b) is the density of the saturated sample,
Shipboard measurements of physical properties are used to characterize lithologic units and alteration processes, for correlating cored material with downhole logging data, and for interpreting seismic reflection profiles.
After recovery, the cores were allowed to come to room temperature (22°–23°C), then thermal conductivity, natural gamma radiation (NGR), and magnetic susceptibility were measured in a series of nondestructive tests. Additional measurements of P-wave velocity, bulk and grain density, porosity, and water content were made on right circular cylinders, cubes, and chips cut from the split core. Most of these samples were also used for paleomagnetic measurements.
Four sets of measurements (magnetic susceptibility, gamma ray attenuation [GRA] density, P-wave velocity, and NGR) can be made in sequence on whole-core sections on the MST. MST data are sampled at discrete intervals, with sampling intervals and count times chosen to optimize the resolution of the data in the time available to run each core section through the device. During Leg 209, no advanced piston corer or extended core barrel sediment cores were taken; consequently, only magnetic susceptibility and NGR measurements were made on the rotary core barrel (RCB) cores.
The MST includes a Bartington susceptibility meter (model MS2C) that has an 8-cm loop and operates at 0.565 kHz with a field intensity of 80 A/m. Volume susceptibility, k, is a dimensionless measure of the degree to which material can be magnetized in an external magnetic field,
where M is the magnetization induced in the material by an external field of strength H. Magnetic susceptibility is sensitive to variations of the type and concentration of magnetic grains in the rocks and is thus an indicator of compositional variations. During Leg 209, we sampled RCB cores at 2.5-cm intervals. On the Bartington MS2C sensor, all readings >0.1 SI are clipped, so that the most significant digit of the susceptibility value is not recorded. The data from the MST system are given in 10–5 SI. They represent the volume susceptibility for a core with a diameter of 6.6 cm. The diameter of the Leg 209 cores was always smaller (~5.5–6.0 cm); therefore, the true susceptibility is underestimated.
The GRA densitometer was not used during Leg 209.
The P-wave logger was not used during Leg 209.
Natural gamma radiation was logged on the MST using a 30-s counting period at 10-cm intervals.
Thermal conductivity is measured by transient heating of the sample with a known heating power generated from a source with assumed infinite length, finite radius, and assumed infinite thermal conductivity (7-cm-long needle) and measuring the temperature change with time using the TK04 system described by Blum (1997). The variation of temperature with radial distance from the source depends on the thermal conductivity of the sample, which is assumed to be homogeneous. The needle probe method is used in half-space mode (Vacquier, 1985) to measure thermal conductivity in pieces of the split core taken from the archive half. Samples were taken at irregularly spaced intervals, depending on the availability of pieces long enough to be measured and on the lithologic variability. Measurements are made at room pressure and temperature; they are not corrected to in situ conditions. The shipboard measurement conditions are not optimal, as the measurement setup is not always in perfect thermal equilibrium because of room temperature changes due to frequently opened doors and circulating people. Each reported thermal conductivity value is an average of four measurements, with the smallest possible standard deviation (<0.03 W/[m·k] in most cases). In the site chapters, Leg 209 data are compared to previous data for gabbros and peridotites from Legs 118 (Robinson, Von Herzon, et al., 1989), 147 (Gillis, MJvel, Allan, et al., 1993), 153 (Cannat, Karson, Miller, et al., 1995), and 176 (Dick, Natland, Miller, et al., 1999). However, these (except for Leg 176) were acquired with the Thermcon-85 system, which was replaced on board the JOIDES Resolution by the TK04 system in 1996 (Leg 168). The latter probably gives more accurate and consistent results, as the new measurement technique is less user-dependent than the older one (Blum, 1997).
The measurement method used on board the JOIDES Resolution (Blum, 1997) assumes the sample is isotropic. However, thermal conductivity is an intrinsic tensor material property that depends on porosity, density, mineral composition, and fabrics. Most known single-crystal thermal diffusivities are anisotropic (e.g., Kobayashi, 1974; Tommasi et al., 2002). Therefore, thermal conductivity is expected to be anisotropic. To evaluate the anisotropy of thermal conductivity in the peridotites and gabbros, we ran, whenever possible, three measurements on the cut face of each sample, one parallel to the core and two at angles of ~35° to the core axis (Fig. F8).
