Previous   |   Next

doi:10.2204/iodp.proc.319.102.2010

Paleomagnetism

Note: This section was contributed by Hirokuni Oda (Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Japan) and Xixi Zhao (Earth and Planetary Sciences Department, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA).

The cryogenic magnetometer was off the ship for repairs during Expedition 319, so paleomagnetic analyses were performed during Expedition 322.

Paleomagnetic and rock magnetic investigations on cores taken during Expedition 319 were carried out on the Chikyu at the beginning of IODP Expedition 322. The measurements were primarily designed to determine the characteristic remanence directions for use in magnetostratigraphy and to reorient cores for structural analysis. To accomplish these goals, paleomagnetic measurements were performed on archive halves.

Laboratory instruments

The paleomagnetism laboratory on board the Chikyu is a relatively large (7.3 m × 2.8 m × 1.9 m) magnetically shielded room with an internal total magnetic field that is ~1% of Earth's magnetic field. The room is oriented with its long axis transverse to the long axis of the ship and houses a superconducting rock magnetometer (SRM) and other magnetically sensitive instruments. The room is large enough to comfortably handle standard IODP core sections (~150 cm).

Superconducting rock magnetometer

The long-core SRM (2G Enterprises, model 760) unit is ~6 m long with an 8.1 cm diameter access bore. A 1.5 m split core liner can pass through a magnetometer, an alternating-field (AF) demagnetizer, and an anhysteretic remanent magnetizer. The system includes three sets of superconducting pickup coils, two for transverse moment measurement (x- and y-axes) and one for axial moment measurement (z-axis). The noise level of the magnetometer is <10^–7 A/m for a 10 cm³ volume rock. The magnetometer includes an automated sample handler system (2G804), consisting of aluminum and fiberglass channels designated to support and guide long-core movement. The core itself is positioned in a nonmagnetic fiberglass carriage that is pulled through the channels by a pull rope attached to a geared high-torque stepper motor. A 2G600 sample degaussing system is coupled to the SRM to allow automatic demagnetization of samples up to 100 mT. The system is controlled by an external computer and enables programming of a complete sequence of measurements and degauss cycles without removing the long core from the holder.

The measurements on cores taken during Expedition 319 were made during the beginning of Expedition 322. One of the main reasons for this is that the SRM was sent back to the manufacturer to replace y-axis superconducting quantum interference device (SQUID) during Expedition 319. The SRM was then transported from the manufacturer, set up, refilled with liquid He and cooled down to 4.2 K during the port call of Expedition 322 at Yokkaichi, Japan. However, leakage caused the liquid He to evaporate more quickly than usual. We managed to measure the whole archive halves taken during Expedition 319 up to 20 mT AF demagnetization. After the measurements, liquid He in the dewar of the SRM evaporated completely and the SRM stopped working.

Sampling coordinates

Magnetic axes from the long-core magnetometer are reported relative to the double line marked on the plastic liner of the working half of the split core with +x, +y, and +z representing north, east, and downcore, respectively (Fig. F33). The "flipping" function of the control software (Long Core version 3.4) enables 180° rotation of the x- and y-axes about the z-axis. By using this flipping function, working and archive halves can be measured in the same coordinate system.

Measurements

Remanent magnetization of archive halves was measured at 5 cm intervals using the shipboard 2G Enterprises (model 760R) long-core cryogenic magnetometer equipped with direct-current superconducting quantum interference devices (DC-SQUIDs) and an in-line automated AF demagnetizer capable of reaching a peak field of 80 mT. The response curve from the sensor coils of the cryogenic magnetometer corresponds to a region ~20 cm wide; therefore, only measurements taken every 20 cm are independent from each other. Measurements at core and section ends, whole-round locations and voids, and within intervals of drilling-related core disturbance were not measured or were removed from the data set during data processing.

Top of page   |   Previous   |   Next