The physical properties of the peridotites and gabbros cored in Hole 1272A 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 multisensor track (MST) system and thermal conductivity, compressional wave velocity, density, and porosity. The rock names reported in data tables correspond to the primary lithologies determined by the igneous core description group. The data are summarized, as a function of depth, in Figure F61.
All cores recovered during Leg 209 were measured for 30 s using the NGR logger on the MST at intervals of 10 cm. Results are output in counts per second and are shown in Figure F62. The cores from Hole 1272A mostly display natural radioactivity in the same range as the background radiation in the core laboratory on board the JOIDES Resolution. One peak >10 cps was recorded in the breccia recovered in Section 209-1272A-5R-1.
Magnetic susceptibility values were acquired on the MST at 2.5-cm intervals for all recovered cores. Core from Hole 1272A has high magnetic susceptibility (Fig. F62), which is related to the presence of magnetite in the recovered rocks. The magnetic susceptibilities of the peridotites at Site 1272 are comparable to those at Sites 1270 and 1271 (see Fig. F105 in the "Site 1270" chapter, and Fig. F65 in the "Site 1271" chapter). They are also comparable to the magnetic susceptibilities of the peridotites from Hess Deep (Gillis, Mével, Allan, et al., 1993) and from the MARK area (Cannat, Karson, Miller, et al., 1995) (see Fig. F88 in the "Site 1268" chapter).
Thermal conductivity measurements were made at irregularly spaced intervals along the Hole 1272A gabbro, diabase, and peridotite cores. The data are summarized in Table T5. The thermal conductivities of the peridotite samples range 1.98–2.23 W/(mˇK) (mean = 2.09 W/[mˇK]); the thermal conductivity of the gabbros and diabases range 1.71–1.97 W/(mˇK) (mean = 1.88 W/[mˇK]). These values are lower than the thermal conductivities of peridotite and gabbroic rocks from ODP Holes 735B, 894G, and 923A at Atlantis Bank (Robinson, Von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999), Hess Deep (Gillis, Mével, Allan, et al., 1993), and MARK (Cannat, Karson, Miller, et al., 1995). They are also lower than the values measured at Sites 1269, 1270, and 1271 (Fig. F63). The low thermal conductivities of the peridotites are probably related to their high degree of serpentinization, as is also shown by their relatively low velocities and densities (see below).
As described in "Thermal Conductivity" in "Physical Properties" in the "Explanatory Notes" chapter, measurements were taken in three directions on the cut face of the archive sample half, whenever possible. The purpose of these measurements was to determine the degree of apparent anisotropy. The apparent thermal conductivity anisotropy of peridotites, gabbros, and diabases measured in cores from Site 1272 ranges 0.1%–7.1% (Fig. F61; Table T5). Apparent thermal conductivity anisotropies measured from the beginning of Leg 209 (including Sites 1268, 1270, 1271, and 1272) are compiled in Figure F64. The apparent thermal conductivity anisotropy ranges 0.1%–12.6% (mean = 4.23%) in gabbros, diabases, troctolites, and peridotites.
Many of the core pieces in which we measured thermal conductivity were also sampled for measurements of porosity, density, velocity, and magnetic susceptibility. In Figure F65, mean values of thermal conductivity are plotted against bulk density for all Leg 209 sites through Site 1272, together with reference single crystal and monomineralic rock data (Clark, 1966; Clauser and Huenges, 1995). As expected from their high degree of alteration, the conductivities of the Leg 209 peridotites and troctolites are close to those of serpentine and talc. Gabbro and diabase values are similar to values reported for anorthite and anorthosite.
Bulk density, grain density, and porosity were measured on small sample chips (~3–6 cm3) from Hole 1272A. P-wave velocity and wet bulk density were measured in cube samples, as described in "P-Wave Velocity," and "Porosity and Density" in "Physical Properties" in the "Explanatory Notes" chapter. These data are summarized in Table T6.
