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doi:10.2204/iodp.proc.324.202.2013

Methods

A total of 135 samples were measured in the Pacific Northwest Paleomagnetism Laboratory, which contains a Lodestar Magnetics Shielded Room, at Western Washington University (WWU) (USA). All samples were 2 cm × 2 cm × 2 cm cubes cut from the working half of Expedition 324 basalt cores. Magnetic susceptibility was measured on all samples prior to demagnetization, using an AGICO KLY3-S magnetic susceptibility bridge. Subsequently, the natural remanent magnetization (NRM) was measured in an AGICO JR6 dual-speed spinner magnetometer. Because the demagnetization results from some samples of massive flows were erratic in shipboard measurements, samples from massive units were dunked in liquid nitrogen for 20 min 1–4 times to remove overprint magnetizations resulting from multidomain magnetite and hematite. All samples were demagnetized using either alternating field (AF) demagnetization with a DTech D2000 AF demagnetizer or thermal demagnetization in an ASC Model TD48 or ASC Model TD48-SC thermal demagnetizing oven. A vibrating sample magnetometer (VSM; Princeton Measurements Corporation MicroMag Model 3900) was used to produce hysteresis loops on four samples from each site that were judged representative of the site’s flow units.

Pilot batches of samples using AF and thermal demagnetization from all sites showed little difference in the efficacy of removing overprint magnetizations, so approximately half of the samples from each site were demagnetized thermally (62 samples) and the other half using AF demagnetization (73 samples). Initially, thermal pilot samples were separated into two batches of 12 samples, one set being demagnetized in an argon atmosphere and the other in a natural atmosphere to compare the effects of oxidation. Because there was no difference in the results from the two treatments, the remaining samples were demagnetized in a natural atmosphere. AF demagnetization started at 5 mT, with steps of 5 mT up to 60–120 mT. Thermal demagnetization started at 80°C, with steps of 40°C up to 500°C and steps of 20°–25°C above 500°C. Complications arose with the JR6 magnetometer in that some of the thermally demagnetized samples had magnetizations too low to measure with this magnetometer above a certain step. If it was not possible to demagnetize a sample past 70% of the original magnetic intensity, the result was not considered a valid measurement of the characteristic remanence.

After samples were demagnetized, the data were analyzed using Remasoft 3.0 software. Equal angle spherical projections, demagnetization intensity plots, and Zijderveld plots (see Tauxe, 1998) were made for each sample. Principal component analysis (PCA) directions (Kirschvink, 1980) were calculated from demagnetization data, typically using higher level demagnetization steps that trend toward the Zij-derveld plot origin. This result is considered the characteristic or primary magnetization for the sample. PCA solutions were calculated both anchored to the origin and not anchored (Tables T1, T2, T3). VSM hysteresis loops were analyzed with the Micromag AGM-VSM program. A Day plot (Day et al., 1977) was produced using the hysteresis values saturation remanent magnetization (M_rs)/saturation magnetization (M_s) vs. remanent coercivity (H_cr)/coercivity (H_c).

Measurements done during Expedition 324 included AF and thermal demagnetization and bulk magnetic susceptibility measurements recorded during the thermal demagnetization process. Although paleomagnetic work was done on board the ship for all sites, in this report we concentrate only on the data from Sites U1346–U1348 and U1350.

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