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doi:10.2204/iodp.proc.327.103.2011

Physical properties

Physical property measurements in the basement section of Hole U1362A included whole-round magnetic susceptibility, density, and natural gamma radiation (NGR) emissions measured on every section greater than 50 cm in length; section-half measurements of magnetic susceptibility; and discrete measurements of thermal conductivity, P-wave velocity, and moisture and density (MAD) properties (e.g., bulk density, grain density, and porosity) (Figs. F33, F34; Table T9). Sampling frequency for discrete samples was generally two per section, with extra samples collected where there were visibly significant changes in lithology, alteration, or texture.

Magnetic susceptibility

Whole sections were run through the WRMSL after the core liner was split, with sampling resolution set at 1 cm regardless of section continuity. No effort was made to correct for incomplete filling of the core liner or for discontinuous rock characteristic of this formation or to correct for voids caused by the removal of whole-round sections for microbiological analysis. Magnetic susceptibility is as high as ~3300 × 10^–6 SI, with the highest values corresponding to massive lava flows recovered in Cores 327-U1362A-17R and 18R at 458 and 463 mbsf, respectively (Fig. F34C). Other lithologies (pillow lava and sheet flows) generally yield values of <1500 × 10^–6 SI, partly because of highly fractured layers and poor core recovery, both of which reduce magnetic susceptibility.

After sections were split, archive halves were run through the Section Half Multisensor Logger (SHMSL). Point measurements were made at 1 cm intervals. Magnetic susceptibility trends determined through this method are similar to those obtained from the WRMSL; however, the discontinuous nature of the core sections is more apparent in these data (Fig. F34C). Point susceptibility rarely exceeds 1500 × 10^–6 SI in pillow lava (Unit 3 and Subunits 1A, 1B, 5A, and 5B). Thin sheet flows (Unit 2 and Subunits 7A and 7B) yield similar values, with occasional isolated excursions to higher values in larger pieces. Massive flows (Subunits 6A and 6B) with high recovery, mainly Cores 327-U1362A-17R and 18R, have the highest point susceptibility values, often exceeding 3000 × 10^–6 SI (Fig. F34C). Among cores with large continuous pieces (Cores 327-U1362A-12R and 16R), point susceptibility values often vary widely from those determined by the WRMSL, sometimes by as much as 500 × 10^–6 SI. It may be possible to use the point susceptibility records as a filter for the WRMSL susceptibility data; however, no attempt was made to do so during this expedition.

Gamma ray attenuation density

The bulk density of the core is estimated by GRA measured on the WRMSL. Density results differ by as much as 15% between GRA and MAD property samples, with GRA results typically providing a lower bound on true values. In general, the data remain fairly consistent at ~2.5 g/cm³, dropping only when poor recovery creates gaps that interfere with measurements (Fig. F34A).

Natural gamma radiation

The integrated counts per second (cps) detected by the NGRL serves as a rough estimate of the variability of radioactive elements in the cores. NGRL data are fairly constant with depth (~2–4 cps), typically peaking only in massive lava flow sections where core was more likely to fill core liners (Cores 327-U1362A-17R and 18R, in particular) (Fig. F34D).

An initial attempt was made to correlate integrated NGRL counts with degree of alteration, but no significant correlation was identified. A more detailed analysis making use of the full spectra collected by the NGRL may yield more information regarding relative abundances of radioisotopes.

Thermal conductivity

Thirty individual measurements were made on three basalt samples over the interval of 349–355 mbsf (Fig. F35; Table T9), with samples being chosen on the basis of size, continuity, and the absence of fractures or veins. A minimum length of 8 cm was required to achieve full contact between the sample and the thermal conductivity probe. Although we planned for a much more extensive testing program, only three samples produced reliable data because of the inconsistent performance of the thermal conductivity system, which gave wildly variable (and often unreasonably low or high) values and often would not get beyond the preheating “drift” stage of testing. Some of the difficulty was eventually traced to a corroded connector embedded in the half-space puck, but the acquisition and processing software also contributed to these problems.

The three samples that yielded thermal conductivity data came from the uppermost section of pillow basalt, providing values of 1.63, 1.67, and 1.72 W/(m·K) at depths of 349, 354, and 355 mbsf, respectively. These values compare well with data collected at similar depths into basement in nearby Hole U1301B (1.70 ± 0.09 W/[m·K]) (Fig. F35).

