The physical properties of the peridotites and gabbros cored in Hole 1268A were characterized through a series of measurements on whole-core sections, split-core pieces, and discrete samples as described in "Physical Properties" in the "Explanatory Notes" chapter. We measured natural gamma ray (NGR) activity and magnetic susceptibility on the MST and thermal conductivity, compressional wave velocity, density, and porosity. The rock names reported in data tables correspond to the primary lithologies assigned by the igneous core description group. Most of the peridotites are highly serpentinized, and some have been pervasively altered to talc. The gabbros recovered from this site are also highly altered. The data are summarized as a function of depth in Figure F85.
All cores recovered during Leg 209 were measured using the NGR logger on the MST at intervals of 10 cm and a time period of 30 s. Results are output in counts per second. A few intervals in Hole 1268A appear to display natural radioactivity significantly higher than the background radiation (Fig. F86). The two highest peaks in the NGR data record range 15–25 cps (corrected) and are related to highly altered serpentinite and pyroxenite vein intervals in Sections 209-1268A-5R-2 and 16R-1 (Fig. F86).
Magnetic susceptibility is particularly sensitive to the presence of magnetite and can be used to identify iron-rich zones in the rock, such as oxide-rich gabbros and magnetite-rich serpentinized peridotites. Magnetic susceptibility values were acquired on the MST at 2.5-cm intervals for all recovered cores. High–magnetic susceptibility intervals (Fig. F87) correspond to serpentinized magnetite-rich, relatively massive altered harzburgites and dunites. Except for these few intervals, the magnetic susceptibility of peridotites in Hole 1268A cores is at least two orders of magnitude lower on average than that of peridotite cores recovered during previous Legs 147 and 153 (Fig. F88).
Thermal conductivity measurements were made at irregularly spaced intervals (Fig. F85). The data are summarized in Table T6. Thermal conductivity is a tensor property, and most known single-crystal thermal diffusivities are anisotropic (e.g., Kobayashi, 1974; Tommasi et al., 2002). We tried to evaluate the anisotropy of thermal conductivity in the measured peridotites and gabbros as described in "Thermal Conductivity" in "Physical Properties" in the "Explanatory Notes" chapter. The average thermal conductivity is 2.84 W/(m·K) in serpentinized peridotites and 2.13 W/(m·K) in gabbros. The apparent measured anisotropy ranges 0.8%–12.0%. High apparent anisotropy is found in both serpentinized peridotites and gabbros. Because of their complex structural pattern, combining one or several networks of cracks and veins and possible preferred mineral orientations, it is not easy to infer the cause of the measured anisotropy from the shipboard observations. In several samples, the thermal anisotropy is lower when the needle is aligned subparallel to the foliation (i.e., in the plane across the foliation).
Thermal conductivity data are shown in Figure F89 with the measured thermal conductivities of gabbros and peridotites recovered from other ODP sites (Robinson, Von Herzen, et al., 1989; Cannat, Karson, Miller, et al., 1995; Gillis, Mével, Allan, et al., 1993; Dick, Natland, Miller, et al., 1999). The thermal conductivities of the altered peridotites from Hole 1268A range 2.30–3.51 W/(m·K) and are similar to the properties of peridotites from ODP Legs 147 and 153. Thermal conductivities in the altered gabbros from Hole 1268A range 1.84–2.52 W/(m·K). This range of values is considerably smaller than the range of thermal conductivities exhibited by the gabbros recovered from ODP sites on Atlantis Bank (Robinson, Von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999), Hess Deep (Gillis, Mével, Allan, et al., 1993), and the MARK area (Cannat, Karson, Miller, et al., 1995). Whether this reflects the different type of alteration of the gabbros from Hole 1268A, or that our data set is small, is unknown.
