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doi:10.2204/iodp.proc.343343T.102.2013

Paleomagnetism

Paleomagnetic and rock magnetic investigations on board the Chikyu during Expedition 343 were primarily designed to determine the characteristic remanence directions for use in a tectonic study. Paleomagnetic measurements were performed only on discrete minicores and cube samples taken from the working halves. Routine measurements on archive halves collected during Expedition 343 were conducted with the superconducting rock magnetometer (SRM) during Expedition 343T.

Laboratory instruments

The paleomagnetism laboratory on board the Chikyu houses a large (7.3 m × 2.8 m × 1.9 m) magnetically shielded room, with its long axis transverse to the length of the ship. The total magnetic field inside the room is ~1% of Earth’s magnetic field. The shielded room houses the magnetometers and other instruments described in this section and is large enough to comfortably handle standard IODP core sections (~150 cm in length).

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 handling 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 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.

Prior to Expedition 343, the SRM was upgraded to enable operation without liquid helium. Installation of the upgraded SRM onto the Chikyu was planned during the port call at Shimizu, Japan, before Expedition 343. However, because of the shortened port call, the installation was postponed until after the completion of Expedition 343. Thus, for paleomagnetism studies during Expedition 343, we could only measure remanent magnetization of discrete samples with the spinner magnetometer. Continuous paleomagnetic measurements using the archive halves of the core were performed during Expedition 343T in July.

Spinner magnetometer

A spinner magnetometer, model SMD-88 (Natsuhara Giken Co. Ltd.), was utilized during Expedition 343 for remanent magnetization measurement. The noise level was ~5 × 10^–7 mAm², and the measurable range was from 5 × 10^–6 to 3 × 10^–1 mAm². Two holders were prepared for the measurements, one (small or short) for weak samples and the other (large or tall) for strong samples. Five standard samples with different intensities were prepared to calibrate the magnetometer. Standard cylindrical samples (2.5 cm diameter × 2.2 cm long) taken with a minicore or 7 cm³ Natsuhara cubes were measured in three or six positions with a typical stacking of 64–256 spins. The measurement sequence took ~1 min for three positions.

Alternating field demagnetizer

The DEM-95 AF demagnetizer (Natsuhara Giken Co. Ltd.) is set for demagnetization of standard discrete samples of rock or sediment. The unit is equipped with a sample tumbling system to uniformly demagnetize up to a peak AF of 180 mT.

Thermal demagnetizer

The TDS-1 thermal demagnetizer (Natsuhara Giken Co. Ltd.) has a single chamber for thermal demagnetization of dry samples over a temperature range of room temperature to 800°C. The chamber holds up to 8 or 10 cubic or cylindrical samples, depending on the exact size. The oven requires a closed system of cooling water, which is conveniently placed next to the shielded room. A fan next to the µ-metal cylinder that houses the heating system was used to cool samples to room temperature. The measured magnetic field inside the chamber was <20 nT.

Pulse magnetizer

The MMPM10 pulse magnetizer (Magnetic Measurement Ltd., United Kingdom; www.magnetic-measurements.com/) can produce a high magnetic field pulse in a sample. The magnetic field pulse is generated by discharging a bank of capacitors through a coil. A maximum field of 9 T with a 7 ms pulse duration can be produced by the 1.25 cm diameter coil. The other coil (3.8 cm diameter) generates a maximum field of 2.9 T.

Anisotropy of magnetic susceptibility

The Kappabridge KLY 3S (AGICO Inc.) magnetic susceptibility meter is designed for anisotropy of magnetic susceptibility (AMS) measurements. Data are acquired from spinning measurements around three mutually perpendicular axes. The deviatoric susceptibility tensor can then be computed. An additional measurement for bulk susceptibility completes the sequence. The sensitivity for AMS measurements is 2 × 10^–8 SI. Intensity and frequency of the applied field are 300 mA/m and 875 Hz, respectively. This system also includes a temperature control unit (CS-3/CS-L) for temperature variation of low-field magnetic susceptibility of samples.

Discrete samples and sampling coordinates

A total of 56 discrete cubic samples (~7–8 cm³) or minicores (~11 cm³) were taken from the working halves in order to determine paleomagnetic direction, primarily for tectonic studies. Sampling frequency depended on the properties of the core material (e.g., to avoid flow-in, coring disturbances, etc.) and the distribution of interbedded lithologies. For the most part, paleomagnetic sampling concentrated on hemipelagic mudstones as the dominant lithology. The standard IODP core coordinate system was used, in which the +x-axis is the vertical upward direction when the core (archive half) is on its curved side, the +y-axis is the direction to the left along the surface of the archive halves when looking upcore, and the +z-axis is the downcore direction (Fig. F19).

Measurements

Stepwise demagnetization experiments on discrete paleomagnetic samples were conducted using the AF demagnetizer to isolate various magnetic components of the samples. Whenever possible, demagnetization was continued up to 120 mT until an unambiguous and reliable determination of direction of the stable component of magnetization was achieved.

Remanent magnetization of archive halves was measured at 2 cm intervals using the 2G long-core SRM during Expedition 343T. We adopted AF demagnetization levels of 15 and 20 mT because AF demagnetization results of discrete samples showed that AF demagnetization below 15 mT was insufficient for removing drilling-induced magnetization of the cores drilled during this expedition. However, we adopted AF demagnetization at 5, 10, 15, and 20 mT for Core 343-C0019E-17R, from which no discrete paleomagnetic samples were measured.

In addition to standard paleomagnetic measurements, bulk magnetic susceptibility and AMS of the discrete paleomagnetic samples were measured using the Kappabridge KLY 3S magnetic susceptibility meter.

Paleomagnetic reorientation of cores

Azimuthal orientation of drilled core material is of prime importance when modeling directional properties of rock formations. Paleomagnetic core reorientation has been successfully used for a number of years (e.g., Fuller, 1969; Kodama, 1984; Shibuya et al., 1991; Parés et al., 2007). The procedure is based on determining the direction of stable remanent magnetization (either viscous remanent magnetization or primary magnetization) with respect to a common reference line that is marked lengthwise along the core. Assuming a moderate sedimentation rate of ~5 cm/k.y. and a magnetization lock-in depth of ~10 cm, a typical sample depth interval of 2.5 cm might be enough to average the secular variation of the geomagnetic field, and the paleomagnetic direction roughly points in the direction of geographic north. During Expedition 343, the characteristic remanent magnetization (ChRM) direction, calculated using principal component analysis (Kirschvink, 1980), was used to reorient the blocks with important deformation structures (see “Structural geology”).

Data reduction and software

Data reduction (Zijderveld demagnetization plots and equal area projections) was conducted using visualization software called “Progress” for Mac OSX programmed by H. Shibuya (Kumamoto University, Japan). Principal component analysis (Kirschvink, 1980) was also performed by using Progress software to determine ChRM directions.

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