P-wave velocity

Seventy-three discrete samples were collected for measurement of P-wave velocity. The samples were carefully cut and polished in order to ensure good contact between the sample and transducers. Forty-five samples were cut from oriented pieces as 2 cm × 2 cm × 2 cm cubes, allowing two horizontal velocities (x- and y-axes) to be measured in addition to vertical velocity (z-axis). From unoriented pieces we extracted cylinder-shaped samples, allowing P-wave velocity measurement in one direction. We were unable to measure P-wave velocities on 3 cylindrical samples because they cracked or split after extraction, but measurements were made on the remaining 70 samples.

We checked the velocity of the calibration standards after every measurement on saturated samples because the Gantry transducer pairs were unstable, as described in “Physical properties” in the “Methods” chapter. If the calibration value was not within tolerances (2.75 ± 0.25 km/s), we calibrated repeatedly until the values converged. In addition, we measured velocities four times on each axis by rotating in 90° increments and averaging the results. After we finished the overall velocity measurements we remeasured velocities for selected samples. We found that even though we frequently calibrated the measurement device the second set of velocities often differed from the first set. We decided to resaturate and remeasure all samples with a manual picking method (see “Physical properties” in the “Methods” chapter) in order to check the validity of the velocity data measured with the automated method. We also calculated P-wave velocities in dry conditions using both methods.

P-wave velocities of saturated samples measured with the automated method range from a minimum of 4.8 km/s to a maximum of 6.3 km/s, with an average of ~5.6 km/s. P-wave velocities of saturated samples determined by manual picking range from a minimum of 4.5 km/s to a maximum of 6.0 km/s, with an average of ~5.45 km/s (Fig. F33). Although calculating P-wave velocities determined by manually picking the first arrival were slower than those determined using the automatic picking method, the overall velocity trends derived from the two methods are consistent. Both averages are greater than the values obtained from similar core samples in Holes 1026B, 1027C, and U1301B (Shipboard Scientific Party, 1997; Expedition 301 Scientists, 2005). Furthermore, the average values are faster than regional values determined using seismic reflection techniques (Rohr, 1994) and vertical seismic profile (VSP) experiments (Expedition 301 Scientists, 2005). One might expect regional and VSP values to be slower because of large-scale fractures. The P-wave velocities measured in saturated conditions are ~200 m/s faster than those in dry conditions because the rock elastic bulk modulus usually stiffens when air in dry pores is replaced with less compressible seawater.

The lowest velocity (~4.5 km/s) was measured on a heavily altered sample (327-U1362A-14R-1, 8–10  cm). A test of nearby unaltered material yielded much higher velocity, which demonstrates the influence of rock alteration on P-wave velocity. The lithology difference (i.e., margin part of pillow, central part of pillow, basalt flow, and sheet flow) generates the local velocity variation. However, we found no statistically significant overall velocity trend with depth or overall velocity anisotropy.

Moisture and density

The same 73 discrete samples collected from Hole U1362A for P-wave velocity measurements were used in the determination of MAD properties, including cubic samples and cylindrical samples. Because we had doubts about the vacuum pump saturating samples completely, we measured the wet and dry mass of all samples twice.

Bulk density values range from 2.20 to 2.90 g/cm³ (mean = ~2.7 g/cm³). Grain density exhibits a range of 2.41–3.00 g/cm³ (mean = ~2.9 g/cm³) (Fig. F34). Porosity values span a range of 2.8%–15.0% (mean = 7.9%) (Fig. F33). The highest porosity value was from a highly altered sample that also had the lowest P-wave velocity. This implies that high porosity may be a proxy for increasing rock alteration. The data clearly show P-wave velocity and porosity to be inversely correlated (Fig. F36), which may be influenced by several factors: primary porosity, sample cracking, rock alteration, or core disturbance. Although the velocities in Hole U1362A are higher than those in Hole U1301B, the slope of the velocity-porosity relationship is similar (Expedition 301 Scientists, 2005). Because the velocities for constant porosity are significantly influenced by pore geometry as well as crack aspect ratio (Wilkens et al., 1991; Tsuji and Iturrino, 2008), the microcrack geometry in Hole U1362A appears to be similar to that of samples from Hole U1301B.

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