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doi:10.2204/iodp.proc.330.105.2012

Physical properties

Characterization of physical properties was conducted for rocks recovered from Hole U1374A through measurements on whole-round and split-core sections and discrete samples. Measurements of gamma ray attenuation (GRA) bulk density, whole-round and point magnetic susceptibility, laser height, and color reflectance were conducted on all 397 core sections recovered from this hole. Whole-round core sections longer than ~50 cm (385 of 397 available sections) were also run through the Natural Gamma Radiation Logger (NGRL). Discrete measurements included compressional wave (P-wave) velocity and moisture and density measurements on 212 discrete oriented rock cubes. Most of these discrete samples were also used for paleomagnetic measurements of alternating-field demagnetization (see “Paleomagnetism”). Thermal conductivity measurements were made at 10 locations along the hard rock split-core face from the upper portion of the hole before being discontinued because of equipment failure. In accordance with core depth below seafloor Method A (CSF-A) conventions for referencing cores to depth (see “Procedures” in the “Methods” chapter [Expedition 330 Scientists, 2012a]), data for cores with >100% recovery (e.g., Cores 330-U1374A-44R through 46R) are shown as overlapping in figures. Generally, all physical property data sets are mutually consistent and show distinctions and trends that often correlate with lithologic changes (see “Igneous petrology and volcanology”) and with petrologically determined alteration trends (see “Alteration petrology”).

Whole-Round Multisensor Logger measurements

Throughout the lithified sediments and igneous basement of Site U1374, individual sections generally contain multiple discrete pieces, as is typical of hard rock coring. In order to remove spurious 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

Magnetic susceptibility is sensitive to the mineralogical composition of the rock. The results of whole-round magnetic susceptibility measurements are shown in Figure F65. Significant deviations from a median value of 3.93 × 10^–3 SI occur as distinct short wavelength peaks in magnetic susceptibility, with values that exceed 1.00 × 10^–2 SI, as well as broad regions of distinctly lower values.

Stratigraphic Unit I, which consists of foraminiferal ooze, has the lowest magnetic susceptibility of all Hole U1374A units, with an average value of 1.76 × 10^–3 SI. However, not all units identified as being sedimentary necessarily exhibit low values. The sharp peaks found in the sedimentary conglomerate (Unit XI) exhibit some of the highest values of magnetic susceptibility in the entire hole. These peaks are attributed to large boulders of massive basalt present at the base of the unit. Elsewhere, peaks in magnetic susceptibility tend to correlate with lava flows and dikes. Stratigraphic Units IV–VII, along with the uppermost 6 m of Unit VIII, mark the largest depth interval of relatively high values and are associated with the thick lava flows and brecciated basalts in these units. The remaining portion of Unit VIII is dominated by low magnetic susceptibility, similar to the uppermost 22 m of Unit XIII. Other large peaks in magnetic susceptibility coincide with intrusive sheets or dikes, which are present in Units XVI–XIX. These dikes are limited in depth extent in the core but yield high magnetic susceptibility values, with an average of 8.89 × 10^–3 SI. In Unit XVI, however, this signal is less obvious; in contrast with Units XVII–XIX, magnetic susceptibility values of the dike and lava flows within this unit are not considerably larger than those of the surrounding volcaniclastics. A notable region of low values occurs from ~480 mbsf to the end of the hole, with a further decrease in the area immediately surrounding the dikes that make up lithologic Units 139 and 140. These values are associated with volcanic sand in the interval identified as having a frothy, glassy nature (see “Igneous petrology and volcanology”). This interval, excluding the dikes, has an average magnetic susceptibility of 3.43 × 10^–3 SI.

Gamma ray attenuation bulk density

The results of GRA-derived bulk density are shown in Figure F66. A correction factor of 1.138 was applied to the hard rock cores (Sections 330-U1374A-1R-6 through 73R-1) to account for the smaller diameter (58 mm) of hard rock cores compared to the full core liner diameter of 66 mm (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. Bulk densities range from 1.02 to 3.01 g/cm³, with an average of 2.38 g/cm³. Although some scatter was observed, as is typical of breccia and conglomerate, the GRA-derived bulk density is consistent throughout most of Hole U1374A, averaging 2.35–2.45 g/cm³. The unconsolidated sediments of stratigraphic Unit I have a much lower bulk density, with an average of 1.55 g/cm³. Moderately lower densities were also observed in stratigraphic Units VII–VIII and in the intervals containing frothy, glassy clasts from 363 to 379 mbsf and 470 to 522 mbsf, excluding the dikes of lithologic Units 139, 140, 146, and 148 (see “Igneous petrology and volcanology”). These lighter intervals average 2.14, 1.97, and 2.08 g/cm³, respectively. The dikes in Units XVI–XIX and the larger lava flows of Units IV and VI have consistently higher densities, with averages of 2.57 and 2.76 g/cm³, respectively.

