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doi:10.2204/iodp.proc.335.103.2012

Physical properties

During Expedition 335, we carried out shipboard physical property measurements to investigate the physical characteristics of rocks recovered from 1372.80 to 1521.6 mbsf in Hole 1256D. We conducted physical property measurements on three groups of rock samples: legacy core from Expedition 312, Expedition 335 core, and non-core material collected during Expedition 335. We conducted the following measurements:

• Analyses using the multisensor core loggers on Expedition 312 archive section halves (Sections 312-1256D-202R-1A through 234R-1A) and Expedition 335 whole-round sections and section halves (Sections 335-1256D-235R-1 through 238R-1);

• Thermal conductivity measurements of archive section half pieces from Expeditions 312 and 335, as well as non-core material from Expedition 335;

• Measurements of discrete cube samples (~8 cm³) from Expeditions 312 and 335 for P-wave velocity, density, and porosity; and

• Magnetic susceptibility measurements of non-core material from Expedition 335.

Multisensor core logger data

Expedition 312 cores revisited

At the beginning of Expedition 335, we remeasured Expedition 312 archive section halves with the Whole-Round Multisensor Logger (WRMSL), Natural Gamma Radiation Logger (NGRL), and Section Half Multisensor Logger (SHMSL) (see “Physical properties” in the “Methods” chapter [Expedition 335 Scientists, 2012b]). Although both the WRMSL and NGRL are calibrated for whole-round sections, these section-half measurements qualitatively indicate the downhole trend of properties. Measurements were made at a higher resolution than during Expedition 312. For presentation and comparison to the measurements made on whole-round sections during Expeditions 312 and 335, measurements on the archive halves have been scaled.

Overall, remeasured and processed Expedition 312 archive section half data reveal significant variations in physical properties within Gabbros 1 and 2. Data from the granoblastic dike intervals between 1406 and 1507 mbsf appear relatively uniform, given the limited recovery (Fig. F88).

Within the gabbro units, the largest peaks in multisensor core logger data correspond to the occurrence of evolved intrusions of diorite, oxide diorite, and oxide quartz diorite (see “Igneous petrology”). The highest WRMSL magnetic susceptibility value was >15,000 instrument units (IU), observed in an oxide-rich vein in interval 335-1256D-230R-1, 48–60 cm. Peaks in natural gamma radiation (NGR) appear to be associated with the occurrence of narrow intrusions of evolved dioritic rock, and NGR data are broadly consistent with the concentrations and general geochemical trends in K, U, and Th analyzed from Hole 1256D (Fig. F89) (Gao et al., 2009; Neo et al., 2009; Yamazaki et al., 2009). High counts associated with evolved dioritic rocks do not always correspond to high concentrations in all three radiogenic elements. Although in the upper part of Gabbro 1 oxide quartz diorites are enriched in K, U, and Th, at the base of Gabbro 2 (1494–1495 mbsf) dioritic intrusions have no corresponding peak in the K window.

Variations within the gabbros are most clearly seen in magnetic susceptibility and color reflectance data, with minor variations in NGRL counts and gamma ray attenuation (GRA) density. Magnetic susceptibility and color reflectance data are interpreted to be relatively sensitive to the oxide and olivine content of rocks. At the top of Gabbro 1 (1410–1420 mbsf), b* (red–green value) and magnetic susceptibility decrease downhole, whereas a* (blue–yellow value) increases; this observation may correspond to decreasing oxide and/or olivine content. Similarly, at the base of Gabbro 1 (1430–1455 mbsf), as olivine mode increases downhole, b* values increase (Fig. F90) and L* values decrease.

Similar observations can be made at the top of Gabbro 2 (1482.5–1486 mbsf); very high magnetic susceptibility values (>15,000) correspond to an oxide vein, but the oxide gabbro host rocks also have relatively high magnetic susceptibility. Magnetic susceptibility then decreases downhole as lithology changes to disseminated oxide gabbros and then gabbronorite, together with a coincident increase in L* and a* reflectance values and a decrease in b* values and NGR counts. The lower section of Gabbro 2 (1488–1495 mbsf) shows more complicated trends in physical properties that may reflect minor changes in the concentrations of oxides in the disseminated oxide gabbronorites. At a broad scale, magnetic susceptibility and b* reflectance values initially decrease downhole from 1488 to 1491 mbsf before increasing toward the base of Gabbro 2. In contrast, L* and a* reflectance values show the opposite trend. A similar trend in magnetic susceptibility and b* values is also observed at the base of Gabbro 2, where a small decrease in olivine mode corresponds with a decrease in b* values and magnetic susceptibility. These observations suggest that color reflectance data can be used for gabbroic rock samples to evaluate relative modal changes in composition.

Expedition 335 shipboard results

During Expedition 335, we measured four whole-round and archive-half core sections recovered while deepening Hole 1256D, as well as non-core material that was obtained during the cleaning of the hole. Pieces in Sections 335-1256D-235R-1 and 236R-1 were too short to be measured with the NGRL. Section 335-1256D-237R-1 was empty. The three pieces in Section 335-1256D-238R-1 are also too small to obtain any reliable signals from both the WRMSL and NGR (a total length of <20 cm). Core from Section 335-1256D-239R-1 was analyzed using all instruments.

