Paleomagnetic investigations conducted on board the JOIDES Resolution during Leg 203 consisted of routine measurements of natural remanent magnetization (NRM) and of magnetic susceptibility (MS) of igneous rocks. NRM was measured on all archive split cores of recovered rocks and on discrete samples of basement rocks taken from the working halves. Stepwise alternating-field (AF) demagnetization was carried out on all archive-half cores and on some discrete samples in an attempt to isolate stable components of remanence. A few discrete samples were thermally demagnetized in an effort to obtain their primary remanent magnetization and identify magnetic carriers. MS was measured for whole cores, archive-half core sections, and, in a few cases, discrete samples. To investigate rock magnetic properties, we also conducted isothermal remanent magnetization (IRM) experiments and thermal demagnetization of the IRM on some of the discrete samples. Magnetic properties were compared with lithologic units.
The remanence of archive-half sections and oriented discrete samples from working-half sections was measured using a 2-G Enterprises pass-through cryogenic direct-current superconducting quantum interference device (DC-SQUID) rock magnetometer (model 760R). This pass-through cryogenic magnetometer is equipped with an in-line AF demagnetizer (2-G model 2G600) that allows for demagnetization of samples up to peak fields of 80 mT with a 200-Hz frequency. 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 personal computer (PC) and are controlled by the 2G Long Core software by National Instruments. A Molspin spinner magnetometer was also available on the ship for measuring the remanence of discrete samples. For stepwise demagnetization of discrete samples, the laboratory contains 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 to 200 mT and 700°C, respectively. An Analytical Services Company (ASC) model IM-10 impulse magnetizer (capable of pulsed fields from 0.02 to 1.35 T) was available for IRM acquisition studies of discrete samples.
MS was measured for all whole-core sections at 2-cm intervals using a susceptibility meter attached to the multisensor track (MST) (see "Physical Properties"). The susceptibility values are 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. The MS of archive halves was routinely measured using the AMST (see "Core Descriptions") at 1-cm intervals. The measurements were recorded automatically by the AMST, which permits measurements only at evenly spaced intervals along each section of core. For the two types of susceptibility measurements (MST and AMST), the same type of MS meter (Bartington Instruments model MS2) was used but with a different sensor. The sensor for whole-core measurements (MS2C) has an 80-mm inner diameter, and the core passes through the sensor coil. A Geofyzika Brno Kappabridge KLY-2 MS meter was available for MS measurements of discrete samples.
To investigate rock magnetic characteristics of some of the discrete samples, IRM acquisition experiments were conducted. IRMs were imparted to the discrete samples by a direct current field generated in the ASC impulse magnetizer. The magnetization acquired by samples in progressively stronger applied fields, usually to saturation, was measured with the cryogenic magnetometer. We conducted thermal demagnetization of IRMs on a few samples to delineate the nature of magnetic remanence in the rocks.
The standard ODP core-orientation convention was applied for paleomagnetic work during Leg 203. This convention can be described as follows: the z-axis is downhole parallel to the core, the x-axis forms a line perpendicular to the split face of the core and is directed into the working half, and the x-axis is used as the reference "geomagnetic north" for the definition of magnetic declination values (Fig. F5). Discrete minicores were marked with an arrow in the negative z-direction (uphole) on the plane representing the split surface of the working half. The plane marked with the arrow is the y-z plane. Because only the RCB was used for drilling, we were unable to use the Tensor tool, which mounts on the APC core barrel to orient cores.
Oriented discrete samples were taken from the working half of each section (typically one or two per section for shipboard analyses) for on board pilot demagnetization studies. Cylindrical minicores (10.5 cm3) were drilled from igneous rocks using a water-cooled nonmagnetic drill bit attached to a standard drill press. To minimize using core material for shipboard studies, most minicores were shared with the shipboard physical properties laboratory.
The NRM of the archive-half sections was analyzed on the cryogenic magnetometer at 1-cm intervals for igneous rocks when sufficiently long continuous pieces were available. To isolate the characteristic remanent magnetization (ChRM), archive halves were AF demagnetized up to a maximum peak field of 70 mT. Stepwise thermal demagnetization of up to 580°C was also applied to some discrete samples. IRM acquisition experiments were performed on some selected rocks in order to define their dominant magnetic mineralogy.
The stability of remanence levels within the archive cores and the discrete samples were determined by both Zidjerveld (1967) plots and equal-area stereographic projections. The ChRM of basalt samples was then obtained using principal component analysis (Kirschvink, 1980) on the results of the AF or thermal demagnetization.
Results from the remanent magnetization and low-field susceptibility measurements were compared with lithologic units and/or geologic structures based on sedimentary, petrologic, and structural features (see "Core Descriptions").