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doi:10.2204/iodp.proc.324.103.2010

Physical properties

Igneous rock, sedimentary rock, and sediments in Hole U1346A were characterized for physical properties as described in "Physical properties" in the "Methods" chapter. Whole-round sections of core with continuous intervals longer than 10 cm were run through the Whole-Round Multisensor Logger (WRMSL) for measurement of gamma ray attenuation (GRA) density and magnetic susceptibility. Sections longer than 50 cm were measured with the Natural Gamma Ray Logger. Twenty-five discrete oriented cubic samples were cut from the working half of the cores for measurement of moisture and density (MAD) properties as well as compressional (P-wave) velocities in three directions.

Whole-Round Multisensor Logger measurements

Measurements of magnetic susceptibility and bulk GRA density were obtained on whole-round cores. Hard rock coring often results in recovery of pieces rather than continuous sections. Measurements on the WRMSL are sensitive to gaps in the core or to small-diameter pieces (e.g., pebbles). Therefore, the raw data from the WRMSL were filtered to remove underestimated data as described in "Physical properties" in the "Methods" chapter.

Magnetic susceptibility

Magnetic susceptibility results are summarized in Figure F46 and reported in 10^–5 SI units. The raw data were corrected using a Bartington correction factor (Blum, 1997). The correction assumes that the core liner is filled. Because the liner is always less than filled, the data must be treated as minima. Magnetic susceptibility in the upper sedimentary sequence in Section 324-U1346A-4R-1 is <10 x 10^–5 SI units with a notable peak of >60 x 10^–5 in Section 5R-1 between 129.8 and 130.2 mbsf (Fig. F47B). The sediment/basement interface is characterized by a sharp increase in magnetic susceptibility to >100 x 10^–5 SI units at 139 mbsf. Throughout the basaltic sequence cored from Section 324-U1346A-6R-1 to 16R-2, magnetic susceptibility shows a wide range from 30 x 10^–5 to ~1000 x 10^–5 SI units. A decrease in magnetic susceptibility in Section 324-U1346A-8R-1 is observed at 149 mbsf (Fig. F47C), which corresponds to a distinct color change associated with differing alteration styles (see "Alteration and metamorphic petrology"). In other cores, there appears to be statistically significant variation in magnetic susceptibility, independent of edge effects or gaps. A good example is Section 324-U1346A-16R-1 from 187.0 to 187.8 mbsf (Fig. F47D). This variation might be used as an indicator of changing mineralogy of the basalt.

Gamma ray attenuation bulk density

The results for GRA bulk density measurements are summarized in Figure F46. Caution should be used when interpreting the absolute density values from the whole-round track, since RCB drilling results in cores that do not fill the lining, thus yielding an underestimation of density. In general, the sedimentary sections have lower GRA density than the igneous sections. The turbiditic sequence (see "Sedimentology") in Section 324-U1346A-4R-2 from 121.1 to 121.8 mbsf has uniform density (Fig. F47A). The igneous material has generally uniform density throughout the core despite pervasive and variable degrees of alteration, veining, and cracks to which the material has been subjected (see "Alteration and metamorphic petrology" and "Structural geology").

Natural Gamma Ray Logger

NGR measurements are summarized in Figure F46. These measurements correlate well with those from downhole logging tools (see "Downhole logging"). Measurement count time was maximized whenever possible to produce the highest resolved spectrum.

NGR data are output in two forms: one is total counts per second for all of the naturally occurring radioactive elements (such as ²³⁸U and ⁴⁰K) and the second form is a measure of the counts per second versus channel, where channel can be correlated to energy using "known" peaks. Basalts from Hole U1346A average ~22 cps (total counts). These relatively high counts could reflect alteration remobilization and enrichment of potassium. This is in general agreement with inductively coupled plasma (ICP) geochemical data showing K₂O contents as high as 4.79 wt% (see "Geochemistry").

As a first-order observation, sedimentary sequences are dominated by contribution of uranium and its decay products, whereas the NGR spectrum data for the underlying basaltic sequence are dominated by potassium and its daughter nuclides (Fig. F48). Section 324-U1346A-5R-1 has an extreme enrichment in uranium from 129.6 to 129.9 mbsf. Figure F47B illustrates the correlation of the high NGR counts with magnetic susceptibility. There is a 20 cm offset that may represent a true offset or could be due to core motion within the liner, as this section was partially unconsolidated.

Moisture and density

Measurements of bulk density, dry density, grain density, void ratio, water content, and porosity on discrete samples are given in Table T8. The average bulk density determined from discrete samples was 2.48 g/cm³, whereas grain density averaged 2.79 g/cm³. These are higher values compared to the GRA data from the WRMSL, confirming that whole-round measurements underestimate density since the core does not completely fill the core liner. There is a slight decrease in water content and void ratio with depth; however, other MAD properties have similar ranges throughout the core (Fig. F49).

Compressional (P-wave) velocity

Compressional wave velocities were measured on 24 basaltic discrete samples and one limestone discrete sample (Fig. F46; Table T9). Oriented cubic samples had P-wave velocities measured in three orthogonal directions (x, y, and z). The average values throughout the hole were 4.54 km/s (x-direction), 4.57 km/s (y-direction), and 4.55 km/s (z-direction), which are within error of the 20 m/s precision of the measurements. The limestone from Section 324-U1346A-5R-1 (130.37 mbsf) had values of 3.82 m/s (x-direction), 3.93 km/s (y-direction), and 3.77 km/s (z-direction). Maximum values in all three directions were 5.6–5.7 km/s, whereas minimum velocities were 3.5 km/s in all three directions. None of the samples showed appreciable anisotropy. There is no trend in P-wave velocity with depth. As expected, P-wave velocity correlates well with bulk density determined from MAD measurements (Fig. F50)

Thermal conductivity

Thirteen measurements of thermal conductivity were made on the archive half of core sections (Fig. F46; Table T10). Determinations were made at irregular intervals, depending on the availability of material homogeneous and continuous enough for measurement, resulting in a sampling frequency of 1–2 per core. Thermal conductivity in basaltic material ranged from 1.37 to 1.68 W/(m·K) and averaged 1.52 W/(m·K). There was no apparent trend of thermal conductivity with depth. In order to preserve the few recovered sedimentary sections, destructive thermal conductivity measurements were not performed on sedimentary samples.

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