The physical properties of the peridotites cored in Hole 1274A 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, 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 F55.
All cores recovered during Leg 209 were measured using the NGR logger on the MST at 10-cm intervals with a time period of 30 s. Results are output in counts per second and shown in Figure F56. The cores from Hole 1274A display natural radioactivity in the same range as the background radiation in the core laboratory on board the JOIDES Resolution.
Magnetic susceptibility values were acquired on the MST at 2.5-cm intervals for all recovered cores, except Core 209-1274A-28R. The variation of magnetic susceptibility with depth in Hole 1274A is shown in Figure F56. Magnetic susceptibility is comparatively low at the top of the hole and increases gradually to a maximum near 0.1 SI in Core 209-1274A-20R (~103 mbsf). Magnetic susceptibility then decreases to the bottom of the hole (Core 209-1274A-27R; Core 28R was not measured with the MST). This trend is correlated with the observed increase of alteration downhole toward a fault zone at ~100–130 mbsf (Fig. F30). Therefore, it is probably related to a gradual increase of the abundance of magnetite in the upper 100 m of Hole 1274A followed by diminishing magnetite content below 130 mbsf.
Measurements of thermal conductivity in peridotite samples from Hole 1274A were made at irregularly spaced intervals. The data are summarized in Table T6. The thermal conductivities of the peridotite samples range 2.1–3.0 W/(m·K) (mean = 2.51 W/[m·K]). These values are similar to the thermal conductivities of peridotites from Sites 895 and 920 at Hess Deep (Gillis, Mével, Allan, et al., 1993) and the Kane Fracture Zone (Cannat, Karson, Miller, et al., 1995), respectively. They are also similar to the values measured at Sites 1269 through 1272 (Fig. F57).
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 the peridotites measured in cores from Site 1274 ranges 0.8%– 8.6% (Fig. F55; Table T6). Apparent thermal conductivity anisotropies measured since the beginning of Leg 209 (including Sites 1268, 1270, 1271, 1272, and 1274) are compiled in Figure F58. The apparent thermal conductivity anisotropy ranges 0.1%–12.6% (mean = 4.23%) in gabbros, diabases, basalts, 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 F59, mean values of thermal conductivity are plotted vs. bulk density for all Leg 209 sites through 1274, 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 1274A. 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 T7.
The density and velocity data are compared with data from Legs 147 and 153, as well as Sites 1268, 1270, 1271, and 1272 (see "Physical Properties" in the site chapters for Sites 1268, 1270, 1271, and 1272) in Figure F60. Apparent compressional wave velocity anisotropy in the serpentinized peridotite samples from Site 1274, which ranges 0.2%– 7.0%, is comparable to the anisotropy in samples from Sites 1268, 1270, 1271, and 1272.
Bulk densities in the peridotites from Hole 1274A range 2.48–2.78 Mg/m3, whereas the average P-wave velocities range 3.82–5.14 km/s. The lowest velocities and densities in the peridotite samples from Site 1274 overlap the highest densities and velocities from Sites 1268, 1270, 1271, and 1272 and are comparable to the densities and velocities of peridotite samples from Hole 920B from Leg 153 (Cannat, Karson, Miller, et al., 1995) (Fig. F60). An interesting feature of the data from Hole 1274A is that P-wave velocities and densities tend to decrease downhole while the bulk porosity increases, as shown in Figure F55. Between the top of the hole and 90 mbsf (Core 209-1274A-17R), densities decrease from >2.70 to ~2.5 Mg/m3, velocities decrease from 5 to <4 km/s, and porosities increase from ~2% to slightly more than 4%. These trends, together with the gradual increase of magnetic susceptibility with depth (Fig. F55), are consistent with the observed downhole increase in the intensity of alteration in the upper 100 m of Hole 1274A (Fig. F30) (see "Metamorphic Petrology").