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doi:10.2204/iodp.proc.330.104.2012

Physical properties

Characterization of physical properties was conducted for Hole U1373A. Measurements included gamma ray attenuation (GRA) bulk density, whole-round and point magnetic susceptibility, laser height, and color reflectance. Whole-round core sections longer than 50 cm (42 of 44 available sections) were also run through the Natural Gamma Radiation Logger (NGRL). Discrete measurements included 12 thermal conductivity measurements at representative locations along the core and compressional wave (P-wave) velocity and moisture and density measurements on 33 discrete oriented rock cubes. Most of these discrete samples were also used for paleomagnetic measurements of alternating-field demagnetization (see “Paleomagnetism”). Following the core depth below seafloor Method A (CSF-A) depth scale convention, data from cores with >100% recovery (e.g., Cores 330-U1373A-7R and 8R) are shown as overlapping in figures and tables. Generally, all physical property data sets are mutually consistent and show clear trends correlating with stratigraphic units based on petrographic descriptions (see “Igneous petrology and volcanology”) and with alteration trends (see “Alteration petrology”).

Whole-Round Multisensor Logger measurements

Throughout the lithified sediments and igneous basement of Site U1373, individual sections usually contain multiple discrete pieces, as is typical of hard rock coring. To remove Whole-Round Multisensor Logger (WRMSL) and Section Half Multisensor Logger (SHMSL) data that were affected by the gaps and edge effects from these discontinuities, we applied a data filtering and processing algorithm (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 are shown in Figure F44. Magnetic susceptibility is sensitive to the mineralogical composition of the rock. In sedimentary stratigraphic Units I and III, magnetic susceptibility averages 1.25 × 10^–2 SI. Magnetic susceptibility increases to an average of 1.64 × 10^–2 SI in the brecciated lava flows of Unit II, reflecting the greater percentage of mafic material in this unit. Units V and VI and the top of VII (lithologic Unit 14) are composed of lava flows with peperitic tops and have an average magnetic susceptibility of 1.53 × 10^–2 SI. This value falls between the average magnetic susceptibility values for conglomerate and the brecciated lavas, which is consistent with the mixing of soft sediment and basalt. This interval (35–50 mbsf) is characterized by a series of highs and lows in magnetic susceptibility. The peaks typically occur near the top of the flow, close to the peperitic boundary (Fig. F45), which may indicate a gradational variation in oxygen fugacity that affected which iron-bearing mineral formed.

For stratigraphic units dominated by continuous lava flows (Units IV and VII), significant variability exists within and between these units. For example, Unit IV has the lowest average magnetic susceptibility (3.86 × 10^–3 SI) at Site U1373, whereas the 22 m thick lava flow that dominates Unit VII has an average magnetic susceptibility of 1.97 × 10^–2 SI, a factor of five higher. There is also significant variability within Unit VII, including a short spatial wavelength high-amplitude peak at 44.5 mbsf (interval 330-U1373A-9R-2, 110–120 cm), which is approximately 20–30 cm below the base of the peperitic flow top (Fig. F45). This variability appears to correspond with a thin interval of abnormally high magnetite content that again may be caused by variation in oxygen fugacity near the top of the flow (Fig. F46). Inductively coupled plasma–atomic emission spectroscopy Sample 330-U1373A-9R-2, 115–117 cm, taken from this interval, also contains 2 wt% more iron than Sample 9R-3, 41–43 cm, taken in the interval of low magnetic susceptibility immediately below the peak (Table T8). There are several more gradual oscillations in Cores 11R and 12R in this stratigraphic unit. These changes may reflect processes associated with repeated injections of material within this inflated flow sheet (see “Igneous petrology and volcanology”).

Gamma ray attenuation bulk density

The results of GRA-derived bulk density are shown in Figure F47. A correction factor of 1.138 was applied to all cores to account for the smaller diameter (58 mm) of hard rock cores (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 were removed. Bulk density values range from 1.12 to 3.06 g/cm³, with an average of 2.52 g/cm³. The densities of sediments (Units I and III), brecciated lava flows (Unit II), and peperitic lava flows (Units V, VI, and top of VII) are very similar, averaging 2.34, 2.36, and 2.29 g/cm³, respectively. The massive basalts in Units IV and VII have a much higher average density of 2.74 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 F48. NGR ranges from 5.28 to 33.75 counts per second (cps), with an average of 17.62 cps. Brecciated and peperitic lavas (Unit II and Units V, VI, and top of VII, respectively) generally have higher NGR values, with averages of 23.21 and 20.11 cps, respectively, which may represent increased alteration in these units. A strong uphole increase in NGR was seen from 20 to 12 mbsf. This increase most likely represents an uphole-increasing trend in alteration to minerals richer in U, Th, and K. A high weight loss on ignition value obtained from Sample 330-U1373A-2R-3, 135–137 cm (13.6 mbsf), by geochemical analysis suggests this region contains significant alteration to clay, which provides some support for this interpretation (see “Geochemistry”). Another possible explanation could be an enriched component in the brecciated lava flow, seen first in fragments that reached the site before the actual flow and were incorporated into the conglomerate at the top of Unit III, immediately below the flow itself.

