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doi:10.2204/iodp.proc.324.102.2010

Paleomagnetism

During Expedition 324, we conducted routine demagnetization measurements of discrete samples from working-half sections. At each site, a set of samples was treated by alternating-field (AF) demagnetization and another set was treated by thermal demagnetization. We also measured bulk magnetic susceptibility on discrete samples after each temperature step during thermal demagnetization to determine whether the carrier of characteristic remanent magnetization (ChRM) of a sample remains intact or undergoes any alteration caused by the repeated heating steps.

Magnetometer

Usual shipboard paleomagnetic measurements are carried out with the automated pass-through cryogenic magnetometer with a direct-current superconducting quantum interference device (2G Enterprises model 760-R) in order to demagnetize and measure remanent magnetization. The magnetometer is equipped with an inline AF demagnetizer (2G Enterprises model 2G600) capable of producing peak fields of 80 mT with 200 Hz frequency. The magnetometer is run and data are acquired by a program called Remanent Magnetism Determination (SRM version 3.23) written by D. Hornbacher (IODP-TAMU) in LabView (version 8.5) programming language.

However, during transit to the first site, preliminary measurements allowed us to detect some problems in the AF demagnetizing by the 2G magnetometer. We found that samples would behave as if they were acquiring an anhysteretic remanent magnetization (ARM) in the course of the AF demagnetization when the demagnetizing field was larger than ~20 mT (Fig. F18). This ARM causes a bias of the measured paleomagnetic inclination, which is critical to our study. Unfortunately, the critical threshold above which ARM was detected varied for each sample. Moreover, the AF fields required to demagnetize basalt samples are usually much higher than 20 mT. After numerous pilot tests using half-cores and discrete samples, we communicated to 2G and concluded that the aging degausser coil may be malfunctioning. However, the 2G magnetometer was used to measure weak magnetization samples (such as some volcaniclastic samples).

As a result, the magnetization measurements were usually carried out with a Molspin Minispin magnetometer. This magnetometer is a slow-speed spinner fluxgate device in which the samples are spun at 6 Hz about a vertical axis in a fluxgate surrounded by a mu-metal shield. To obtain the three components of the magnetization vector, it is necessary to perform a sequence of measurements with the specimen in four orientations.

Core orientation

The standard IODP paleomagnetic coordinate system was used. In this system, +x is perpendicular to the split core surface and into the working half, +z is downcore, and +y is orthogonal to x and z in a right-hand sense (i.e., it points left along the split-core surface when looking upcore at the archive half) (see "Structural geology").

Magnetic overprints

Several types of secondary magnetization were acquired during coring, which sometimes hampered interpretation. The most common secondary magnetization was a steep downward-pointing overprint attributed to the rotating drill string. This was also seen as a bias for 0° declinations in archive-half sections, which has been observed during many previous cruises (e.g., Leg 206, Shipboard Science Party, 2003a) and has been interpreted as a radially inward overprint.

Working-half (discrete sample) measurement

During the expedition, oriented discrete samples representative of the lithology were taken from the working halves of selected sections. Cubic samples with a volume of ~7 cm³ were collected (typically one or two per section) for shipboard magnetic analysis. We drew an arrow on the split-core face pointing uphole (z-axis) and used the rock saw to cut the sample. The same samples were also used for physical property measurements. We carried out AF demagnetizations on approximately one-third to one-half of the samples and thermal demagnetizations on the remaining samples.

Susceptibility measurements

In addition to standard paleomagnetic measurements, bulk magnetic susceptibilities were determined for all the discrete samples using the Kappabridge KLY 4S (Geofyzika Brno). Values have been corrected for the true cubic sample volume. Bulk magnetic susceptibilities were also measured after each heating step in order to track possible magneto-mineralogical changes that could happen because of the repeated heating.

Thermal demagnetizations

Discrete samples from the working-half sections were progressively demagnetized using a Schonstedt Thermal Demagnetizer (model TSD-1) in steps of 50° or 25°C for the higher temperature steps, up to a temperature usually between 500° and 600°C. The temperature steps were adjusted as a function of the unblocking temperature spectra of each batch of samples. Each batch consisted of an ensemble of 6–12 individual samples. After each demagnetization step, samples were cooled in a low–magnetic field environment and the remaining magnetic intensity and orientation were measured with the Molspin Minispin magnetometer (or the 2G magnetometer for weak samples).

For a few samples, the thermal demagnetization procedure was preceded by AF demagnetization at 10 mT to partly remove the drilling overprint, with the hope that it would remove the viscous component before the thermal procedure and increase the success rate of thermal demagnetizations. Because this was usually not the case, this procedure was not used routinely.

Alternating-field demagnetizations

As we could not use the 2G magnetometer to perform AF demagnetizations, we used the D-Tech demagnetizer. Samples were demagnetized in three orthogonal directions by the application of an alternating field with a decay rate of 0.01 mT per half-cycle. The maximum field intensity was 150 mT. Demagnetizations were carried out in field steps of 2 or 3 mT up to 15 mT, then 5 mT up to 30 mT, then 10 or 20 mT up to 80–150 mT, depending on the coercivity spectrum of each sample. The remaining magnetization was measured after each demagnetization step with the Molspin Minispin magnetometer (or the 2G magnetometer for weak samples).

Data analysis

During Expedition 324, ChRM was estimated by principal component analysis (PCA) (Kirschvink, 1980) of three or more of the stable endpoint directions. PCA analysis was conducted using the Progress08 program written by H. Shibuya. The quality of the fit is quantified with the maximum angular deviation, which measures how consistently the demagnetization data fit a line. This parameter is routinely used as a quality index for the calculated ChRM. Maximum angular deviation values >8° were typically considered to signify an ill-defined mean direction, and therefore such samples were rejected from further analysis.

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