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doi:10.2204/iodp.proc.336.104.2012 Physical propertiesThe igneous rock and sediment recovered from Hole U1382A were characterized for physical properties as described in the “Methods” chapter (Expedition 336 Scientists, 2012). All recovered cores were processed with the Whole-Round Multisensor Logger (WRMSL) for measurements of gamma ray attenuation (GRA) density and magnetic susceptibility. All basement rocks were analyzed for NGR total counts and potassium weight percentage. The short sections of sediment recovered in Core 336-U1382A-8R were not processed with the Natural Gamma Radiation Logger (NGRL) because the samples remaining after water and microbiological sampling were too small to produce representative data. All archive core halves were analyzed for color reflectance and point magnetic susceptibility (MSP) on the Section Half Multisensor Logger (SHMSL). Twenty-three discrete cube-shaped samples were cut from the working core halves for P-wave compressional velocity and moisture and density (MAD) measurements. When the piece was oriented, P-wave measurements were taken along each axis. Gamma ray attenuation 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. The raw data from this instrument were filtered to remove underestimated data, as described in the “Methods” chapter (Expedition 336 Scientists, 2012). Hard rock cores generally have a smaller diameter (~58 mm) than the internal diameter of the core liner (66 mm). This discrepancy can be corrected by multiplying the system output by Magnetic susceptibilityMagnetic susceptibility (MS) measurements of whole-round and archive core halves are summarized in Figure F33. The raw data obtained from the WRMSL were filtered using the same criteria as those for GRA filtering because both sensors are installed on the same track and have a similar range of detection. Magnetic susceptibility 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 was 1.012 for igneous sections and 0.687 for sediment sections. 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 SHMSL along with 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, 2012). No correction is needed for MSP measurements. Both whole-round and point magnetic susceptibility measurements present similar peaks, with a maximum (MSP = 3257 × 10–5 SI; MS = 2388 × 10–5 SI) located in the first piece of Section 336-U1382A-6R-3 at ~144.6 mbsf in lithologic Unit 3, which corresponds to a massive lava flow. This aphyric medium-grained basalt piece is the longest piece recovered in Hole U1382A. A secondary maximum (MS = 2010 × 10–5 SI; MSP = 1939 × 10–5 SI) was found at ~117.4 mbsf, corresponding to Section 336-U1382A-3R-3, which presented the same main lithology in lithologic Subunit 1-4. Natural gamma radiationNGR and potassium concentration data obtained from the NGRL are summarized in Figure F34. The count time for each core section (2 h) was maximized to increase the resolution of the data obtained. The total counts per second (cps) represent all of the radioactive elements. 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 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 0.18 to 3.22 cps once the background is subtracted (Fig. F34). The concentration of potassium was calculated by using GRA bulk density as the effective density for the corrections. Shipboard ICP-AES potassium data have values similar to those from NGR data (Fig. F34), with only three outliers (located at 126.20, 153.85, and 172.52 mbsf). The difference between the registries is due to the NGR data being from an average of >46 cm of sample versus the discrete samples for ICP-AES data. The ICP-AES potassium concentration values are in the range of 0.07–0.26 wt%, with an average of 0.17 ± 0.05 wt%. Potassium concentrations range from <0.01 to 0.35 wt%, with an average of 0.16 ± 0.05 wt%. In general, the concentration of uranium in basalt detected by NGR is very small, and 40K is the main contributor to the total counts. However, a strong enrichment in uranium was detected in Sections 336-U1382A-8R-4 and 9R-1, which have low concentrations of potassium (<0.1 wt%). The unusual composition in these sections is related to harzburgite (lithologic Subunit 5-5) that has undergone oxidative seawater alteration of olivine, resulting in U enrichment (e.g., Niu, 2004). In most of the cases where there was core recovery, NGR potassium concentrations are similar to the values obtained by the HNGS logging tool (see Fig. F35). The trend of increasing gamma ray intensities found near the bottom of the hole by the logging tool is not reflected by the NGR data, probably because of the lack of core recovery from the