Section Half Multisensor Logger measurements

Color reflectance spectrometry

Color reflectance spectrometry results are summarized in Figure F49. The L* (lightness) of the recovered core averages 37.6. L* is generally uniform throughout the hole, but two short intervals of lower L* occur at 10.25–10.75 mbsf and 14.75–15.75 mbsf, corresponding to intervals with a visibly smaller proportion of white calcite cement.

Figure F49 also shows values of a* and b*, which correspond to redness (positive) versus greenness (negative) and yellowness (positive) versus blueness (negative), respectively. The strongly red spectrum in Units I–III and VI correlates well with observations of red and brown alteration, including alteration of olivine to iddingsite (Figs. F19, F28). Positive b* values, corresponding to a more yellow spectrum, were observed predominantly in Units I, II, V, and VI and correlate well with the occurrence of brown alteration (Fig. F28). Trends in b* do not correlate with those in a* (e.g., b* increases with depth in Unit II, whereas a* is nearly constant, and b* subtly decreases with depth in Units V and VI, whereas a* shows the opposite trend).

A slightly greener spectrum was observed in Units IV and V and in two intervals in Unit VII (45–50 mbsf and 55–65 mbsf), which correlates well with observations of green clay in veins and vesicles, moderately altered olivines, and olivines altered to green clay minerals (Figs. F19, F29, F33).

Point magnetic susceptibility

Point magnetic susceptibility results are shown in Figure F44 with whole-round magnetic susceptibility data. Both data sets agree well and show clear distinctions between brecciated lava flows, sediments, and peperitic units. Average point magnetic susceptibility values are 9.02 × 10^–3 SI for sediment (Units I and III), 1.27 × 10^–2 SI for brecciated lava flows (Unit II), and 1.04 × 10^–2 SI for peperitic units (Units V and VI and top of Unit VII). Unit IV and the inflated sheet flow in Unit VII again show distinct magnetic susceptibility profiles, with averages of 2.49 × 10^–3 SI and 1.46 × 10^–2 SI, respectively.

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 T9. Bulk density ranges from 2.23 to 3.10 g/cm³, with an average of 2.60 g/cm³. Porosity ranges from 1.13% to 36.94%, with an average of 16.2%. As illustrated in Figure F50, 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 F51. The near one-to-one linear relationship between the two supports our 1.138 volume correction factor 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, slight variations in core radius (approximately ±1–2 mm), and distortions of the core’s cylindrical shape near the ends of pieces or from large voids. These factors can cause overestimates of the total volume used in 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 F47 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. A high average bulk density (2.77 g/cm³) characterizes the massive basalt flows of Units IV and VII, whereas the brecciated lavas of Unit II and the peperitic lavas that are predominant in Units V and VI and the top of Unit VII have lower average bulk densities of 2.41 and 2.36 g/cm³, respectively.

The percent porosity measured in the discrete samples also changes distinctly with depth (Fig. F52). These changes correlate with stratigraphic units and changes in other physical properties, particularly bulk density. In general, porosity is low (average = 7.7%) in Unit IV and the lower 22 m of Unit VII. In contrast, high porosity characterizes the brecciated lava flows of Unit II (25.3%) and the peperites of Units V and VI and the top of Unit VII (24.2%). Porosity and bulk density in the conglomerate (Units I and III) are highly dependent on the proportion of matrix to clast in the sample and are thus highly variable, with ranges of 1.1%–31.9% and 2.34–3.11 g/cm³, respectively.

Compressional wave (P-wave) velocity

The measured P-wave velocity of discrete samples shows a strong linear relationship with bulk density (Fig. F53). Downhole variations in discrete P-wave velocity are shown in Figure F47 and Table T10. P-wave velocities range from 3.14 to 7.05 km/s, with an average of 4.84 km/s. In Units I and III, P-wave velocities are widely scattered, ranging from 3.24 to 6.98 km/s reflecting lithologic variations between the matrix and different basaltic clast types. Among the igneous units, the massive basalts of Unit IV and the lower 22 m of Unit VII have a high average velocity of 5.90 km/s. The brecciated Unit II and peperitic rocks that characterize Units V and VI and the top of Unit VII have average velocities of 3.83 and 4.01 km/s, respectively, reflecting the higher porosity and higher percentage of sedimentary material in these rocks. Most samples show no statistically significant anisotropy; of those that do, the anisotropy has no consistent relationship 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 U1373 range from 1.18 to 2.29 W/(m·K), with an average of 1.54 W/(m·K). Downhole variations in thermal conductivity are shown in Figure F52 and listed in Table T11. Although there is no clear trend in the downhole variations, Figure F54 shows a strong linear relationship between thermal conductivity and GRA-derived bulk density, which suggests that the variation in thermal conductivity is primarily due to variation in porosity rather than mineralogical composition and that the scatter visible in Figure F52 reflects the heterogeneity of the rocks recovered. Unit VII is the most homogeneous unit; however, it still contains vesicular intervals. The thermal conductivity measurement at 58 mbsf was taken in such an interval, explaining its lower thermal conductivity. Sample 330-U1373A-12R-2, 61–63 cm, was taken in the same vesicular interval and, as expected, has a higher porosity, lower density, and lower P-wave velocity than the more characteristic samples taken from the unit.

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