As noted above, we measured P-wave velocity, wet bulk density, and porosity on selected samples, as described in "P-Wave Velocity" and "Porosity and Density" in "Physical Properties" in the "Explanatory Notes" chapter. For unknown reasons, the penta-pycnometer was unable to measure the volume of some of the minicores of highly serpentinized peridotite. Therefore, some porosities and densities were estimated from 1- to 2-cm3 chips trimmed from minicores sampled for paleomagnetic measurements. A possible explanation for this problem is that some helium is trapped in the very low permeability claylike structure of serpentine and/or talc. As a result, the pycnometer seems to be unable to equilibrate the helium pressure in the cell for sample volumes as large as the minicores (~10 cm3). Interestingly, this problem has not been reported for previous measurements on serpentinized peridotites (Gillis, Mével, Allan, et al., 1993; Cannat, Karson, Miller, et al., 1995); it might be related to the specific type of alteration in this core. All the minicores measured in the pycnometer were samples that were logged by the metamorphic alteration shipboard description team as containing little or no talc. We encountered no problem measuring the volumes of the gabbro minicore samples.
Results are presented in Figure F85 and Tables T7 and T8. For most minicore samples, the volume was measured in the helium pycnometer, using either chips or minicores when possible (gabbros and a few peridotites), giving a direct estimate of grain density. The bulk volume of all minicores was also computed from length and diameter measurements using a caliper, giving a direct estimate of bulk density. The two methods give comparable results for bulk density, within error, but there is some difference between the grain density results. Except for one sample (Sample 209-1268A-24R-2, 90–92 cm), all grain densities derived from volumes measured in the pycnometer are higher than those calculated from the bulk volume. For the six minicores measured in the pycnometer, the difference is low (0.03–0.12 Mg/m3). The difference is higher (0.14–0.17 Mg/m3) for those samples for which we used chips for the pycnometer volume measurements. The grain densities estimated from dry masses and pycnometer volume measurements are probably overestimated, possibly a result of the small volume of the chips (2 cm3 on average) and/or of the difficulties encountered in using the pycnometer to measure the volumes of highly serpentinized peridotites (see above).
In both altered gabbros and serpentinized peridotites, the calculated porosities are more than two times higher when using the pycnometer technique than determined using the bulk volume technique. Unlike bulk density, this difference is apparently not related to the volume of the samples or to the rock type. Therefore, it is difficult to infer the real porosities of these rocks and these estimates should be taken with extreme caution.
Aside from the minicores and cubes in which we measured velocity and density, we also determined the grain density of some powders prepared for ICP-AES analyses by measuring their dry mass and volume. The very fine grained powders were prepared as described in "Sample Preparation" in "ICP-AES Analyses of Major and Trace Elements" in "Geochemistry" in the "Explanatory Notes" chapter. The grain densities range 2.78–4.01 Mg/m3. These densities are difficult to reconcile with the ICP-AES and XRD analyses that indicate the samples are composed of ~99% talc and/or lizardite with little or no magnetite. All densities measured this way appear to be unreasonably high in view of the composition of the samples.
We tested the procedure by using silicon carbide powders of known density (3.21 Mg/m3) and different grain sizes (120 grit/110 µm and 600 grit/15 µm). The density measured on the coarse-grained sample agrees with the nominal density within error, whereas the density measured on the fine-grained sample is always too high (~0.1 Mg/m3 higher than the nominal density for ~16 g of powder). This overestimation of the density seems to result from an underestimation of fine-grained powder volumes by the helium pycnometer. The reasons for this effect are unknown but are likely related to the properties of very fine grained powders.
We measured P-wave velocity and wet bulk density of ~8-cm3 cube samples (Fig. F85; Table T8) and minicores that were also used for paleomagnetic measurements (Fig. F85; Table T7). Velocity was measured in three directions in each of the cube samples (Table T8) in the conventional x-, y-, z- core reference frame (see Fig. F7 in the "Explanatory Notes" chapter; i.e., normal to the core axis and to the cut face [Vx], normal to the core axis and parallel to the cut face [Vy], and parallel to the core axis [Vz]). P-wave anisotropy ranges 0.0%–9.2% (mean = 3.8%).