Natural Gamma Radiation Logger

Natural gamma radiation (NGR) measurements reflect the combined total amount of uranium, thorium, and potassium present in the rock. Because of the exceptionally high recovery at this site, count times were reduced to 60 min for sections containing potential lava flows or lobes and 30 min for all other sections beginning with Section 330-U1374A-42R-3 to keep up with core flow. Results from the NGRL are shown in Figure F67. NGR ranges from 0.90 to 40.94 counts per second (cps), with an average of 15.20 cps. The unconsolidated sediments of Unit I have consistently low levels of NGR, with an average of 6.90 cps. Units II–XIII are characterized by an essentially featureless band of NGR between 13 and 21 cps, with an average of 16.20 cps. This overall pattern ends abruptly at the base of Unit XIII, below which Units XIV–XIX have a lower overall average of 14.31 cps. These lower units exhibit distinctly less short-wavelength scatter but greater overall variability, with several well-defined peaks and troughs. Many of the peaks correlate with the intrusive dikes observed in Units XVI–XIX, which average 19.47 cps. Other peaks and lows also appear primarily connected to the basaltic components of the units; the very low levels of NGR observed below 490 mbsf correspond to an almost complete absence of basaltic clasts in the volcanic sands between lava flows. In contrast, the interval of high NGR from 450 to 470 mbsf contains a high percentage of basalt and increased magnetic susceptibility, which may indicate a distinct volcanic or alteration history affecting the material in this interval. Finally, the peak at 278–288 mbsf correlates with a zone of increased alteration.

Section Half Multisensor Logger measurements

Color reflectance spectrometry

Color reflectance spectrometry results are summarized in Figure F28. L* (lightness) of the recovered core averages 33.1. L* is fairly uniform throughout the hole but shows a gradual reduction in scatter downhole beginning at ~240 mbsf, decreasing more rapidly between 280 and 370 mbsf, and continuing to slowly decrease to the bottom of the hole.

Figure F28 also shows values of a* and b*, which correspond to redness versus greenness and yellowness versus blueness, respectively. Overall, a* gradually changes from a predominantly red spectrum in Units I–XV, to a transitional zone in Units XVI and XVII, followed by a predominantly green spectrum in Units XVIII and XIX. This is very well correlated with the transition from red and brown alteration to green alteration (Figs. F38, F39) and likely represents the transition through time from a submarine reducing environment to a subaerial or shallow-marine oxidizing environment. The distinct red interval from 353 to 362 mbsf at the base of the transition zone is characterized by red alteration in the smaller basaltic clasts, possibly indicating subaerial eruption or shoaling in the surrounding region. Another strongly red interval occurs from 278 to 295 mbsf, which may indicate local shoaling (see “Sedimentology” and “Igneous petrology and volcanology”).

Other distinct features include a strong shift toward more green and yellow spectra in the frothy hyaloclastite interval from 470 to 522 mbsf. A moderately green spectrum was also observed in the conglomerate of Unit XI and the olivine-plagioclase-augite-phyric rocks of Unit XII. The lava flow of Unit IV has a distinctly blue spectrum and very uniform, neutral a* values.

Point magnetic susceptibility

Point magnetic susceptibility results are shown in Figure F65 together with whole-round magnetic susceptibility data. This data set agrees well with the whole-round data and shows clear contrasts between the background signal of the breccia and the lava flows and dikes.

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 T12. Bulk density ranges from 1.89 to 2.93 g/cm³, with an average of 2.47 g/cm³. Porosity ranges from 0.9% to 47.1%, with an average of 18.3%. As illustrated in Figure F68, 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 F69. 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. For Hole U1374A, a slight decrease (2–3 mm) in average core diameter resulting from bit wear was observed near the base of the hole. However, variability in diameter also increased, so these changes could not be incorporated into the volumetric correction. 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. The two outliers with anomalously low GRA-derived bulk densities appear to be affected by underestimates resulting from measurements taken in partial cores with a highly noncylindrical shape.

Figure F66 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. Regions of consistently high density mark stratigraphic Units IV and VI and the lowermost dikes of Unit XIX, whereas the majority of the basalt breccia units have a significant amount of scatter in the data. Distinct intervals of lower density values exist in Unit VIII, near the boundary between Units XVII and XVIII, and surrounding the dikes of the lower portion of Unit XIX. These low-density volcaniclastics in Unit XIX (starting at 470 mbsf) contain clasts and fragments of frothy, glassy basalt and have an average density of 2.08 g/cm³. The nearby dikes have a significantly higher average density of 2.57 g/cm³.

The percent porosity measured in the discrete samples also shows distinct changes with depth (Fig. F70), which agrees well with the density observations discussed above. Although porosity values vary widely throughout most the basalt breccia units, Units IV, VI, and the upper portion of Unit X are marked by consistently low values. Many of the lowest values can be attributed to lava flow lobes within the breccia. Additionally, the dikes that appear intermittently in Units XVI–XIX are also typically low in porosity (average = 9.7%) but have values that are generally higher than those obtained from lava flow lobes. A region of high porosity was observed in the lower two-thirds of Unit XIX in material identified as containing frothy basalt glass. This interval begins at 470 mbsf and has an average porosity of 37.4%.

