Laboratory measurements of the physical properties of igneous and metamorphic rocks are used to indicate major lithologic changes, to help identify intervals of high magnetic susceptibility, to recognize changes in paleomagnetic character, and to allow preliminary interpretation of seismic velocity profiles. During Leg 179, when only gabbroic rocks were sampled, measurements were made of magnetic susceptibility, rock magnetism, bulk density (wet and dry), porosity, compressional wave (P-wave) velocity, and natural gamma-ray emission. We made measurements on whole core sections (magnetic susceptibility and gamma ray emission), split core (remanent magnetization), and on discrete minicore samples (P-wave velocity at ambient pressure and index properties).
The multisensor track (MST) is an automated and rapid core-conveying and positioning system with multiple in-line sensors/detectors that allow a user-defined series of measurements. At preselected sampling intervals, this system provides a display of measured values and allows rapid data archiving in the ODP database for evaluation of these properties with depth. Inasmuch as the P-wave velocity and gamma-ray attenuation porosity evaluator devices were designed for optimum performance when core liners are completely full (as is common in sediment coring, but which never occurs with hard-rock recovery), these measurements were not made during Leg 179. Additionally, although we expected little response above baseline values for natural gamma emission from the lithologies recovered, these measurements were intermittently made as a systems check. During Leg 179, we routinely sampled magnetic susceptibility at a constant 4-cm interval to optimize time and minimize the potential of missing thin intervals of oxide-bearing lithologies.
Magnetic susceptibility (k) is the ease or degree to which a material is magnetized in an external field and is a measure of the concentration of ferromagnetic grains. The Bartington MS2C susceptometer loop on the ODP MST has a measurement range of 1 × 10-5 to 9999 × 10-5 (SI, volume specific) or 1 × 10-8 to 9999 × 10-8 (SI, mass specific). This sensor operates at a frequency of 0.565 KHz and an alternating field intensity of 80 A/m. Temperature drift effects are less than 10-5 SI/hr and the resolution of the loop sensor is 2 × 10-6 SI at 9-s measurement intervals.
Gamma rays are spontaneously emitted by the atomic nuclei of elements during natural radioactive decay. In rocks, the elements of interest are most commonly potassium (half-life = 1.3 × 109 years), thorium (half-life = 1.4 × 1010 years), and uranium (half-life = 4.4 × 109 years). The emission of natural gamma radiation is routinely measured on ODP cruises in electron volts (eV) or GAPI (gamma-ray, American Petroleum Industry) units for correlation with downhole logging measurements. Details of the measurement system are available in ODP Technical Note 26 (Blum, 1997). Natural gamma radiation measurements were made for calibration and system-check purposes only during Leg 179.
Archive halves of the split core were processed in the 2-G Enterprises pass-through magnetometer. With this instrument, sensors measured intensity and direction of natural remanent magnetization at stepped demagnetization intervals to 20 mT.
One-inch diameter minicore samples were extracted from the working half of the core on a routine basis for measurements of physical properties. Measurement techniques for index properties (bulk density, grain density, water content, porosity, and dry density) are documented in ODP Technical Note 26 (Blum, 1997), and specific techniques for hard-rock measurements were applied as described in Shipboard Scientific Party (1999).
P-wave velocity at ambient pressure was determined by the pulse transmission method utilizing piezoelectric transducers as sources and detectors in a screw-press modified Hamilton Frame, as described by Boyce (1976). All measurements were made on seawater-saturated minicores cut perpendicular to the axis of the core (diameter = 2.54 cm; approximate length = 2 cm) at zero confining pressure.
Minicores used for the P-wave velocity measurements were resaturated with seawater in a vacuum for 24 hr before measurement. Flat ends of the minicores were carefully smoothed with 240 carbide grit on a glass plate to ensure parallel faces. The length of each minicore was checked using a caliper along its circumference, and grinding continued until all length measurements were within 0.02 mm. Before measurement, the grit was removed by thoroughly cleaning the samples in an ultrasonic bath. Distilled water was used to improve the acoustic contact between the sample and the transducers.
Calibration measurements were performed during the cruise using polycarbonate standard minicores of varying lengths in order to determine the zero-displacement time delay inherent in the measuring system. Results were output in meters per second.