origin.

Shore-based measurements

Shore-based measurements were made in the paleomagnetic laboratory at Western Washington University with the intent of better understanding the source and characteristics of the magnetization in Hole U1301B basalts. Measurements of Curie temperature and magnetic hysteresis parameters were made on 32 discrete ~15 cm³ cube samples (Table T2). Magnetization direction studies were carried out on these and four additional cube samples. In addition, magnetic hysteresis and isothermal remanent magnetization (IRM) acquisition measurements were conducted on eight pairs of samples in which the two samples of each pair come from an alteration halo and the adjacent slightly altered rock (Tables T3, T4).

All samples were treated to detailed AF or thermal demagnetization routines, with all but 4 of the 36 samples being thermally demagnetized. AF demagnetization was carried out in a D-Tech D2000 AF demagnetizer. Compared with previous shipboard analysis, demagnetization steps were more closely spaced at low field values (2–3 mT steps to 10 mT), the same 5 mT steps were used from 10 to 100 mT, and 20 mT steps were used to 200 mT. Thermal demagnetization was also generally more detailed than shipboard analyses, with steps of 10°–20°C being commonly used. Thermal demagnetization was carried out to 450°–580°C, depending on results from any particular sample. Heating was done in an argon-filled environment to prevent oxidation from occurring at high temperatures. Magnetization direction analysis for each of these samples was carried out in the same manner as for the shipboard samples.

Hysteresis loops were measured using a MicroMag vibrating sample magnetometer (VSM) at room temperature in fields of up to 10 kOe. Hysteresis loops before and after heating were also taken for samples for which Curie temperatures were measured in the presence of a magnetic field. Paramagnetic contributions to the hysteresis curves were minimized in most cases by a slope correction using MicroMag software. IRM and backfield curves were also determined with the VSM.

To determine Curie temperature, magnetic susceptibility was measured as a function of temperature using a Kappabridge magnetic susceptibility meter with a furnace attachment. Chips from each sample were powdered and cycled from room temperature to 700°C and back to 40°C in the presence of argon gas to minimize oxidation. Chips from two representative samples were then cut and powdered for closer examination. These samples were run in the same manner as previously described but to a maximum temperature of 320°C. Curie temperatures were determined by the intersecting tangents method from graphs of magnetic susceptibility versus temperature. To gauge the accuracy of susceptibility-based measurements, four paired samples were selected for direct Curie temperature measurement with a furnace-equipped VSM. Curie temperatures for these samples were determined by the second derivative method outlined by Tauxe (1998). Thin sections from these sample pairs were also made, polished, and coated in carbon for scanning electron microscope (SEM) examination.

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