During Leg 196, 5.19 m of sediment and basalt was recovered from Hole 1173B between 737.1 and 756.6 mbsf. Physical properties data at this site provide important information about the shallowest oceanic basement before it experiences any potential deformation due to subduction.
Whole-round core sections were run on the multisensor track to collect magnetic susceptibility, gamma ray attenuation density, and natural gamma radiation measurements at 2- to 10-cm intervals. However, the data are considered unreliable because these measurements are sensitive to sample volume and the core pieces were relatively small and highly variable in diameter. Thus the results are not presented in this report.
Nine basalt minicores and two sediment samples were taken. The minicores were taken from unfractured parts without veins to avoid breaking. The minicores were then passed through the parallel saw blades to allow velocity to be measured on all three directions. Moisture and density (MAD) properties were measured on all samples and P-wave velocity was measured on the minicore samples.
MAD properties are shown in Table T8 and Figure F36. Porosities are 31% and 38% for the two sediment samples and 2.6% to 6.2% for the basalt samples. Bulk density for the sediment samples is 2.13 g/cm3, ~0.5 g/cm3 lower than for basalt. Although this difference in bulk density is primarily due to the porosity contrast, the difference in mineral composition is also significant because the grain densities of sediment and basalt are 2.73 and 2.83 g/cm3, respectively.
P-wave velocities were measured on basalt samples using the P-wave sensor 3 (PWS3) contact probe system. Velocities range between ~4.7 and ~5.5 km/s (Table T8; Fig. F36). No significant anisotropies are observed. Velocity shows a negative correlation to porosity. Moderately plagioclase-olivine-phyric basalt samples (Samples 196-1173B-2R-1, 21-24 cm, and 3R-2, 80-82 cm) show high velocities and low porosities, whereas sparsely plagioclase-phyric basalts, which tend to be significantly altered, show lower velocities and higher porosities.
Thermal conductivities were measured on archive half-rounds (Table T9; Fig. F36). Nine of the measurements show quite uniform values (1.69 ± 0.02 W/[m·K]). The tenth sample (Sample 196-1173B-2R-1 [Piece 4]) has higher conductivity (1.83 W/[m·K]). This sample has a very low porosity (3.3%) as compared with other basalts (5%-6%). Although we do not have enough porosity and conductivity data to apply a geometrical mean model to calculate the grain thermal conductivity, the large conductivity values could in part be explained by the small porosity of the sample.