Overall, GRA density values are surprisingly low (maximum = ~2.2 g/cm³) for Sections 335-1256D-235R-1 and 236R-1 (Fig. F88), although measurements of the standard water sample were still within ~1.5% of its certified value. These values are probably related to the relatively small core diameter of the recovered pieces, typically <50 mm. A correction has been applied to these cores as described above to allow comparison of filtered data with those acquired on Expedition 312 cores.

Both whole-round magnetic susceptibility and point magnetic susceptibility (MSPOINT) data show generally high magnetic susceptibility values consistent with observations from granoblastic basalt in Expedition 312 cores and from the non-core material (see “Expedition 335 non-core material” in “Compressional wave velocity”). The highest observed magnetic susceptibility and GRA density values are observed in interval 332-1256D-235R-1, 22–24 cm (Piece 5), which hosts a tonalite dike. Whether the peak in magnetic susceptibility reflects the tonalite dike or the altered host rock is unclear. In contrast, MSPOINT data display a low value over the same piece, although there may have been poor sensor contact, as the piece is small (~2.5 cm).

Thermal conductivity

Expedition 312 cores revisited

Thermal conductivity was measured on a total of nine archive section half pieces (Table T10). The results of these measurements complement the Expedition 312 data (Fig. F91). We tentatively interpret an increase in thermal conductivity downhole in Gabbro 1 to be linked to a corresponding increase in olivine mode (see “Igneous petrology”), whereas the uniform thermal conductivity of Gabbro 2 appears to be broadly consistent with the more uniform mineralogy of Gabbro 2.

Expedition 335 shipboard results

Thermal conductivity was measured on Section 335-1256D-235R-1 (Piece 1), on a 12 cm long piece of dolerite ~5 cm in diameter (Table T11). Tests were conducted to assess the spatial response curve for the half-space thermal conductivity probe, as, given the low recovery, we considered measuring small pieces of core. Measurements were made at different positions across and along the core piece to determine the edge effect of the measurements to each piece (Fig. F92). Results were within the reported 2% instrument accuracy of measurements made at the center of the core until the center of the detector was within 15 mm of the edge of the sample. Although this length scale is likely to be dependent on the conductivity of the core (the average is 2.27 W/[m·K] for measurements away from piece edges) and the heating power applied (3 W/m), the results suggest that robust measurements can be obtained from relatively small core pieces. This inference is consistent with the estimated characteristic length scale (l) of thermal conduction of ~8 mm for the experiment, where the characteristic length scale is derived from the equation for the diffusion of heat:

assuming a thermal conductivity (k) of 2.3 W/(m·K), a period of heating (t) of 80 s, a density (ρ) of 2.9 g/cm³, and specific heat capacity (c_p) of 900 J/(kg·K). For low-porosity mafic rocks, these results appear to indicate that robust thermal conductivity measurements might be attained on half cores with a radius of >15 mm and a length of >30 mm, although further tests are required to refine minimum reliable piece size.

Thermal conductivity of Expedition 335 non-core material

Thermal conductivity measurements were made on two slabs cut from Samples 335-1256D-Run12-RCJB-Rock C and 335-1256D-Run13-RCJB-Rock B (Table T10). Both samples were granoblastic basalts; Sample RCJB-Rock C is a granoblastic basalt cut by several alteration veins, whereas Sample RCJB-Rock B is a uniform, unaltered granoblastic basalt. Both samples yielded thermal conductivities consistent with measurements of cored granoblastic basalt (2.2–2.3 W/[m·K]).

Thermal conductivity measurements were also made to assess the anisotropy of thermal conductivity due to the veins in Sample 335-1256D-Run12-RCJB-Rock C (Fig. F93). As the angle between the needle probe and the veins increased, the observed thermal conductivity also increased (from 2.21 to 2.23 W/[m·K]), consistent with greater thermal conduction to the veins (1% anisotropy).

Discrete sample measurements

We measured a total of 11 cubes, which were shared with the Paleomagnetism group, in order to (1) compare data acquired during Expedition 335 to that acquired with a different instrument during Expedition 312 and (2) to complement and add to the existing downhole sample data.

Compressional wave velocity

Expedition 312 cores revisited

Compressional wave velocity (V_P) was measured on 11 seawater-saturated hard rock samples from gabbroic sections of the Expedition 312 cores (Table T12).

V_P measurements were initially made on cubes using Expedition 312 protocols; however, we found that variations in the surface saturation of the minicubes had a dramatic effect on velocity values, resulting in differences of as much as 900 m/s (~15%) for measurements on a single cube (see the “Methods” chapter [Expedition 335 Scientists, 2012b]). To obtain more reliable, reproducible values, we developed a new approach for measuring velocities in which minicubes are submerged in a seawater bath during measurement (see “Physical properties” in the “Methods” chapter [Expedition 335 Scientists, 2012b]). This procedure led to considerably more stable results.