In Figure F66, the density and velocity data are compared with data from Legs 147 and 153, as well as Sites 1268, 1270, and 1271 (see "Porosity, Density, and Seismic Velocity" in "Physical Properties" in the "Site 1268" chapter, "Porosity, Density, and Seismic Velocity" in "Physical Properties" in the "Site 1270" chapter, and "Porosity, Density, and Seismic Velocity" in "Physical Properties" in the "Site 1271" chapter). Apparent compressional wave velocity anisotropy in the serpentinized peridotite samples from Site 1272 (1.1%–11.8%) is comparable to the anisotropy in samples from Sites 1268, 1270, and 1271. Similarly, the apparent compressional wave velocity anisotropy in the gabbro and diabase samples from Site 1272 (0.7%–8.2%), is comparable to the anisotropy in samples from Sites 1268, 1270, and 1271.
Velocities and densities in the peridotite samples from Site 1272 are lower than the densities and velocities of the ultramafic samples from Sites 1268, 1270, and 1271 and much lower than the velocities and densities of samples from Legs 147 (Gillis, Mével, Allan, et al., 1993) and 153 (Cannat, Karson, Miller, et al., 1995) (Figs. F61, F66). P-wave velocity and density, as well as thermal conductivity (Fig. F61), tend to increase downhole in the lower two recovered cores, pointing to progressively decreasing alteration at the bottom of the hole. The P-wave velocities and densities of the gabbro and diabase samples are comparable to the properties of gabbros from Leg 153 and to those of gabbros from the previous Leg 209 sites, except for the oxide-rich gabbros of Hole 1270B.
Densities and velocities in discrete samples from Site 1272 are compared to the downhole density and sonic velocity logs in Figure F72 (see "Downhole Measurements"). There is excellent agreement between the velocities in laboratory samples and the sonic log and good agreement between the densities determined from chip samples and the density log, whereas the densities estimated from the wet masses and dimensions of the cube samples are systematically lower than the wireline logging densities. Seismic velocities are particularly sensitive to thin (low porosity) cracks in the medium, whereas equant voids affect the density of the medium. Hence, the agreement between the laboratory data and the downhole logs indicates that the properties of the formation are not greatly affected by pervasive large-scale cracks or voids and that the samples may be taken as representative of the cored interval.
The discrepancy between the densities of the cube samples and the downhole logs led us to review the laboratory procedure. There are two potential sources of systematic error in these measurements. One is that some of the cube samples have small chips, particularly on their corners, that will cause the calculated volumes to be larger than the real volumes of the sample—but chips should affect only the densities of samples that are chipped, not the entire data set. The other possible source of error is systematic error in the measured dimensions of the cubes. We used the Hamilton Frame velocimeter (PWS3) caliper to measure the dimensions of the cubes, which, though not strictly cubic, are all quite regular, right-rectangular parallelepipeds.
We tested several calipers, including PWS3, by measuring a machinist's standard (with steps of 0.5, 0.4, 0.3, 0.2, and 0.1 in) and a set of polycarbonate standards (with lengths of 20, 30, 40, and 50 mm). The results are summarized in Table T7. Of the several devices, calipers 1 and 2 show no statistically significant error, caliper 3 shows a slight systematic error, and PWS3 exhibits a comparatively large systematic error. Measurements made on the polycarbonate standards, which are used to calibrate the PWS3 caliper, suggest that the true lengths of the cylinders differ from their reported lengths.
Having determined that caliper 1 is not a source of systematic error, we remeasured all cubes from holes at Sites 1268, 1270, 1271, and 1272 to make better estimates of their densities, which we compare to the previous measurements made using PWS3 in Table T8. We find that the previous measurements are too low by ~1.6% on average. The tables and figures of these chapters as well as those of the "Physical Properties" sections in previous site chapters (Sites 1268, 1270, and 1271) were updated to take these revised density estimates into account.