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doi:10.2204/iodp.proc.324.104.2010

Physical properties

Igneous rock, sedimentary rock, and sediments from Hole U1347A were characterized for physical properties as described in "Physical properties" in the "Methods" chapter. Core sections with continuous intervals longer than 8 cm were run through the Whole-Round Multisensor Logger for measurement of gamma ray attenuation (GRA) density and magnetic susceptibility. Data from whole-round measurements were filtered by a MATLAB code to remove data associated with gaps and small pieces as described in "Physical properties" in the "Methods" chapter. The filtered data were then visually double-checked against images of the core section halves. Sections longer than 50 cm were measured with the Natural Gamma Ray Logger. Fifty-nine measurements of thermal conductivity were performed. Fifty-five discrete oriented cubic samples of igneous material 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

Magnetic susceptibility

The results for whole-round magnetic susceptibility in Hole U1347A are summarized in Figure F33 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 usually less than filled, the data should be treated as minima.

A first-order observation is that magnetic susceptibility in Hole U1347A is much higher than in Hole U1346A, which was subjected to pervasive alteration. Average values in igneous material in Hole U1347A range from ~1500 x 10^–5 to ~2400 x 10^–5 SI. Two exceptions occur in massive flow Units VII and XV, which show a two-fold increase in magnetic susceptibility, yielding values up to 3800 x 10^–5 SI (Fig. F33).

Recovered sedimentary interbeds are characterized by markedly lower magnetic susceptibility values, generally <100 x 10^–5 SI, making them useful for correlative purposes (see also "Downhole Logging").

Gamma ray attenuation density

The results for GRA bulk density measurements are summarized in Figure F33. Caution should be used when interpreting the absolute density values from the whole-round track because RCB drilling results in cores that generally do not fill the liners, thus underestimating density. As with magnetic susceptibility, values from Hole U1347A are higher than those at Site U1346. This is in agreement with the fresher nature of the material at Site U1347. Sedimentary interlayers are again easily distinguishable by their lower density relative to igneous material.

Natural Gamma Ray Logger

Natural gamma ray (NGR) measurements are summarized in Figure F33. NGR in Hole U1347A averaged 2–4 cps, in stark contrast to the 20–30 cps routinely measured at Site U1346. This observation is again in keeping with the fresher nature of the material in Hole U1347A versus Hole U1346A. With such low count rates, the spectra in Hole U1347A are not as well defined as in Hole U1346A. Geochemical shipboard analyses of igneous material confirm low K₂O content, with samples yielding values of 0.1–0.2 wt% (see "Geochemistry"). The sedimentary section in the bottom of Core 324-U1347A-16R to the top of 17R shows the only appreciable increase in total NGR counts, reaching values of almost 10 cps (Fig. F33).

Moisture and density

A summary of bulk density, dry density, grain density, void ratio, water content, and porosity measurements on discrete samples is listed in Table T8. The densities and porosities are shown in Figure F54. The bulk density of igneous material ranges from 2.49 to 2.94 g/cm³, with a porosity range of 2.36% –22.89%. When porosity is low, the bulk, dry, and grain densities converge. Porosity is low in the interiors of flows, where presumably there is less vesicularity. The recovered igneous material is relatively well preserved (see "Igneous petrology" and "Alteration and metamorphic petrology"), and porosity displays a tight negative correlation with bulk density (Fig. F55).

The MAD determinations are also correlated with identified stratigraphic units (see "Igneous petrology" and "Sedimentology"). Massive flows (stratigraphic Unit XV) and thick upper pillow basalts (Unit X) have the lowest porosity (<5%) and highest bulk densities measured at the site (Fig. F54).

Compressional (P-wave) velocity

Downhole variation of compressional wave velocity is summarized in Figure F54 and listed in Table T9. P-wave velocity shows several intervals of interest. In particular, there is a sharp increase in downhole velocity at ~220 mbsf. This corresponds to the beginning of a ~30 m thick pillow lava (stratigraphic Unit X; see "Igneous petrology"). The increase is accompanied by a drop in porosity and increase in bulk density. The other interval of note is the massive igneous flow (Unit XV) recovered below ~ 290 mbsf. This ~20 m thick unit has consistently high P-wave velocities, low porosities, and high bulk densities (as measured by discrete sampling).

Figure F56 illustrates the relationship between the stratigraphic units with bulk density versus P-wave velocity. The massive flows appear to be distinguishable from one another by their physical properties. Massive flow Units IV–IX have lower P-wave velocities and bulk densities and display very little overlap with massive flow Unit XV, which has the highest measured bulk densities and P-wave velocities. The pillow basalts (stratigraphic Units X–XIV) span the entire range of bulk density and P-wave velocity. One outlier on Figure F56 is from Section 324-U1347A-16R-5. This sample has an apparently low density for its P-wave velocity. This sample had a large vesicle in one corner of the discrete sample cube. This likely results in a lower density without affecting the P-wave velocity because the vesicle is not continuous throughout the cube and the P-wave measurement records the first (fastest) arrival time.

Thermal conductivity

Fifty-nine measurements of thermal conductivity were performed (58 igneous and 1 sedimentary sample). The results are summarized in Table T10 and Figure F57. The sedimentary measurement yielded 1.007 ± 0.017 W/(m·K) (2σ). The average of all igneous measurements was 1.59 ± 0.200 W/(m·K) (2σ). The massive flow Unit XV, recovered below ~290 mbsf, has consistently higher thermal conductivity than the rest of the hole (average = 1.733 ± 0.092 W/[m·K]; 2σ, N = 11). Unit XV also has the highest magnetic susceptibility (Fig. F57), indicating a greater proportion of magnetic, thermally conductive minerals (e.g., titanomagnetite).

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