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doi:10.2204/iodp.proc.330.103.2012

Physical properties

Characterization of physical properties was conducted on samples from Hole U1372A. All whole-round core sections were run through the Whole-Round Multisensor Logger (WRMSL) for gamma ray attenuation (GRA) bulk density and magnetic susceptibility. Whole-round core sections longer than 50 cm (119 of 125 available sections) were also run through the Natural Gamma Radiation Logger (NGRL). All 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. From the working half of the cores, 83 discrete oriented rock cubes were cut for compressional wave (P-wave) velocity measurements (in three orthogonal directions), as well as moisture and density measurements. Most of these discrete samples were also used for paleomagnetic measurements of alternating-field demagnetization (see “Paleomagnetism”). Nondestructive measurements of thermal conductivity were made at 25 representative locations along the working half of the split core. Generally, all physical property data sets are mutually consistent and show clear trends correlating with the stratigraphic units defined for Hole U1372A on the basis of petrographic descriptions (see “Igneous petrology and volcanology”) and with alteration trends (see “Alteration petrology”).

Whole-Round Multisensor Logger measurements

Throughout the lithified sediment and igneous basement of Site U1372, the core is fractured and broken, as is typical of hard rock coring. As a result, individual sections in most cases contain multiple discrete pieces, and gaps between sections and edge effects near piece boundaries led to spurious values in the data collected from the WRMSL and SHMSL. In order to remove the affected data, we applied a data filtering and processing algorithm (see “Physical properties” in the “Methods” chapter [Expedition 330 Scientists, 2012]). 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 LIMS database (iodp.tamu.edu/tasapps/).

Magnetic susceptibility

Whole-round magnetic susceptibility measurements are shown in Figure F54. Magnetic susceptibility is sensitive to the mineralogical composition of the rock. Magnetic susceptibility is low in the uppermost ~13.5 m of core associated with the sandy foraminiferal ooze of stratigraphic Unit I, with an average value of 2.03 × 10^–3 SI. This value is an order of magnitude higher than that typical of pelagic foraminiferal ooze, probably because of volcanic glass and lithic fragments in the sediment (see “Sedimentology”), reflecting the depositional environment on the volcano. Magnetic susceptibility increases to an average of 1.13 × 10^–2 SI in the basalt conglomerate of stratigraphic Unit II. Magnetic susceptibility is also high in the deeper igneous basement (Units III–XVII) but varies between different lithologic units, with the strongest contrast being between basalt and hyaloclastite. At depths of ~130 mbsf, magnetic susceptibility drops considerably, with very low values in the hyaloclastite-dominated units (Units IX and XI–XVI). Average magnetic susceptibility in these volcaniclastic units is 2.33 × 10^–3 SI, in contrast to an average of 1.23 × 10^–2 SI in the units dominated by lava flows (Units III–VIII, X, and XVII).

Gamma ray attenuation bulk density

The results of GRA-derived bulk density are shown in Figure F55. A correction factor of 1.138 was applied to the hard rock cores (330-U1372A-4R through 38R) to account for their smaller diameter (58 mm) (see “Physical properties” in the “Methods” chapter [Expedition 330 Scientists, 2012]). Values of <1.00 g/cm³ were attributed to empty portions of core liner and removed. Bulk density ranges from 1.00 to 3.07 g/cm³, with an average value of 2.32 g/cm³. Density, however, is consistently 2.3–2.6 g/cm³ throughout the majority of the cores. Notable exceptions are in the sandy ooze of Unit I, which has an average density of 1.65 g/cm³, and the hyaloclastite-dominated volcaniclastic units, particularly Units XII and XV, which both have an average density of 2.04 g/cm³.

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 F56. NGR ranges from 0.55 to 27.46 counts per second (cps), with an average of 13.18 cps. A sharp increase from 1–2 to 15–20 cps occurs at ~12 mbsf near the base of Unit I, suggesting that there is very little U, Th, or K in the foraminiferal ooze. Other notable trends are seen in the hyaloclastite-dominated units. Unit XVI has the lowest average count of the igneous basement rocks (6.64 cps) and is overlain by Unit XV, which has an average of 14.03 cps, with a distinct uphole-increasing trend. Unit XIV then decreases to an average of 13.08 cps, with no apparent trend in the unit itself. These differences are likely due to evolving compositions during the emplacement of these volcaniclastic sequences.

Section Half Multisensor Logger measurements

Color reflectance spectrometry

Color reflectance spectrometry results are summarized in Figure F43. The L* (lightness) of the recovered core averages 40.4, with a slight downhole decrease starting at ~90 mbsf. The hyaloclastites of Units XII and XV are darkest and are delimited by marked L* lows (averages of 33.5 and 35.0, respectively). A short interval of higher L* at 181 mbsf corresponds to a flow lobe of basalt in the hyaloclastite.

