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doi:10.2204/iodp.proc.336.105.2012

Physical properties

The igneous rocks recovered from Hole U1383C were characterized for physical properties as described in the “Methods” chapter (Expedition 336 Scientists, 2012a). All recovered cores were processed with the Whole-Round Multisensor Logger (WRMSL) for measurements of gamma ray attenuation (GRA) density and magnetic susceptibility (MS). All basement rocks were analyzed for NGR total counts to derive potassium weight percentage. Cores 336-U1383C-15R and 18R were shorter than 50 cm, so no NGR measurements were performed on them. All archive core halves were analyzed for color reflectance and point magnetic susceptibility (MSP).

Twenty-three discrete cube-shaped samples were cut from the working core halves for P-wave compressional velocities and moisture and density (MAD) measurements. Discrete samples were taken from sections of the core showing marked differences in alteration intensity. For each sample, we tried to take subsamples with uniform alteration; fractures and veins were avoided as much as possible. For each main lithology, a sample of altered rock and an unaltered specimen were taken, when available.

Gamma ray attenuation and bulk density

The WRMSL was used to measure magnetic susceptibility and bulk density on the whole-round cores. The GRA measurements are sensitive to gaps and cracks in the material, as well as to underfilled liners and unsaturated samples. Raw data from the WRMSL were filtered to remove underestimated data as described in the “Methods” chapter (Expedition 336 Scientists, 2012a). Briefly, the filter eliminates measurements with a variation in density >0.15 g/cm³ in four consecutive measurements. Hard rock cores generally have a smaller diameter (~58 mm) than the internal diameter of the core liner (66 mm) considered by the instrument. This discrepancy can be corrected by multiplying the system output by = 1.138 (Jarrard and Kerneklian, 2007). The pieces recovered from Hole U1383C were generally smaller in diameter than the ones recovered from Hole U1382A. This is reflected in the reduced number of data points left after filtering. The basalt recovered from Hole U1383C also exhibits a higher extent and more variable alteration than basalt from Hole U1382A. These differences are reflected in the GRA measurements, which present a larger range than those for Hole U1382A. The GRA bulk density values range from 1.55 to 3.15 g/cm³, however, both extremes of this range should be used with caution because values for rocks having a diameter smaller than 58 mm are underestimated and those for rocks with an uncommonly large diameter may be overestimated. All MAD bulk density values lie within the range obtained by GRA density (Fig. F30). The average value obtained for GRA bulk density is 2.71 ± 0.02 g/cm³.

Magnetic susceptibility

The raw MS data obtained from the WRMSL were filtered on the basis of the GRA data because both sensors are installed on the same track and have a similar range of detection (Fig. F31). The filtering helps ensure that unrepresentative values from piece ends and cracks are eliminated from the final data. MS data were also corrected for the diameter of the core (58 mm for hard rock and 66 mm for sediment) based on the equation given by the manufacturer (see MS2 Magnetic Susceptibility System operation manual, www.bartington.com/​operation-manuals.html). The correction factor applied for the igneous sections was 1.012. However, this measurement assumes that the core liner is filled and does not take into consideration the gaps between samples; because this assumption is rarely correct for hard rock cores, the values measured need to be considered as minimums.

The MSP sensor is installed on the same track as the color reflectance spectroscopy sensor, and MSP data were filtered using the same criteria as those for color reflectance as explained in the “Methods” chapter (Expedition 336 Scientists, 2012a). No correction was applied after filtering the measured data. Both whole-round and point magnetic susceptibility measurements present similar peaks, with a maximum (MSP = 1391 × 10^–5 SI, MS = 905 × 10^–5 SI) located at ~145.4 mbsf in lithologic Unit 2, corresponding to a massive basalt flow (Fig. F31). A second and subordinate peak (MS = 669 × 10^–5 SI, MSP = 980 × 10^–5 SI) was found at ~139.4 mbsf, corresponding to another basalt flow in lithologic Subunit 2-1. Numerous smaller peaks were identified on the basis of MSP data. The magnetic susceptibility sensor in the whole-round track did not detect all of these peaks.

Natural gamma radiation

NGR total counts and potassium concentration data obtained from the Natural Gamma Radiation Logger (NGRL) are summarized in Figure F32. Count time for each core section (2 h) was maximized to increase the resolution of the data obtained. The total counts per second represent all of the radioactive elements detected by the system. Spectral data were recorded, and the counts per second for each channel indicate the abundance of a particular element. When these values are related to a radioactive standard with known composition, the concentration of the element in the core section can be calculated. The total counts from the whole rounds range from 0.65 to 4.63 cps once the background is subtracted (Fig. F32). Potassium concentration was calculated by using GRA bulk density as the effective density for the corrections.

Shipboard analyses of bulk rock composition by inductively coupled plasma–atomic emission spectrometry (ICP-AES) obtained potassium concentrations similar to the NGR data collected (Fig. F32). NGR potassium concentrations are in the range of 0.1–0.46 wt%, with an average of 0.19 ± 0.05 wt%. Potassium concentrations from ICP-AES range from 0.11 to 0.25 wt%, with an average of 0.16 ± 0.05 wt%. These results document a marked variability in potassium concentrations for all samples from Hole U1383C. In general, uranium and thorium concentrations detected by NGR are very low in basalt (Fig. F32), but they are higher than those detected in rocks from Hole U1382A. The main contributor to total gamma ray counts is ⁴⁰K.

