The physical properties of the gabbros, troctolites, and diabases cored in Holes 1275B and 1275D 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 assigned by the igneous group. The data are summarized as a function of depth in Figures F80 and F81.
All cores recovered during Leg 209 were measured using the NGR logger on the MST at intervals of 10 cm for a period of 30 s. Results are output in counts per second. The cores from Hole 1275B do not show natural radioactivity significantly higher than background (mean = 2.02 cps); the two highest peaks (~10 cps) (Fig. F82) correspond to gabbro pieces that, macroscopically, do not differ significantly from the surrounding rocks. NGR values for cores from Hole 1275D are in the same range as those in Hole 1275B (mean = 2.07 cps) and counts in the range 8–16 cps from six veined gabbros and troctolites (Fig. F82).
Magnetic susceptibility values were acquired on the MST at 2.5-cm intervals for all recovered cores. The variation of magnetic susceptibility with depth in Holes 1275B and 1275D is shown in Figure F83. Magnetic susceptibility is comparatively low at the top of Hole 1275B and increases gradually to ~0.05 SI in Section 209-1275B-6R-1 (~28.5 mbsf). From that depth, magnetic susceptibility then remains high (0.01–0.1 SI) from 28.5 mbsf to the bottom of the hole. These high magnetic susceptibility values reflect the high oxide content of the gabbros. The gabbros from the upper part of Hole 1275B display lower magnetic susceptibility than those recovered from Hole 735B during Leg 176 (Dick, Natland, Miller, et al., 1999) and Leg 179 (Pettigrew, Casey, Miller, et al., 1999) (Fig. F84). The oxide-rich gabbros of the lower part of Holes 1275B and 1275D are similar to the oxide-rich gabbros from Hole 735B (Dick, Natland, Miller, et al., 1999; Pettigrew, Casey, Miller, et al., 1999), with magnetic susceptibility ranging 0.01–0.1 SI (Fig. F83) (Natland, 2002) or higher, as shown by the discrete sample measurements (see "Paleomagnetism"). On the Bartington MS2C sensor, all readings >0.1 SI are clipped such that higher values are recorded without the first digit. Thus, the highest magnetic susceptibilities of oxide-rich gabbros recorded are ~0.1 SI. The magnetic susceptibilities of oxide-rich gabbros in Holes 1275B and 1275D are similar to those of the oxide-rich gabbros recovered from Hole 1270B (Fig. F105 in the "Site 1270" chapter).
Thermal conductivities were measured in gabbro, troctolite, and diabase samples taken at irregularly spaced intervals along Holes 1275B and 1275D. The data are summarized in Table T5. The thermal conductivities of the gabbro and diabase samples range 1.97–3.35 W/(m·K) (mean = 2.15 W/[m·K]). These values are similar to those measured at previous Leg 209 sites. The thermal conductivities of the troctolite samples from Holes 1275B and 1275D range 3.06–3.83 W/(m·K) (mean = 3.32 W/[m·K]). All of the thermal conductivity values measured during Leg 209 are shown in Figure F85 and are compared to samples from ODP sites at 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).
As described in "Thermal Conductivity" in "Physical Properties" in the "Explanatory Notes" chapter, wherever possible, measurements were taken in three directions on the cut face of the archive half of the core. The purpose of these measurements was to determine the degree of apparent anisotropy. The apparent thermal conductivity anisotropy of the diabases, gabbros, and one troctolite sample measured in cores from Hole 1275B is not very strong (0.3%–5.8%) (Fig. F80; Table T5). (Anisotropy measurements were not made on samples from Hole 1275D, due to the lack of time at the end of the cruise.) Apparent thermal conductivity anisotropies measured during Leg 209 (including Sites 1268, 1270–1272, 1274, and 1275) are compiled in Figure F86. The apparent thermal conductivity anisotropy ranges 0.1%–12.6% (mean = 3.92%) in diabases, gabbros, 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. Mean values of thermal conductivity are plotted against bulk densities in Figure F87 for Leg 209 sites 1268, 1270–1272, 1274, and 1275, 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. Thermal conductivities in gabbro and diabase samples 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 Holes 1275B and 1275D. 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.
The variations of P-wave velocity, seismic anisotropy, bulk density, and porosity in gabbros and troctolites with depth in Holes 1275B and 1275D are shown in Figures F80 and F81. Through Core 209-1275B-6R (~30 mbsf), the physical properties of these rocks are variable. The bulk densities and porosities of the three troctolite samples are lower than those of the gabbros from higher in the hole. Apparent compressional wave velocity anisotropy in the upper part of Hole 1275B is in the range 4%–10%. Below Core 209-1275B-6R, the properties of the gabbros are less variable. Bulk densities are generally in the range 3.1–3.2 Mg/m3, porosities are commonly 1.5%–2.5%, velocities are mostly 4.7–5.4 km/s (mean = 5.2 km/s), and the apparent seismic anisotropy is generally <2.5% (mean = ~2%).