Figure F43 also shows values of a* and b*, which correspond to redness versus greenness and yellowness versus blueness, respectively. At 95–100 mbsf the core changes markedly from a predominantly red spectrum (a* > 0) to a predominantly green spectrum (a* < 0) downhole. This change correlates well with the change in alteration color from red to green, likely representing the transition through time from a submarine reducing environment to a subaerial or shallow-marine oxidizing environment (see “Alteration petrology”). The sandy foraminiferal ooze (Unit I) shows a strong yellow spectrum (b* > 0), but within the conglomerate, basalts, and hyaloclastites variations in b* are relatively small. Values of b* gradually decrease in yellowness with depth, with the exception of strongly positive (yellow) b* values in hyaloclastite-dominated Unit XV, which correlate well with observations of brown alteration (see Fig. F35).

Point magnetic susceptibility

Point magnetic susceptibility results are shown in Figure F54 together with whole-round magnetic susceptibility data. Both data sets agree well; point magnetic susceptibility averages 1.68 × 10^–3 SI in the foraminiferal ooze (Unit I), 8.26 × 10^–3 SI in the conglomerate (Unit II), 8.83 × 10^–3 SI in the lava flow units (Units III–VIII, X, and XVII), and 2.19 × 10^–3 SI in the volcaniclastic units (Units IX and XI–XVI).

Moisture and density

Results of bulk density, dry density, grain density, void ratio, water content, and porosity measurements on discrete samples are listed in Table T10. Bulk density ranges from 1.94 to 3.07 g/cm³, with an average of 2.49 g/cm³. Porosity ranges from 0.52% to 51.7%, with an average of 19.8%. As illustrated in Figure F57, a strongly linear negative correlation between bulk density and porosity was observed. Bulk density measurements from discrete samples also agree well with GRA-derived bulk density measurements, as shown by Figure F58. The near one-to-one linear relationship between the two supports our 1.138 volume correction for GRA-derived bulk density. GRA-derived bulk density values may be affected by the presence of fractures and cracks in the whole-round cores, decreasing core radius as the drill bit wears down, and distortions of the core’s cylindrical shape. These factors can cause overestimates of the total volume used in the GRA-derived bulk density calculations even after the correction factor is applied, thus explaining why some GRA-derived bulk densities remain slightly lower than the corresponding results from discrete samples.

Figure F55 shows the variation of bulk density with depth based on both discrete samples and GRA-derived bulk density and further illustrates the strong correlation between the two. The hyaloclastite-dominated Units XII and XV are characterized by low bulk density, having averages of 2.19 and 2.18 g/cm³, respectively. Discrete samples for moisture and density Method C measurements were not taken in Unit I because of severe drilling disturbance.

The percent porosity measured in the discrete samples also changes distinctly with depth (Fig. F59). These changes correlate with stratigraphic units and changes in other physical properties. In general, porosity is low in Units II–X (above the hyaloclastite-dominated units), with an average of 14.4% and localized increases as high as 43.1%. In contrast, background porosity in Units XI–XVI is generally high (average = 27.9%), although less porous lava flows with porosities of 3.2%–9.6% occur occasionally (e.g., Samples 330-U1372A-22R-2W, 108–110 cm, and 17R-2W, 106–108 cm). Discrete samples from the hyaloclastite-dominated Units XII and XV have consistently high porosities, averaging 29.9% and 34.5%, respectively, which explains their characteristically low bulk density values.

Compressional wave (P-wave) velocity

The measured P-wave velocity of discrete samples shows a strong linear relationship with bulk density (Fig. F60). Downhole variations in P-wave velocity are shown in Figure F55 and Table T11. P-wave velocities range from 2.24 to 7.05 km/s, with an average of 4.55 km/s. In Unit II, P-wave velocities are widely scattered, ranging from 3.37 to 6.46 km/s, reflecting the lithologic variations between the matrix and different basaltic clast types. After the transition to igneous basement, the velocities are then consistently high, averaging 5.39 km/s throughout Units III–V. For Units VI–XVI, P-wave velocities are characterized by two distinct populations: one with an average of 3.21 km/s, generally corresponding to volcaniclastic samples, and another population with a higher average P-wave velocity of 5.52 km/s, corresponding to samples from basaltic clasts or flows within the hyaloclastite. These higher velocities also dominate in Unit XVII, which has an average velocity of 6.86 km/s. Most samples show no statistically significant anisotropy; among those that do, the anisotropy has no consistent trend with depth or lithology.

Thermal conductivity

Thermal conductivity is largely a function of the porosity and mineralogical composition of the rock. Thermal conductivity values for Site U1372 range from 0.99 to 2.10 W/(m·K), with an average of 1.49 W/(m·K). Downhole variations in thermal conductivity are shown in Figure F59 and Table T12. Thermal conductivity is generally 1.33–2.10 W/(m·K) in units dominated by lava flows, with an average of 1.67 W/(m·K). In contrast, thermal conductivity in volcaniclastic units ranges from 0.99 to 1.63 W/(m·K), with an average of 1.15 W/(m·K). Values are particularly low in Units XII and XV, likely because of the high porosity of these units. Figure F61 shows a strong linear relationship between thermal conductivity and bulk density, suggesting that the variation in thermal conductivity is primarily due to variation in porosity rather than mineralogical composition.

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