NGR potassium concentrations and total counts per second do not show any major trends or differences with depth. This was also observed by the downhole logging tools (Fig. F33). However, potassium concentrations measured in the cores are lower than those obtained from downhole logging.

Moisture and density

Results of measurements of bulk density, dry density, grain density, void ratio, and porosity on 23 discrete samples are presented in Table T9. These values were determined using Method C (see “Physical properties” in the “Methods” chapter [Expedition 336 Scientists, 2012a]). Samples were chosen to be representative of the different levels of alteration in the lithologic units. Thin sections were taken in areas adjacent to the areas used for physical properties measurements in order to identify relationships with petrographic properties. The extent of alteration estimated for thin sections (thin section number indicated, see “Petrology, alteration, structural geology, and hard rock geochemistry” in the “Methods” chapter [Expedition 336 Scientists, 2012a] for more information) from adjacent pieces is presented in Table T9. This wide range is in agreement with the large spread in density data. Average bulk density from discrete samples ranges from 2.43 to 2.90 g/cm³ (Fig. F30). Dry density values range from 2.26 to 2.87 g/cm³, and grain densities vary between 2.71 and 2.96 g/cm³. Porosity ranges from 1.87% for to 16.61%, and the void ratio varies between 0.02 and 0.20. Strong correlations were found between porosity and bulk density and P-wave velocity (Fig. F34). The relationships between porosity, alteration intensity, and LOI data are shown in Figure F35. The presence of particular secondary minerals (e.g., carbonates) may explain some of the scatter in the relationship between porosity and LOI and alteration degree, but further studies are needed to resolve this. The percentage of vesicles in basalt from Hole U1383C is low and is not considered to play a major role on porosity for these rocks (see “Petrology and hard rock geochemistry” for values on vesicularity). The MAD values obtained are within the range of values found in the literature for basalt from the Mid-Atlantic Ridge near 37°N (Hyndman and Drury, 1976).

Compressional P-wave velocity

All of the discrete samples were used to determine the compressional P-wave velocity of the main lithologic units. P-wave velocities were measured along the three axes (x, y, and z) in oriented cube-shaped samples. In nonoriented samples, only x-axis velocities were measured. All samples were measured three times. Measurements were done as quickly as possible for each saturated sample to avoid desiccation of the sample, which would affect the measured P-wave velocity. A summary of these measurements is presented in Figure F36. P-wave velocity shows a good direct correlation with bulk density and an inverse correlation with porosity determined by MAD measurements (Fig. F34).

The mean value for P-wave velocity measurements ranges from 4741 to 7631 m/s along the x-axis. Along the y-axis, the measurements range from 4749 to 7064 m/s, and along the z-axis, the P-wave measurements range from 4825 to 6983 m/s. A summary of all averaged values for each sample is presented in Table T9. The mean P-wave velocity values correlate strongly with porosity and bulk density values (Fig. F34). Low values seem to be consistent with an elevated degree of alteration of the samples. The values obtained for P-wave velocities of Hole U1383C fresh basalt samples are within the range of values found in the literature for basalt of the Mid-Atlantic Ridge (Hyndman and Drury, 1976; Miller and Christensen, 1997; Johnson and Semyan, 1994).

Two samples (336-U1383C-10R-2W, 10–12 cm, and 28R-1W, 40–42 cm) seem to present some anisotropy (standard deviation = 498 and 119 m/s, respectively) when P-wave tests were done on three axes. These samples correspond to avesicular plagioclase-olivine-phyric basalt and aphyric basalt, respectively.

Thermal conductivity

Thirty measurements were taken on archive pieces longer than 6 cm (Fig. F37). The uneven distribution of samples within the recovered section is the result of the limited availability of samples of the right size. Most of the samples have similar values (1.680 ± 0.056 W/[m·K]). Three samples, located at ~78.11, ~87.65, and ~313.15 mbsf, have lower thermal conductivity values (1.410 ± 0.004 W/[m·K] for the first two depths and 1.435 ± 0.003 W/[m·K] for the last depth). The samples with lower thermal conductivity correspond to pieces with high presence of veins.

Color reflectance spectroscopy

Color reflectance L* for Hole U1383C varies between 11.90% and 66.50%, with a mean of 37.48% ± 4.49% (Table T10; Fig. F38). In general, basalt reflectance values are homogeneous, without pronounced differences between cores (Fig. F38). The values of a* and b* close to zero reflect the dark color of the basalt. The higher values obtained for L* (>50) and the outliers represented in the tristimulus scale indicate the presence of sedimentary breccia. Color reflectance was not measured in small pieces and fragments because of the lack of flat surfaces (see “Physical properties” in the “Methods” chapter [Expedition 336 Scientists, 2012a]). Altered pieces have positive b* values, contrasting with the negative b* values obtained in fresh basalt pieces (Fig. F39).

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