The PWS3 (Hamilton Frame) was used to measure velocities in discrete samples (minicores and cubes) of the materials recovered during Leg 209. The PWS3 is a modified and updated version of the classic Hamilton Frame velocimeter, in which one transducer is fixed and the other is mounted on a screw. The PWS3 is mounted vertically to measure velocities perpendicular to the core axis by placing the sample on the lower transducer and bringing the upper transducer into direct contact with the upper surface. To improve the coupling (i.e., the impedance match) between the transducer and the sample, water is commonly applied to the top and bottom of the sample and transducer heads. Traveltimes are picked manually or automatically by the threshold method, and the transducer separation is recorded by a digital caliper.
The minicores and cubes used for velocity measurements were also used to estimate bulk density, grain density, and porosity from the wet weights and dry weights or volumes of the samples. Volumes were calculated from the dimensions of the samples, measured using either a hand caliper or the PWS3 digital caliper. In the course of our analysis of data from Site 1272, we found that the latter method is subject to a systematic mean error in the length of ~0.5%, which is not enough to affect the PWS3 velocities but leads to a systematic overestimate of sample volume and a corresponding underestimate of bulk density of ~1.5% (~4 kg/m3). Sample mass was determined to a precision of ±0.001 g using two Scientech electronic balances. The balances are equipped with a computer averaging system that compensates for the motion of the ship. The sample mass on one balance is counterbalanced by a known mass on the adjacent balance. The volumes of chips and some minicores were determined using a five-cell Quantachrome helium-displacement pycnometer with a nominal precision of ±0.01 cm3. Measurements were repeated until a standard error <0.02 cm3 was achieved. A standard reference sphere was run sequentially in each of the five operating cells to maintain calibration. The cell volume was recalibrated if the measured volume of the standard was not within 0.02 cm3 of the known volume of the standard. Dry weight and pycnometer volume measurements were made after the samples were oven dried at 105° ± 5°C for 24 hr and allowed to cool in a desiccator. A potential problem with this drying temperature is that, in addition to interstitial water, chemically bound water in some clay minerals can be lost.
Water content, as a fraction of total mass or as a ratio of water mass to solid mass, is determined by standard methods of the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM, 1989). The total water-saturated mass (Mb) and dry mass (Md) are measured using the electronic balance as described above, and the difference is the uncorrected water mass. Measured wet and dry masses are corrected for salt assuming a pore water salinity (s) of 0.035 (Boyce, 1976). The water contents expressed as a percentage of the wet mass or the dry mass (Wb and Ws, respectively) are given by
where Mb = the mass of the saturated sample.
Bulk density (b) is the density of the saturated sample,
b = Mb/Vb,
where Vb = the total sample volume, which is estimated from the dimensions of the sample or from the volume of the dry sample (Vd) and the volume of the pore fluid (Vpw) (see below):
Grain density (g) is determined from the dry mass and dry volume measurements. Both mass and volume must be corrected for the salt content of the pore fluid,
g = (Md – Msalt)/(Vd – Msalt/salt),
where Md = the dry mass of the sample (in grams) and salt = the density of salt (2.257 g/cm3), where
is the mass of salt in the pore fluid and Mpw is the salt-corrected mass of the seawater
The volume of pore water is
pw,
where pw = the density of the pore fluid, which is assumed to be seawater.
Alternatively, the grain density is calculated from the wet and dry masses (Mb and Md) and the sample volume (Vb) is calculated from the measured dimensions of a minicore or cube sample:
g = (Md – Msalt)/[(1 – s) pw/(Mb – Md)].
Porosity (
) is the ratio of pore water volume to total volume and can be calculated from fluid density, grain density, and bulk density of the material:
= 100[(g – b)/(g – w)],
where g = the grain density and b = the bulk density.
The dry density (d) is the ratio of the dry mass (Md) to the total volume (Vb). The dry density is calculated from the corrected water content (Wd) and porosity ():
d = (/Wd) w .