P-wave velocities in troctolite samples from Cores 209-1275D-2R to 10R (10–50 mbsf) from Hole 1275D (Fig. F81) are <4.8 km/s, and the velocity anisotropy is <5%. Densities of these samples are in the range 2.6–2.9 Mg/m3, and porosities range as high as 5%. The transition between the troctolite in Core 209-1275D-10R (47 mbsf) and the gabbros in Core 12R (56 mbsf) is an abrupt increase in bulk density from <2.6 to ~2.9 Mg/m3 and a modest increase in P-wave velocity. From 55 mbsf to the bottom of the hole at 209 mbsf, P-wave velocities of gabbros remain in the range 5–5.5 km/s (mean = ~5.3 km/s) and the degree of anisotropy is low, generally <2%. Bulk densities reach a maximum of 3.0–3.1 Mg/m3 between 110 and 120 mbsf then decrease to 2.9–3.0 Mg/m3 between 130 and 209 mbsf. Porosities in the lower part of the hole range from near 0% to 2.5% (mean = 1.6%).
The density and velocity data for the gabbros and troctolites recovered from Holes 1275B and 1275D are compared with data from Legs 147 and 153, as well as Leg 209 Sites 1268, 1270–1272, and 1274 (see "Physical Properties" sections in the Sites 1268, 1270–1272, and 1274 chapters) in Figure F88. The troctolites have velocities and densities comparable to those of samples from Sites 1268 through 1274 but lower than the densities of gabbroic rocks from Leg 153 Site 923 (Cannat, Karson, Miller, et al., 1995). A few of the gabbros recovered from Holes 1275B and 1275D have densities and velocities comparable to the lowest values reported from Hole 923A, but most have lower velocities and higher densities (>3 Mg/m3). These properties reflect the relatively high oxide contents of the gabbros from Holes 1275B and 1275D.
As suggested by Toft et al. (1990), the magnetic susceptibility of serpentinized peridotites should be inversely correlated with their density because of the increasing contents of magnetite and Fe-bearing brucite and serpentinite during multireaction serpentinization. If so, petrophysical data can potentially be used as a proxy for alteration of abyssal peridotites. This working hypothesis can be tested using magnetic susceptibility and density measurements made on cube and minicore samples taken during Leg 209. The results are shown in Figure F89 and compared to data from the Josephine ophiolite (Toft et al., 1990), the Oman ophiolite (B. Ildefonse, unpubl. data), Site 895 (Gillis, Mével, Allan, et al., 1993), and Site 920 (Cannat, Karson, Miller, et al., 1995). Exponential regression lines are plotted through data from each hole (1268A, 1270A, 1270C, and 1270D), except for those holes for which the data points are too scattered. Regression lines fit to the samples from Holes 1271A and 1271B have slopes similar to those fit to data from the Josephine ophiolite and Site 920 (from the MARK area) and close to the fit to samples from the Oman ophiolite. The regression lines for Holes 1272A and 1274A have steeper slopes, similar to the fit to data from Site 895 (Hess Deep). These two families of trends may point to two different types of serpentinization processes.
The positions of sample points in Figure F89A and F89B also depend on the porosity of the peridotites as we plot the bulk densities. This problem can be avoided by plotting the magnetic susceptibilities against grain densities (Fig. F89C). The Hole 1272A samples, which are the most altered peridotites recovered during Leg 209, plot close to the Hole 1274A samples. However, the data from Hole 1272A are more scattered in this plot and do not show a good correlation between magnetic susceptibility and density. Although we do not have a clear explanation for this phenomenon, it may be due to a larger imprecision in grain density estimates in these very highly altered peridotites, possibly related to the problems experienced in measuring the volumes of highly serpentinized peridotites in the helium pycnometer (see "Physical Properties" in the "Site 1268" chapter).
The peridotite samples from Hole 1268A do not show any inverse correlation between magnetic susceptibility and density (Fig. F89A, F89B). This lack of correlation is probably due to the presence of distinct alteration products in the serpentinized peridotites of this core, with decreasing magnetite and increasing talc contents ("Metamorphic Petrology" in the "Site 1268" chapter), leading to a decrease of magnetic susceptibility while the density is little affected. This type of vertical trend in a magnetic susceptibility vs. density diagram may be indicative of the unusual talc-rich alteration encountered in Hole 1268A.
Data for the gabbros sampled during Leg 209 are plotted in Figure F89D. The samples from Holes 1270B, 1275B, and 1275D show positive correlations between magnetic susceptibility and bulk density that probably reflect variable oxide content. Increasing oxide proportions result in increasing magnetic susceptibility and density of the gabbros.