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doi:10.2204/iodp.proc.345.113.2014

Physical properties

Physical properties of the gabbroic rock pieces recovered in Hole U1415P were characterized through a series of measurements on whole-core sections, half-core sections, half-core pieces, and discrete samples as described in “Physical properties” in the “Methods” chapter (Gillis et al., 2014f). We measured gamma ray attenuation (GRA) density and magnetic susceptibility on the Whole-Round Multisensor Logger (WRMSL); natural gamma radiation (NGR) on the Natural Gamma Ray Logger (NGRL); point magnetic susceptibility, reflectance spectroscopy, and colorimetry on the SHMSL; and thermal conductivity, compressional wave velocity, density, and porosity on discrete samples. The rock names reported in data tables correspond to the primary lithologies assigned by the igneous group (Tables T11, T12). Data are summarized as a function of depth in Figure F64. Discrete sample data from ghost Core 345-U1415P-4G are shown in Figure F64 together with the other data from routine RCB cores because the corresponding interval is well constrained (12.4–19.5 mbsf); WRMSL and SHMSL data from ghost cores are not shown in this figure.

Raw GRA density, magnetic susceptibility, reflectance spectrophotometry, and colorimetry data were uploaded to the Laboratory Information Management System database and subsequently filtered following the procedures described in “Physical properties” in the “Methods” chapter (Gillis et al., 2014f) to remove spurious points that correspond to empty intervals in the liner, broken pieces, and pieces that were too small. Both raw and filtered data are provided in PHYSPROP in “Supplementary material.”

Multisensor core logger data

Natural gamma radiation

In Hole U1415P, 29 of 38 core sections were measured on the NGRL; other sections contained pieces too small to provide reliable data with this instrument. NGR is, overall, very low (0–4.3 cps) and is generally significantly lower than background level (~5 cps), except in Core 345-U1415P-23R where two intervals return significantly higher counts (3.6 cps in Sample 345-U1415P-23R-1, 8–22 cm [Piece 2], and 4.3 cps in Sample 23R-2, 37–50 cm [Piece 4]) (Fig. F64). NGR values in ghost Core 345-U1415P-4G are in the same range as those in routine RCB cores (0.15–1.4 cps).

Gamma ray attenuation density

In Hole U1415P, 33 of 38 core sections were measured on the WRMSL. GRA density measurements are volume dependent, and filtered data range between 1.46 and 2.74 g/cm³, with an average of 2.39 g/cm³ (94% of the values are <2.5 g/cm³ and are not shown in Fig. F64). These values are generally significantly lower than bulk density measured on discrete samples in the same cores.

Magnetic susceptibility

Magnetic susceptibility was measured on both the WRMSL (33 core sections) and SHMSL (35 core sections). The whole-round core measurements are volume measurements that give an average apparent susceptibility value over an 8 cm long interval, whereas the SHMSL values are given by point measurements (see “Physical properties” in the “Methods” chapter [Gillis et al., 2014f]). When measured on whole-round cores, magnetic susceptibility is generally underestimated, with values significantly lower than point magnetic susceptibility (Fig. F64). The mean magnetic susceptibility of rocks recovered in Hole U1415P is generally low (~1350 × 10^–5 ± 1330 × 10^–5 SI for point magnetic susceptibility), reflecting the absence of magmatic Fe-Ti oxides.

Reflectance spectroscopy and colorimetry

Reflectance spectroscopy and colorimetry data were systematically acquired, together with point magnetic susceptibility data, using the SHMSL with a measurement interval of 1 cm. The mean values of reflectance and chromaticity parameters a*, b* and L* are 0.6 ± 0.5, –6.6 ± 1.4, and ~40.7 ± 9.2, respectively.

Discrete sample measurements

Moisture and density

Bulk density, grain density, and porosity were calculated from measurements on 29 cubic samples (2 cm × 2 cm × 2 cm) taken from the working-half sections (Table T11; Fig. F64). These samples comprise two rock types, olivine gabbro (including some orthopyroxene-bearing intervals) and troctolite. The average bulk density and grain density are 2.82 ± 0.06 and 2.84 ± 0.06 g/cm³, respectively (Table T12), and are similar to densities measured at Hess Deep (Ocean Drilling Program [ODP] Leg 147 Site 894) (Fig. F65). Troctolite has average grain densities (2.76 ± 0.03 g/cm³) that are ~0.1 g/cm³ lower than those of olivine gabbro (2.86 ± 0.05 g/cm³). This difference reflects the average differences in background alteration for these lithologies, which is primarily related to olivine contents because olivine is generally more strongly altered than plagioclase and clinopyroxene in olivine-rich rocks (Fig. F66; see also “Metamorphic petrology”). Porosity is generally very low, ranging from 0.4% to 1.8% (Fig. F64), reflecting the low degree of brittle deformation in these rocks (see “Structural geology”).

P-wave velocity

The same 29 cubic samples used for moisture and density analyses were measured for P-wave velocities (V_P) along the three principal directions (x, y, and z) in the core reference frame (see Fig. F2 in the “Methods” chapter [Gillis et al., 2014f]). Results are listed in Tables T11 and T12 and plotted in Figures F64 and F65. Average V_P is 6.25 ± 0.13 km/s, and the apparent anisotropy varies from 0.7% to 5.9%. As detailed in “Physical properties” in the “Methods” chapter (Gillis et al., 2014f) and “Physical properties” in the “Hole U1415I” chapter (Gillis et al., 2014d), the precision of our V_P measurements is ~2%. Hence, the relatively low measured apparent anisotropies should be treated with caution.

In Figure F65, results for V_P and grain density measurements on samples at Site U1415 are compared with those made during previous ODP legs and IODP expeditions on gabbroic samples from fast-spreading and slow-spreading oceanic crust. V_P values are consistent with measurements made at Hess Deep (Site 894). V_P measurements made on board over time show a large dispersion (Fig. F65), which probably cannot be solely explained by petrophysical variations (note, for example, the ~1–1.5 km/s difference in velocity between Hole 735B data and data from other slow-spreading crust locations even though they have similar composition, porosity, and alteration). These data should therefore be treated with caution.

Although the range of measured porosities is small, measured velocities at room pressure show a general inverse correlation with porosity (Fig. F67).

Thermal conductivity

Thermal conductivity was measured in 20 gabbroic rock samples (≥8 cm long and representative of the various recovered lithologies) taken at irregularly spaced intervals downhole in Hole U1415P (Tables T12, T13; Fig. F64). Measured values range from 2.10 to 3.13 W/(m·K) and are averages of 8–10 measurements for each piece, with a standard deviation <1.6%. Overall, thermal conductivity values measured during Expedition 345 on samples from Holes U1415I, U1415J, and U1415P are of very good quality, with a standard deviation of measurement on a single piece that is generally significantly lower than 2%. These values are in agreement with thermal conductivities measured in similar lithologies during previous ODP legs and IODP expeditions (Fig. F68).

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