Compressional wave (P-wave) velocity

The measured P-wave velocity of discrete samples shows a strong linear relationship with bulk density (Fig. F71). Downhole variations in P-wave velocity are shown in Figure F66 and Table T13. The P-wave velocities are widely scattered in general because the majority of the material is breccia with intermittent lava flows and large boulders, leading to large variability over even short depth intervals. For the entire hole, P-wave velocities range from 2.10 to 6.81 km/s, with an average of 4.70 km/s. Many of the highest values can be attributed to lava flow lobes interspersed throughout the basalt breccia, particularly the aphyric basalt flows found in stratigraphic Units IV, VI, and X. The dikes that appear in Units XVI–XIX also demonstrate distinctly high P-wave values, with an average of 5.59 km/s. The most notable grouping of low P-wave velocities begins at 470 mbsf, where clasts of frothy basalt glass were identified. This lowermost interval (470–522 mbsf) of Unit XIX has an average compressional velocity of 3.26 km/s. 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. The limited thermal conductivity values for Site U1374 range from 0.77 to 1.65 W/(m·K), with an average of 1.38 W/(m·K). Note, however, that values were only available for roughly the uppermost 50 m of Hole U1374A because of equipment failure and thus are not representative of the entire hole. Available thermal conductivity measurements are listed in Table T14 and plotted against GRA bulk density in Figure F72.

Large-scale trends in physical properties

In addition to the patterns noted above, most of the physical properties measured show large-scale variations in average and distribution. These variations typically correlate with broad petrologic differences, either between the olivine-dominated package of stratigraphic Units II–XIII and the plagioclase-dominated package of Units XIV–XIX or above and below the first appearance of hyaloclastites at 327 mbsf. The unconsolidated sediment of Unit I and the intrusive dikes in Units XVI–XIX were also treated separately because they are petrologically and potentially temporally distinct.

The distinction between the plagioclase- and olivine-dominated packages is reflected in the magnetic susceptibility and NGR data, properties likely to be sensitive to subtle mineralogical variations. NGR values average 16.47 cps in the olivine-dominated Units II–XIII, compared with 13.63 cps in the plagioclase-dominated Units XIV–XIX. For magnetic susceptibility data, variation in background magnetic susceptibility better reflects large-scale changes than do the localized peaks caused by individual pieces of basalt or alteration zones. This background level is best reflected in the median magnetic susceptibility, with the olivine-dominated package having a median whole-round magnetic susceptibility of 5.74 × 10^–3 SI, which is almost twice the median value of the plagioclase-dominated package of 2.91 × 10^–3 SI. Point magnetic susceptibility shows a similar distinction, with median values of 3.74 × 10^–3 SI and 2.05 × 10^–3 SI for the olivine- and plagioclase-dominated packages, respectively.

The differences between units containing a high proportion of hyaloclastites, those deeper than 327 mbsf, and those above 327 mbsf are reflected in L* (which measures the lightness versus darkness of the core), P-wave velocity, and porosity. A further difference is also seen at 470 mbsf in the hyaloclastite package, marking the beginning of the interval containing frothy, glassy clasts. L* in the hyaloclastite units (327–522 mbsf) is characterized by a lower average of 29.5 and a smaller standard deviation of 6.8 compared to units above the hyaloclastites, which have an average of 35.1 and a standard deviation of 8.9. Despite the presence of both high and low outliers, median P-wave velocities decrease downhole from a median of 4.99 km/s above the hyaloclastites, to 4.35 km/s in the first hyaloclastite package (327–470 mbsf), to 3.42 km/s in the interval containing clasts of frothy basalt glass (470–522 mbsf). Porosity exhibits the opposite trend, increasing downhole from 13.4% to 24.4% to 36.2%, respectively. This marked increase in porosity is clearly reflected in the density of the interval containing frothy, glassy clasts and fragments (median density = 2.07 g/cm³) but is less clear between the upper hyaloclastite package and those units without hyaloclastites, whose median densities are higher and more similar (2.36 and 2.46 g/cm³, respectively).

The intrusive dikes, which were excluded from the above analysis, are characterized by high median density (2.58 g/cm³), high median magnetic susceptibility (9.16 × 10^–3 SI for whole-round magnetic susceptibility), high median NGR (19.3 cps), high median P-wave velocity (5.38 km/s), and low median porosity (10.7%). The unconsolidated sediment is characterized by low median density (1.59 g/cm³), low median magnetic susceptibility (1.51 × 10^–3 SI for whole-round magnetic susceptibility), high L* (42.7), and low NGR (6.97 cps).

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