Previous   |    Next

doi:10.2204/iodp.proc.345.110.2014

Physical properties

Physical properties of the gabbroic rock recovered in Hole U1415J 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., 2014e). 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, P-wave velocity (V_P), density, and porosity on discrete samples. Rock names reported in data tables correspond to the primary lithologies assigned by the igneous group (Tables T12, T13). Data are summarized as a function of depth in Figure F104; data from ghost cores (i.e., WRMSL, SHMSL, and only one discrete sample from Core 345-U1415J-7G) are not shown in this figure.

Raw GRA density, magnetic susceptibility, reflectance spectrometry, 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., 2014e) 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 U1415J, 16 of the 30 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 (0–1.53 cps), which is significantly lower than background level (~5 cps). NGR values in ghost cores are in the same range as those in routine RCB cores.

Gamma ray attenuation density

In Hole U1415J, 18 of the 30 core sections were measured in the WRMSL. GRA density measurements are volume dependent, and filtered GRA data range between 1.27 and 2.85 g/cm³, with an average of 2.35 g/cm³ (85% of the values are <2.5 g/cm³ and are not shown in Fig. F104). 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 (16 core sections) and SHMSL (26 core sections). The whole-round core measurements are volume measurements that give an average apparent susceptibility value over an 8 cm long interval, whereas SHMSL values are given by point measurements (see “Physical properties” in the “Methods” chapter [Gillis et al., 2014e]). When measured on whole-round cores, magnetic susceptibility is generally underestimated, with values significantly lower than point magnetic susceptibility (Fig. F104). The mean magnetic susceptibility of rock recovered in Hole U1415J is very low (~750 × 10^–5 ± 1450 × 10^–5 SI for point magnetic susceptibility), reflecting the absence of magmatic Fe-Ti oxides. The only exceptions to this are two intervals (345-U1415J-18R-1, 67–71 cm, and 21R-1, 4–16 cm) that contain Fe oxide–rich material (presumably altered chromitite; see “Igneous petrology”), in which WRMSL magnetic susceptibility has been corrected (see Fig. F20 in the “Methods” chapter [Gillis et al., 2014e]) and is as high as ~14,400 × 10^–5 SI in Section 345-U1415J-18R-1 (Piece 9) and ~25,000 × 10^–5 SI in Section 21R-1 (Piece 2).

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. As described in “Physical properties” in the “Hole U1415I” chapter (Gillis et al., 2014d), absolute values of chromaticity parameters a* and b* are not reliable. The mean value of the lightness (L*) is ~43 ± 8.

Discrete sample measurements

Moisture and density

Bulk density, grain density, and porosity were calculated from measurements on 25 cubic samples (2 cm × 2 cm × 2 cm) taken from the working-half sections (Table T12; Fig. F104). These samples comprise four rock types: gabbronorite, gabbro, olivine gabbro, and troctolite. Average bulk density and grain density are 2.83 ± 0.08 and 2.86 ± 0.07 g/cm³, respectively, and are similar to the densities measured at Hess Deep (Site 894) (Fig. F105). Olivine-rich lithologies (olivine gabbro and troctolite) have average grain densities (2.92 ± 0.04 g/cm³) that are ~0.1 g/cm³ lower than gabbro and gabbronorite (2.81 ± 0.06 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 (Fig. F106; see also “Metamorphic petrology”). Porosity is generally low, ranging from 1.7% to 4.1%, and generally increases downhole (Fig. F104).

P-wave velocity

The same 25 cubic samples used for moisture and density analyses were measured for 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., 2014e]). Results are listed in Table T12 and plotted in Figures F104 and F105. Average V_P is 6.12 ± 0.38 km/s, and the apparent anisotropy varies from 0.8% to 5.5%. As detailed in “P-wave velocity” 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.

Results for all samples from Site U1415 measured so far (Holes U1415I and U1415J) are compared in Fig. F105, with V_P and grain density measurements 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, which probably cannot be solely explained by petrophysical variations (note that, for example, the ~1 to 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.

As expected, measured V_P at room pressure depends primarily on porosity (Fig. F107). Because we avoided taking discrete samples with metamorphic or alteration veins, the progressive increase of porosity downhole (Fig. F104) primarily reflects increasing fracturing in the sample groundmass, as is also suggested by the general correlation between V_P/porosity and the macroscopically estimated intensity of cataclastic deformation (Fig. F108).

Thermal conductivity

Thermal conductivity was measured in 11 gabbroic rock samples (≥8 cm long and representative of the various recovered lithologies) taken at irregularly spaced intervals in Hole U1415J (Table T13; Fig. F104). Measured values range from 2.1 to 4.1 W/(m·K) and are averages of 6–20 measurements for each piece, with a standard deviation <2.8% (average = 0.9%). The relatively high thermal conductivity values measured in two troctolite samples from the bottom of the hole (3.4 and 4.1 W/[m·K]) might be explained by the presence of relatively abundant magnetite (~5.3 W/[m·K]) in altered olivine.

We attempted to measure anisotropy in a foliated olivine-bearing gabbronorite and a foliated troctolite (Sections 345-U1415J-5R-2A [Piece 2] and 18R-1A [Piece 14]) by using the shorter probe (see “Physical properties” in the “Methods” chapter (Gillis et al., 2014e), collecting two series of measurements with the probe needle aligned parallel and perpendicular to the foliation. The apparent anisotropies are 3.6% in the olivine-bearing gabbronorite and 0.1% in the troctolite. These values are lower than or close to the measurement standard deviations (Table T13) and are probably not meaningful. The small probe tends to return less stable values than the large probe, which makes the exercise of estimating the anisotropy of thermal conductivity difficult in this type of lithology.

Top of page   |    Previous   |    Next