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doi:10.2204/iodp.proc.330.108.2012

Physical properties

Characterization of physical properties was conducted on cores recovered from Holes U1377A and U1377B at Hadar Guyot. The whole-round core sections (10 from Hole U1377A and 15 from Hole U1377B, not including sections from ghost Core 330-U1377B-3G) were run through the Whole-Round Multisensor Logger (WRMSL) for measurements of gamma ray attenuation (GRA) bulk density and magnetic susceptibility. Whole-round core sections longer than 50 cm (21 of 25 total sections) were also run through the Natural Gamma Radiation Logger (NGRL). The archive-half split-core sections were run through the Section Half Multisensor Logger (SHMSL) for measurement of laser height, color reflectance, and point magnetic susceptibility. Moisture and density measurements were performed on six discrete samples from Hole U1377A only. Compressional wave (P-wave) velocity was measured in three orthogonal directions on 13 discrete oriented rock cubes, including seven samples selected from Hole U1377B. These samples were also used for paleomagnetic measurements of alternating-field demagnetization (see “Paleomagnetism”). Because of equipment failure, no measurements of thermal conductivity were made at this site (see “Physical properties” in the “Methods” chapter [Expedition 330 Scientists, 2012a]). No physical property measurements were made on ghost Core 330-U1377B-3G (see “Operations”).

Whole-Round Multisensor Logger measurements

Throughout the lithified sediments and igneous rocks of Holes U1377A and U1377B the core is fractured and broken, as is typical of hard rock coring. These discontinuities in the recovered core led to spurious values in the data collected with the WRMSL and SHMSL, so we applied a data filtering and processing algorithm to remove the affected data (see “Physical properties” in the “Methods” chapter [Expedition 330 Scientists, 2012a]). In this report we show only the filtered data; for raw data we refer the reader to the visual core descriptions (see “Core descriptions”) and the Laboratory Information Management System (LIMS) database (iodp.tamu.edu/tasapps/).

Magnetic susceptibility

Whole-round magnetic susceptibility measurements for the two holes from Site U1377 are shown in Figure F25. Magnetic susceptibility is sensitive to the mineralogical composition of the rock. Hole U1377A magnetic susceptibility averages 8.42 × 10^–3 SI, and Hole U1377B averages 3.75 × 10^–3 SI. Although magnetic susceptibility is highly variable throughout both holes, three intervals of markedly higher values exceeding 2.50 × 10^–2 SI were observed: 25–27 mbsf and 35–36.5 mbsf for Hole U1377A and 12–12.5 mbsf for Hole U1377B. All of these intervals are associated with aphyric trachybasalts (see “Igneous petrology and volcanology”). No distinct patterns in other types of physical property data were observed at these specific depths.

Gamma ray attenuation bulk density

The results of GRA-derived bulk density for the two holes are shown in Figure F25. A correction factor of 1.138 was applied to account for the average diameter of the hard rock cores (58 mm) being less than the full 66 mm diameter of the core liner (see “Physical properties” in the “Methods” chapter [Expedition 330 Scientists, 2012a]). Values of <1.00 g/cm³ were attributed to empty portions of core liner and removed. The average GRA-derived bulk densities of Holes U1377A and U1377B are 2.24 g/cm³ and 2.33 g/cm³, respectively.

For Hole U1377A, average GRA-derived bulk density increases from 1.90 g/cm³ at 15.3–16.7 mbsf to 2.41 g/cm³ at 43.9–47.2 mbsf, near the bottom of the hole. Two high values averaging 2.60 g/cm³ found at ~6.2 mbsf are likely associated with a piece of limestone with ferromanganese encrustation on its top recovered in Section 330-U1377A-2R-1.

The GRA-derived bulk density profile for Hole U1377B is characterized by relatively low values near the top, representing foraminiferal ooze, and an interval from 19.5 to 21.3 mbsf with an average value of 2.72 g/cm³, an increase over the background average of 2.40 g/cm³ for igneous sections recovered deeper than 9 mbsf. This group of high density measurements corresponds with the olivine-plagioclase-phyric trachybasalt of lithologic Unit 2. The overall density of igneous units is lower than that observed at earlier Expedition 330 sites, possibly reflecting the increased silica content of these magmas (see “Geochemistry”).