The relationship between bulk density and P-wave velocity is summarized in Figure F90. The properties of the serpentinized peridotites are similar to the measured densities and velocities of serpentinites from the MARK area (Cannat, Karson, Miller, et al., 1995), but the gabbros from Hole 1268A have lower velocities and densities than similar rocks recovered during Leg 153. This is likely because the Hole 1268A gabbros are affected by strong hydrothermal alteration. Oceanic gabbros typically have velocities, measured at bench pressure, of >5 km/s and densities >2.5 Mg/m3. In the altered gabbros from Hole 1268A, densities range 2.44–2.82 Mg/m3 and velocities range from 3.4 to just over 4.9 km/s.
Although they were measured at bench pressure, the P-wave velocities in serpentinites and altered gabbros recovered from Hole 1268A have interesting implications for interpreting the seismic structure of the crust in the vicinity of the 15°20´N Fracture Zone, where serpentinites are widely exposed at the seafloor. Two unpublished seismic profiles from 16°N and 15°37´N (J. Collins and R. Detrick, pers. comm., 1998) are shown in Figure F91. Several other profiles are included in this figure; one is a seismic profile through "typical" oceanic crust at 9°N on the East Pacific Rise (Vera et al., 1990). Also shown are velocity-depth profiles calculated from the average P-wave velocities measured in oceanic gabbros from ODP Holes 735B, 894G, and 923A over a range of effective pressures appropriate to the oceanic crust (Iturrino et al., 1991, 1996; Miller and Christensen, 1997) and a set of profiles showing the variation of velocity with depth in partially serpentinized ultramafic rocks as a function of serpentinization. The latter profiles were estimated from the measured properties of ultramafic rocks reported by Christensen (1966) and Miller and Christensen (1997).
Of note is the fact that the East Pacific Rise 9°N profile is a close match to the gabbro profiles, indicating that "typical" seismic profiles are consistent with a lower crust composed of gabbroic rocks, whereas the seismic profiles from 16°N and 15°37´N do not match the gabbro profiles. Instead, these profiles show a gradual and continuous increase of velocity with depth, from ~3 km/s at the top of the crust to 8.0–8.1 km/s, velocities typical of the upper mantle at depths of 5 and 7.5 km.
Whereas the seismic profiles from 16°N and 15°37´N are not consistent with a lower crust composed of comparatively unaltered oceanic gabbros, they can be readily interpreted in terms of the measured seismic properties of partially serpentinized peridotites. At the top of the crust, the in situ seismic velocities are lower than the velocities in completely serpentinized rocks, suggesting that the uppermost 0.5–1.2 km of the crust contains large-scale fractures. This inference is consistent with both our experience of drilling at Site 1268 and with the character of the rocks, which have been heavily altered by hydrothermal circulation through a network of fractures. At comparatively shallow depths the seismic profiles reach the 100% serpentinization contour. Thereafter, the seismic profiles suggest a progressive decrease in proportion of cracks and/or the degree of serpentinization from 100% near the top of the section to fresh ultramafic rocks at depths of 5 and ~7.5 km, where the seismic profiles reach velocities characteristic of the upper mantle; the fact that both seismic profiles lie near the 0% serpentinization contour below the point where they reach ~8 km/s lends support to this interpretation. Variably altered gabbroic rocks, such as those recovered in Hole 1268A, could also form a substantial fraction of the crust, with their degree of alteration and/or their proportion relative to peridotite decreasing downhole.
The seismic properties of the rocks recovered from Hole 1268A are consistent with this interpretation. The average P-wave velocity in the 38 samples measured is 3.84 ± 0.08 km/s. The mean for the 30 serpentinites is 3.69 ± 0.07 km/s, and the mean for the 8 altered gabbros is 4.35 ± 0.16 km/s. These averages are shown in Figure F90, where we note that the average for the whole data set and the average for the serpentinites lie on or near the 100% serpentinization contour, whereas the mean for the gabbros is significantly higher. These observations have two important implications for the interpretation of the seismic profiles. One is that the seismic properties of the rocks recovered from Hole 1268A are arguably consistent with the seismic structure of the crust. The other is that the seismic velocities near the seafloor are consistent with a crust composed of serpentinized peridotite with a small proportion (up to 25%) of altered gabbros.