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doi:10.2204/iodp.proc.330.106.2012

Physical properties

Characterization of physical properties was conducted on cores recovered from Holes U1375A and U1375B at Achernar Guyot. The whole-round core sections (two from Hole U1375A and one from Hole U1375B) were run through the Whole-Round Multisensor Logger (WRMSL) for measurement of gamma ray attenuation (GRA) bulk density and magnetic susceptibility. The three whole-round core sections were also run through the Natural Gamma Radiation Logger (NGRL). The archive-half split-core sections were then run through the Section Half Multisensor Logger (SHMSL) for measurement of laser height, color reflectance, and point magnetic susceptibility. One discrete oriented rock cube was sampled from interval 330-U1375B-1R-1W, 43–45 cm, for compressional wave (P-wave) velocity (in three orthogonal directions), as well as moisture and density measurements. This sample was 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]).

Whole-Round Multisensor Logger measurements

Throughout the lithified sediments and breccia of Hole U1375A and the igneous rocks of Hole U1375B 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 from 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 U1375 are shown in Figure F12. Magnetic susceptibility is sensitive to the mineralogical composition of the rock. In Hole U1375A magnetic susceptibility averages 5.13 × 10^–3 SI. Two intervals of moderately higher values up to 1.10 × 10^–2 SI were observed, and these correspond with the locations of basaltic clasts that appear less altered than elsewhere in the hole. The lithified sediment toward the bottom of the hole between 9.6 and 10 mbsf has the lowest magnetic susceptibility, with an average of 9.6 × 10^–4 SI. In contrast, the igneous microgabbro observed throughout Hole U1375B has a much higher average magnetic susceptibility of 2.30 × 10^–2 SI.

Gamma ray attenuation bulk density

The results of GRA-derived bulk density for the two holes are shown in Figure F12. A correction factor of 1.138 was applied to account for the smaller diameter (58 mm) of hard rock material compared to 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 in Holes U1375A and U1375B are 2.26 g/cm³ and 2.69 g/cm³, respectively. A decrease in GRA-derived bulk density from an average of 2.37 g/cm³ above 9.5 mbsf to 1.90 g/cm³ below was observed in Hole U1375A. This trend is likely a combination of two factors: a downhole-decreasing amount of basalt clasts and the effect of the recovered rock deviating significantly from a cylindrical shape toward the bottom of the hole. These noncylindrical pieces have less volume than is assumed in the density calculation, even after the application of the correction factor for hard rock cores.

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 F12. For Hole U1375A, NGR ranges from 7.18 to 26.00 counts per second (cps), with a trend of decreasing values downhole. This trend likely results from a decrease in basalt clasts but may also be affected by the decreased volume of rocks recovered toward the bottom of the hole. NGR values from Hole U1375B are consistent, averaging 22.13 cps.

Section Half Multisensor Logger measurements

Color reflectance spectrometry

Color reflectance spectrometry results summarized in Figure F12 include L* (lightness of the recovered core) and a* and b* (redness vs. greenness and yellowness vs. blueness, respectively). Cores from both holes generally have more yellow- than blue-colored spectra. Hole U1375A exhibits a distinctly red spectrum, whereas Hole U1375B has neutral to slightly red values.

Point magnetic susceptibility

Point magnetic susceptibility results are shown in Figure F12 along with whole-round magnetic susceptibility data. Both data sets agree well. Hole U1375A has an average point magnetic susceptibility of 2.50 × 10^–3 SI, and Hole U1375B has an average of 2.13 × 10^–2 SI.

Moisture and density

Only one discrete sample was taken from Site U1375 (Sample 330-U1375B-1R-1W, 43–45 cm). This sample represents the microgabbro recovered in Hole U1375B and is characterized by a bulk density of 2.85 g/cm³ and a porosity of 4.4%. The bulk density value is comparable to nearby GRA-derived bulk density measurements, which vary from 2.66 to 2.90 g/cm³. Results of the full moisture and density determinations on this sample, including bulk density, dry density, grain density, void ratio, water content, and porosity, are listed in Table T7.

Compressional wave (P-wave) velocity

Compressional wave (P-wave) velocity was measured on discrete Sample 330-U1375B-1R-1W, 43–45 cm, 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. Measured values are 6.074, 5.954, and 6.095 km/s, respectively, with an average of 6.041 km/s (Table T7).

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