Natural Gamma Radiation Logger

Natural gamma radiation (NGR) measurements reflect the amount of uranium, thorium, and potassium present in the rock. Results from the NGRL are shown in Figure F25. Low NGR was measured in the foraminiferal ooze of Hole U1377B, as would be expected for calcareous sand. However, NGR for igneous rocks from both Holes U1377A and U1377B is quite high relative to previous Expedition 330 sites, with averages of 20.00 counts per second (cps) and 22.58 cps, respectively. For Hole U1377A, NGR ranges from 7.36 to 42.17 cps, with the highest values appearing at 45 mbsf, corresponding with the olivine-phyric trachybasalt of lithologic Units 4–6 (see “Igneous petrology and volcanology”). The igneous rocks of Hole U1377B exhibit a similar NGR range of 3.90–45.52 cps. High NGR (>30 cps) was measured at ~18.5 mbsf, 21.4–24.3 mbsf, and 28.0–28.7 mbsf. These large values may reflect increased alteration compared to other sites or could be related to a more evolved magma composition (see “Geochemistry”).

Section Half Multisensor Logger measurements

Color reflectance spectrometry

Color reflectance spectrometry results showing values of L*, which corresponds to lightness, and a* and b*, which correspond to redness versus greenness and yellowness versus blueness, respectively, are summarized in Figure F26. The uppermost data points for L* in Hole U1377A (6.12–6.19 mbsf) exhibit considerable variability over this narrow interval, with some values of >70. These high values correspond to the small piece of limestone encrusted with ferromanganese oxides recovered in Section 330-U1377A-2R-1. In the igneous basement of Hole U1377A the majority of L* values fall between 25 and 50, with a slight increase downhole. L* values from Hole U1377B have a similar pattern, with high values around 75 at the top, corresponding to an interval of light-colored foraminiferal ooze, and most of the remaining values in the igneous basement falling between 25 and 50, with no significant downhole trend.

Values of a* for Hole U1377A indicate a tendency toward a red spectrum above ~17 mbsf but are generally neutral in the rest of the hole. Color reflectance values for b* are also quite neutral for Hole U1377A, except for the interval from 15 to 17 mbsf, which shows a moderately yellow spectrum. Values of both a* and b* are consistently positive for Hole U1377B, indicating a strongly red and yellow color reflectance spectrum. This strong redness and yellowness is likely related to intense alteration by oxidation apparent along cracks and veins and throughout much of the rock recovered from this hole (see “Alteration petrology”).

Point magnetic susceptibility

Point magnetic susceptibility results are shown in Figure F25, along with whole-round magnetic susceptibility data. The pattern of peaks and troughs is consistent between the two data sets; however, point magnetic susceptibility has a consistently lower average than whole-round magnetic susceptibility. Hole U1377A has an average point magnetic susceptibility of 5.76 × 10^–3 SI, and Hole U1377B has an average of 2.31 × 10^–3 SI.

Moisture and density

Results of bulk density, dry density, grain density, void ratio, water content, and porosity measurements on discrete samples from Hole U1377A are listed in Table T8. Moisture and density measurements on discrete samples were not performed for Hole U1377B because time constraints at the end of the expedition did not allow for the ~50 h measurement procedure. Bulk density in Hole U1377A ranges from 2.19 to 2.53 g/cm³, with an average of 2.43 g/cm³. Porosity ranges from 16.7% to 33.2%, with an average of 23.5%. Figure F25 shows the variation of bulk density with depth based on both discrete samples (Hole U1377A only) and GRA-derived bulk density and further illustrates the strong correlation between the two. Figure F25 shows the variation of porosity with depth for Hole U1377A. Density and porosity are strongly correlated, suggesting that changes in porosity, rather than in composition, are the primary cause of the variations in density.

Compressional wave (P-wave) velocity

Compressional wave (P-wave) velocity was measured on discrete samples in three orthogonal directions: along the x-axis normal to the split-core surface, along the y-axis parallel to the split-core surface, and along the z-axis downcore. P-wave velocity measurements were performed for Hole U1377A on the same discrete samples used for moisture and density testing. For Hole U1377B, discrete oriented rock cubes were selected in collaboration with the paleomagnetism laboratory group, with P-wave testing to be performed following alternating-field demagnetization. Downhole variations in P-wave velocity for Holes U1377A and U1377B are shown in Figure F25 and Table T8. Measured values for Hole U1377A range from 3.22 to 4.39 km/s (average = 3.87 km/s), and those for Hole U1377B range from 3.21 to 4.71 km/s (average = 4.06 km/s). P-wave velocities are generally consistent throughout both holes. The values are typically lower than those observed in lavas at previous sites, possibly reflecting the more evolved composition of these magmas or increased alteration (see “Geochemistry”).

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