The rock magnetic analyses conducted onboard the JOIDES Resolution during Leg 193 consisted of continuous pass-through measurements on both whole and split cores and discrete measurements on minicore samples taken from the working half of the split cores. Once a section of the core containing recovered material was brought into the lab, magnetic susceptibility was measured on the whole core before it was split. This was done using the susceptibility meter mounted on the multisensor track (MST). After splitting the core, natural remanent magnetization (NRM) and magnetic susceptibility were measured on the archive half by passing it through the cryogenic magnetometer. Similarly, NRM and susceptibility were measured on discrete rock samples from the working half. One or more 2.5-cm-diameter minicores were generally taken from each lithologic unit recovered for detailed shipboard analyses. An effort was made to select the samples near important structural features and between lithologic boundaries for possible reorientation using the remanent magnetization direction. Stepwise alternating field (AF) demagnetization was conducted on the archive halves of the cores and on discrete samples to isolate stable components of remanence. The discrete samples were imparted with isothermal remanent magnetization (IRM) using a pulse magnetizer. This was done at increasing field strengths while we monitored the changes in IRM intensity. Once the samples became saturated, they were then thermally demagnetized to study the characteristics of their primary magnetic carrier. The results of the magnetic properties were compared with lithologic units and geological features.
The remanent magnetization of archive halves and discrete samples from working halves was measured using a 2G Enterprises pass-through cryogenic super-conducting quantum interference device (DC-SQUID) rock magnetometer (model 760R). This pass-through cryogenic magnetometer has an in-line AF demagnetizer (2G model 2G600) that allows for demagnetization of samples up to peak fields of 80 mT. The practical limit on the resolution of natural remanence of core samples is imposed by the magnetization of the core liner itself (~0.01 mA/m). The magnetometer and AF demagnetizer are interfaced to a PC-compatible computer and controlled by the 2G Long Core software written using LabVIEW by National Instruments. For many samples, the IRM intensity proved to be too high to be measured by the cryogenic magnetometer. For stepwise demagnetization of discrete samples, the laboratory is equipped with an AF demagnetizer (model D-2000 by DTech Inc.) and a thermal demagnetizer (model TSD-1 by the Schonstedt Instrument Co.) capable of demagnetizing specimens up to 200 mT and 700°C, respectively. An Analytical Services Company (ASC) model IM-10 impulse magnetizer (capable of generating pulsed fields from 0.02 T to 1.35 T) and a PARM-2 system by DTech Inc. were used for IRM acquisition studies of discrete samples.
Whole-core magnetic susceptibility (k) was measured at 2-cm intervals using the Bartington Instrument susceptibility meter (model MS1) attached to the MST (see "Physical Properties"). The susceptibility data were stored in the Janus database as raw data in units of 10-5 SI. The true SI volume of susceptibilities should be multiplied by a correction factor to account for the volume of material that passed through the coils. The standard correction factor for ODP core is ~0.66. Since the susceptibility measurements of the whole core were made following the establishment of the final curated positions and placement of spacers, they are directly comparable with the pass-through measurements on the archive half cores.
The magnetic susceptibility of archive half cores was measured using the archive multisensor track (AMST). Because most of the core samples recovered during Leg 193 were relatively short in length, rather than taking continuous measurements, the AMST was used to take intermittent measurements. Both the MST and the AMST employ the same type of susceptibility meter (Bartington Instruments model MS2), but with a different sensor. The sensor for whole-core measurements (MS2C) is a loop with an 88-mm inner diameter, and the core passes through the sensor coil. The AMST has a cylindrical tip probe (MS2F), and the sensor provides a depth of investigation approximately equal to its diameter (20 mm). The volume susceptibility of the minicores was measured using the Geofyzika Brno Kappabridge KLY-2 magnetic susceptibility meter. The susceptibility values on the Kappabridge are reported relative to a nominal volume of 10 cm3.
The standard ODP core orientation convention (fig. 8, Shipboard Scientific Party, 1991; fig. 8, Shipboard Scientific Party, 1997) was adopted for rock magnetic work during Leg 193. According to this convention, the z-axis is downhole parallel to the core, and the x-axis forms a line perpendicular to the split face of the core and is directed into the working half (Fig. F13). The x-axis is used as the reference "geomagnetic north" for the definition of magnetic declination values. Discrete minicores were marked with an arrow in the negative-z (uphole) direction on the plane representing the split surface of the working half. The plane marked with the arrow is the y-z plane.
Cylindrical minicores (12 cm3) were extracted from the cores recovered during Leg 193 using a water-cooled nonmagnetic drill bit attached to a standard drill press. Minicores were marked with an arrow pointing in the uphole direction. We tried to collect at least one or two discrete samples from each section of the core, and a few samples were used for onboard pilot demagnetization studies.
NRM measurements were
taken on the archive half of the core using the cryogenic magnetometer at 2-cm
intervals when continuous pieces longer than 15 cm were available. To isolate
characteristic magnetization, archive halves were AF demagnetized at 5, 10, 15,
20, and 30 mT. Minicore samples taken from the working half were examined in
greater detail. First, we measured the dimensions of the minicore samples. The
anisotropy of magnetic susceptibility (AMS) was determined for the samples using
the Kappabridge and the program ANI20 supplied by Geofyzika Brno. A 15-position
measurement scheme was employed to obtain the susceptibility tensor (kij)
from which the associated eigenvectors and eigenvalues were derived. We then
measured the NRM of the samples using the cryogenic magnetometer. The samples
were AF demagnetized at 10, 15, 20, 25, 30, 40, 50, 60, and 80 mT. The
characteristic remanent magnetization was derived using principal component
analysis (Kirschvink, 1980). Both Zidjerveld (1967) plots and equal-area
stereographic projections were used to determine the stability of remanence
levels within the archive cores and the discrete samples. The volume
susceptibility in conjunction with the NRM intensity was used to estimate the
Koenigsberger ratio (Q, the ratio of remanent to induced magnetization) of the
samples. Koenigsberger ratios can be calculated using the following equation: Q
= Joµo / kH, where Jo
is the NRM intensity, µo is the permeability of free space (4
× 10-7 H/m), k is the susceptibility, and H is the
ambient geomagnetic field at the site, which according to IGRF is 36.508 mT. The
results from the remanent magnetization and low-field susceptibility
measurements were compared with lithologic units and other geologic structures.
For rock magnetic properties measurements, the minicore samples were subjected to stepwise IRM acquisition experiments. The IRM acquisition experiments (to a peak field of 1.2 T) were performed using the ASC impulse magnetometer (model IM-10). After each impulse field step, we measured the remanence of the sample using the cryogenic magnetometer. Many of the IRM values were too high for the cryogenic magnetometer to measure accurately. As a result, the IRM at high field very often exhibited large fluctuations with increasing field strength. To examine the thermal demagnetization characteristics, a number of minicores were progressively heated from 100° to more than 600°C. This was done by heating the sample to a prescribed temperature, allowing the sample to cool back to the room temperature inside the cooling chamber (model TSD-1 by the Schonstedt Instrument Co.), and then measuring the magnetization intensity using the cryogenic magnetometer. Together with the information on the Curie temperature of different magnetic minerals, we were able to identify the dominant magnetic mineralogy of some of the minicores.