deepest section of the hole. Moisture and densityResults of measurements of bulk density, dry density, grain density, void ratio, and porosity on 23 discrete samples are presented in Table T11. These values were determined using Method C (see “Physical properties” in the “Methods” chapter [Expedition 336 Scientists, 2012]). Samples were chosen to be representative of the different lithologic units and, when possible, were taken in areas adjacent to thin section samples in order to identify possible correlations. The alteration percentages for the adjacent thin sections are indicated in Table T11; when not available, the alteration percentage values of the piece are indicated. These samples generally had smaller volumes because they were not taken from the center of the core but from the side opposite the thin section sample. No samples were taken from the sediment core sections because of drilling disturbance observed on these sections. The average bulk density from discrete samples is 2.87 ± 0.05 g/cm3 (Fig. F32). This value is similar to the average GRA bulk density (2.85 ± 0.17 g/cm3). Dry density values average 2.78 ± 0.06 g/cm3, and grain densities average 2.95 ± 0.05 g/cm3. The mean porosity is 4.05% ± 1.27%, and the void ratio averages 0.04 ± 0.01. Porosity values do not linearly correlate with the percentage of vesicles or alteration estimated visually in core and thin section descriptions (see “Petrology, hard rock and sediment geochemistry, and structural geology” for more detail). Compressional P-wave velocityTwenty-three 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, which can affect the measured P-wave velocity. A summary of these measurements is presented in Figure F36. P-wave velocity correlates well with bulk density determined by MAD measurements (Fig. F37). The mean value for all of the samples and directions is 6199 ± 447 m/s. The mean P-wave velocity values do not show any major trend related to the percentage of alteration or vesicles present in the sample. P-wave velocities inversely correlate with porosity (Fig. F37). P-wave velocity measurements range from 5733 to 7380 m/s along the x-axis, with a mean of 6206 ± 442 m/s. Along the y-axis, measurements range from 5675 to 7436 m/s, with a mean value of 6182 ± 458 m/s. Along the z-axis, P-wave measurements range from 5756 to 7288 m/s, with a mean of 6207 ± 462 m/s. A summary of all averaged values for each sample is presented in Table T11. Only the harzburgite sample from lithologic Subunit 5-5 (Sample 336-U1382A-9R-1W, 102–104 cm) seemed to present some anisotropy (standard deviation = 176 m/s) when P-wave tests were done on its three axes. The basalt with the highest P-wave velocity is Sample 336-U1382A-5R-3W, 29–30 cm, which corresponds to aphyric cryptocrystalline basalt from a pillow flow. Sample 336-U1382A-7R-1W, 123–125 cm, from the same lithology also has an elevated P-wave velocity. Thermal conductivityTwenty-three measurements were taken on archive pieces representative of each section longer than 6 cm (Table T12; Fig. F38). Most of the basalt samples have similar values (1.728 ± 0.077 W/[m·K]). Two samples, located at ~163.91 and ~171.73 mbsf, are peridotites and have markedly different thermal conductivity values (3.384 ± 0.3162 W/[m·K] and 3.386 ± 0.079 W/[m·K], respectively). The higher thermal conductivities of the peridotites are consistent with the olivine-rich nature of these rocks because olivine has a much higher thermal conductivity (5.1 W/[m·K]) (Clauser and Huenges, 1995) than plagioclase (a main constituent of basalt). The uneven distribution of samples in the recovered section is the result of the limited availability of samples of the right size. The thermal conductivity values recorded for Hole U1382A do not present an apparent trend with depth (Fig. F38). Color reflectance spectroscopyColor reflectance L* for Hole U1382A varies between 10.80% and 60.90%, with a mean of 36.06% ± 4.72% (Table T13; Fig. F39). In general, basalt reflectance values are homogeneous, without pronounced differences between cores (Fig. F39). The values of a* and b* close to zero reflect the dark color of the basalt. No obvious trends were detected downhole. 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, 2012]). The main variations in color parameters were observed in lithologic Unit 5 (161.30–173.24 mbsf) because of the more diverse nature of this unit. These variations are particularly easy to see in the tristimulus parameters (Fig. F39). |
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= 1.138 (Jarrard and Kerneklian, 2007). MAD and GRA density measurements give similar values once this correction is done for hard rock sections (Fig.