Velocity measurements using the seawater bath were obtained for four minicubes that had undergone AF demagnetization by the Paleomagnetism group and so were available for repeated measurements. The remaining seven cubes were not reanalyzed using the seawater bath because they had already undergone high-temperature (as high as 600°C) demagnetization, and so the measured V_P may no longer have been representative.

V_P measurements on four gabbro minicubes submerged in the seawater bath range from 6200 to 6800 m/s with standard deviations for each measurements on the order of ~40 m/s (Table T12). In contrast, measurements made on seven minicubes without the seawater bath range between 4922 and 6017 with standard deviations for each direction of 160 m/s. Measurements made without the seawater bath are interpreted to underestimate velocity, with scatter attributed to variations in the coupling between the sample and sensor for each reading.

V_P measurements on the minicubes in three orthogonal directions vary by <2% for each sample, which is insignificant, given they are less than instrumental error. Hence the samples do not display a well-pronounced anisotropy in V_P as might be expected from the isotropic nature of both granoblastic basalt and the Gabbro 1 and 2 recovered. For samples measured using the seawater bath, averaging the velocities measured in each orthogonal direction yields sample average velocities for three of the gabbro cubes analyzed that lie in the range from 6694 to 6759 m/s; the remaining cube has a lower velocity of 6298 ± 170 m/s.

Velocity measurements with the seawater bath are markedly higher than those obtained during Expedition 312 on samples from the same intervals (Fig. F94). We suggest that the seawater bath provides more reliable results and that V_P values for gabbros in Hole 1256D may be higher than reported during Expedition 312. We note that the velocities are generally >6500 m/s, match shore-based measurements between 1440 and 1460 mbsf (Violay et al., 2010; Gilbert and Salisbury, 2011), and are consistent with the downhole sonic log (~6800 m/s) (Guerin et al., 2008). Although measured velocities are >6500 m/s and consistent with seismic Layer 3, an interpretation that the Layer 2/3 boundary has been reached is premature, as the velocities also lie at the upper range of Layer 2 values (see also discussion in Gilbert and Salisbury, 2011).

Expedition 335 non-core material

Two minicubes were cut from each of the granoblastic basalt slabs that were used for thermal conductivity measurements (Samples 335-1256D-Run12-RCJB-Rock C and 335-1256D-Run13-RCJB-Rock B). Density, porosity, and V_P were measured for each cube. Prior to cutting, V_P was also measured perpendicular to the cut faces of the slab from Sample 335-1256D-Run13-RCJB-Rock B. These latter measurements were made without the seawater bath because of sample size. Average V_P for the slab was 6123 ± 20 m/s (1σ error). For the four minicube samples, V_P ranged from 6610 ± 22 to 6907 ± 21 m/s. These velocities are higher than values measured during Expedition 312 because of the improved surface saturation of samples measured using the seawater bath (see the “Methods” chapter [Expedition 335 Scientists, 2012b]) (Fig. F94; Table T12), an inference consistent with the observation that V_P measured in the z-direction of minicubes using the seawater bath is ~300 m/s greater than measurements made on the slab without the seawater bath. The velocities of these granoblastic basalts are generally higher than those of gabbros.

Moisture and density

Expedition 312 cores revisited

Density and porosity results for the 11 minicubes are directly comparable to the results obtained during Expedition 312 (Table T12; Fig. F95). Bulk density values increase slightly with depth from ~2.9 to 3.0 gm/cm³ over the interval from 1400 to 1507 mbsf, whereas porosity decreases from ~2% to 0.5%. These observations are consistent with the general trend of increasing V_P over the same interval. Superimposed on this apparent trend is variation associated with lithology; in particular, porosity and V_P vary more in Gabbro 1 than within the granoblastic basalts and Gabbro 2, an observation that is consistent with the comparably more heterogeneous texture and mineralogy of Gabbro 1.

Expedition 335 non-core material

Density and porosity results for the four minicubes analyzed from non-core material appear to continue the downhole trends observed for samples from rocks recovered during Expedition 312. Bulk densities of 3.01–3.07 g/cm³ are among the highest observed in Hole 1256D, whereas porosities of 0.4%–0.8% continue the decreasing trend in porosity downhole.

Magnetic susceptibility of Expedition 335 non-core material

Unoriented samples recovered during operations to clear Hole 1256D were not analyzed using the logging tracks; however, we measured magnetic susceptibility using a portable Bartington MS2F probe on all large samples from the reverse circulation junk basket (RCJB) runs and on representative samples (approximately >5 cm³) from EXJBs on Runs 11 and 14.

Histograms of magnetic susceptibility indicate that rock samples from Run12 and Run13 have a high susceptibility [mean = (5580 ± 200) × 10^–5 SI; 95% confidence] (Fig. F96). Separating the data by lithology shows that data are dominated by results for the granoblastic basalts [mean = (6190 ± 200) × 10^–5 SI; 95% confidence], which show relatively consistent magnetic susceptibilities. This observation contrasts with the broader distribution but generally lower magnetic susceptibility of the gabbroic samples [mean = (3010 ± 450) × 10^–5 SI; 95% confidence limit]. These results agree with previous measurements of magnetic susceptibility of gabbros and granoblastic basalts in Hole 1256D (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006).

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