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doi:10.2204/iodp.proc.324.107.2010

Physical properties

Igneous and sedimentary rocks from Hole U1350A were characterized for physical properties as described in "Physical properties" in the "Methods" chapter. Forty-three (42 igneous and 1 limestone) discrete oriented cubic samples were cut from the working half of the cores for measurement of moisture and density properties as well as compressional (P-wave) velocities in three directions. No measurements of thermal conductivity were made at this site because of technical difficulties with the instrument.

Whole-Round Multisensor Logger measurements

Magnetic susceptibility

Results for whole-round magnetic susceptibility for Hole U1350A are summarized in Figure F43. Magnetic susceptibility decreases downhole. The uppermost Subunit IIa, made of massive basalt flows and pillowlike inflation units (see "Igneous petrology"), averages 1357 ± 723 x 10^–5 SI (2σ). Subunit IIa also contains three excursions to values >2000 x 10^–5 SI, occurring in Sections 324-U1350A-7R-1, 50 cm (2272 x 10^–5 SI), 9R-3, 61 cm (2268 x 10^–5 SI), and 11R-2, 56 cm (2370 x 10^–5 SI). Magnetic susceptibility remains fairly high throughout the basaltic transition of Subunit IIb (from massive inflation units to smaller pillow units; see "Igneous petrology"), with values averaging 1308 ± 568 x 10^–5 SI (2σ). Subunit IIc is described as pillow basalts (see "Igneous petrology"); however, it can clearly be split into two intervals based on magnetic susceptibility. The upper interval found in Core 324-U1350A-20R from 249.54 to 262.06 mbsf maintains a high magnetic susceptibility value averaging 1407 ± 296 x 10^–5 SI (2σ), whereas material in the remainder of the unit from Cores 22R through 24R (268.96–290.30 mbsf) shows a sharp ~50% decrease in magnetic susceptibility to average values of 725 ± 431 x 10^–5 SI (2σ). The transition to lower magnetic susceptibility corresponds to the end of intercalated sediment and pillows and the onset of a continuous pillow stack devoid of sediments (see "Igneous petrology"). However, since magnetic susceptibility values remain low downhole (see next paragraph) when sedimentary interbeds return, a correlation cannot be made directly with lithology.

Hyaloclastic material composing Unit III (see "Igneous petrology") yields very few data that are not affected by cracks/edge effects (see "Physical properties" in the "Methods" chapter). The unit (290.36–296.78 mbsf; Sections 324-U1350A-24R-3 through 25R-1) is pervasively cracked and/or rubbly. Therefore, it is difficult to assess characteristic values of this unit with whole-round magnetic susceptibility measurements. Averaging filtered data yields a value of 266 ± 474 x 10^–5 SI (2σ), which is an underestimation (see details on filtered data maxima in "Physical properties" in the "Methods" chapter). Unit IV consists of pillow basalts presumably intruded into limestone (see "Igneous petrology" and "Sedimentology"). Unit IV displays low magnetic susceptibility values similar to the lower interval of pillow basalts seen in Subunit IIc, with an average of 623 ± 475 x 10^–5 SI (2σ). Unit IV has been subjected to higher alteration, perhaps accounting for some of the decrease in magnetic susceptibility.

Gamma ray attenuation bulk density

The results for gamma ray attenuation (GRA) bulk density measurements are summarized in Figure F43. Filtering of data does not take into account the differences between recovered core diameter and a full core liner diameter so that the GRA data may still include some artificially low values (Fig. F43); therefore, averages of the filtered bulk density should be considered an underestimate. Overall, density maxima remain fairly constant downhole at ~2.5 g/cm³, irrespective of unit.

Natural Gamma Ray Logger

Measurement of natural gamma radiation (NGR) is summarized in Figure F43. Counts per second of NGR in Hole U1350A ranged from 2 to 15 cps. Stratigraphic units can be somewhat distinguished by their NGR values. Overall, Subunit IIa (massive basalt flows; see "Igneous petrology") has an average NGR signal of 4.05 ± 3.94 cps (2σ). Upon further examination, Subunit IIa can be divided into two intervals based on NGR readings. The upper part of the unit found in Cores 324-U1350A-7R through 10R has counts between 2 and 10 cps, whereas the lower interval in Cores 11R through 16R show a decrease in NGR counts to 2–5 cps. The NGR signal in both the upper and lower part of the unit is due to products of the ⁴⁰K decay chain, as observed in gamma spectra. The increased ⁴⁰K signal could be due to alteration clays, increased plagioclase abundance, or some combination thereof. The basaltic transition (see "Igneous petrology") in Subunit IIb has a uniform NGR signal with an average of 3.46 ± 2.47 cps (2σ). The low NGR counts continue through the underlying Subunit IIc, consisting of pillow basalts (see "Igneous petrology") and displaying an average of 2.51 ± 1.62 cps (2σ).

The hyaloclastic material in Unit III (see "Igneous petrology") has a slightly increased NGR signal, averaging 3.92 ± 2.4 cps (2σ) and showing excursions up to 7 cps. Finally, the lowermost recovered Unit IV of intercalated pillow basalts and limestones yields the highest NGR values in Hole U1350A, with an average of 6.86 ± 4.95 cps (2σ), reaching values of 15 cps. The higher counts are attributed to products of the ⁴⁰K decay chain, and not, for example, uranium derived from the sedimentary material. The increased ⁴⁰K decay chain NGR counts in this lower unit are consistent with indications that this section of the hole is more altered than the overlying material (see "Alteration and metamorphic petrology").

Moisture and density

Table T5 summarizes discrete sample bulk density, dry density, grain density, void ratio, water content, and porosity. Figure F44 shows the downhole variation of bulk density, dry density, and grain density, as well as porosity. Discrete determinations confirm the consistent downhole bulk density inferred from the maximum GRA density measurements (Fig. F43). Discrete measurements of bulk density are also plotted on Figure F43. The average bulk density for the entire hole is 2.61 ± 0.17 g/cm³ (2σ) with averages from all individual units within error of this value. The single limestone discrete sample (Section 324-U1350A-26R-5) has indistinguishable density compared to the surrounding basalts from the same unit.

Porosity varies throughout the hole and is controlled by the vesicularity of the material rather than distinction between stratigraphic units. In general, porosity ranges from 3.43% to 28.45% and shows a good negative correlation with bulk density (Fig. F44). The limestone sample displays half the porosity of the surrounding basalts of Unit IV (8.6% versus 13%–15%).

Compressional (P-wave) velocity

Compressional wave velocities of discrete samples measured in Hole U1350A are shown in Figure F44, and data are listed in Table T6. All samples show no appreciable anisotropy and have an average of 4.793 ± 1.249 km/s (2σ). In analogous fashion to porosity, the P-wave velocity is controlled by amount of vesicles, rather than distinction due to stratigraphic unit, consistent with the low variation of downhole bulk density. The sample set includes one limestone from Unit IV (Section 324-U1350A-26R-5) that has an indistinguishable P-wave velocity (5.585 km/s) from the surrounding basaltic samples taken from the same unit. This value is remarkably high for limestone and might be due to the variable baking the material has undergone (see "Sedimentology").

Figure F45A illustrates the positive correlation between P-wave velocity and bulk density measurements. There is some scatter evident in the figure. In particular, samples from Sections 324-U1350A-16R-1, 26R-1, and 26R-5 lie significantly off the trend. The sample from Section 26R-5 is limestone, so it is not necessarily expected to fall on the same regression as basaltic material. The sample from Section 26R-1 is a mixture of alteration clay and basaltic material, which makes its physical properties distinct from homogeneous igneous material. The sample from Section 16R-1 has an anomalously low density for its P-wave value. Upon examination, the sample was highly vesicular. However, some of the vesicles were in-filled with dark clay material, perhaps allowing for a lower density while maintaining a higher P-wave velocity.

Figure F45B shows the negative correlation between P-wave velocity and porosity. There is one outlier in the figure from Section 16R-1 that has an anomalously high porosity for its measured P-wave velocity. As with the bulk density, this may be related to the abundance of vesicles and their secondary in-filling.

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