At Site 1253, laboratory measurements were made to provide a downhole profile of physical properties at a reference site seaward of the subduction trench, as well as within igneous units not penetrated during ODP Leg 170. All cores were initially passed through the MST before being split. Gamma ray attenuation (GRA) bulk density, noncontact electrical resistivity (NCR), and volumetric magnetic susceptibility measurements were taken at 2-cm intervals, with measurements averaged from three separate 1-s data acquisitions for all cores. P-wave velocity logger measurements were not taken because of the small and variable diameter of RCB cores, which generally leads to poor coupling between the core liner and recovered core. NGR emissions were counted every 10 cm for 20-s intervals above 519.20 mbsf (Core 205-1253A-27R), after which the count time was increased to 60 s to improve data quality within the igneous units. Voids and cracks in hard rock and drilling disturbance in sediment were noted in all cores and degraded the volumetric magnetic susceptibility and GRA bulk density MST measurements. The NCR tool is still in a testing phase (see "Physical Properties" in the "Explanatory Notes" chapter). Data collected with the instrument exhibited significant scatter, and measured resistivity values were consistently and unreasonably low for both sediments and hard rock compared with measurements from Leg 170, suggesting a problem with the instrument or its calibration. These data will not be discussed further.
Moisture and density (MAD) samples were selected from undisturbed core at regularly spaced intervals of two per section in sediments (75-cm resolution) between 370-400 and 430-454 mbsf. In igneous rock, pieces were selected from intact core at a frequency of one per section and soaked in seawater for 24 hr prior to sampling for MAD and measurement of P-wave velocity and thermal conductivity, to ensure that the samples were fully saturated. Thermal conductivity was measured using the half-space needle probe for soft and indurated sediments and hard rock. After soaking, thermal conductivity was measured on hard rock pieces, which were subsequently sampled for MAD and used for P-wave velocity measurement. Measurements of dry volume and wet and dry mass were uploaded to the ODP Janus database and were used to calculate water content, bulk density, grain density, porosity, void ratio, and dry bulk density. P-wave velocities were measured at a frequency of one per section on core pieces in the x-direction (cross-core) using the P-wave velocity sensor system. In sediments, P-wave measurements were taken immediately adjacent to MAD samples. In igneous rock, measurements were made on the same intact piece that was sampled for MAD and thermal conductivity, usually immediately adjacent to the MAD sample location.
Raw and calculated physical property data are available from the Janus database for all MST, MAD, velocity, thermal conductivity, and P-wave velocity measurements.
Sediment porosities determined using the MAD method generally range from 66% to 77%; bulk densities range from 1.39 to 1.77 g/cm3 and show no trend with depth between 370 and 394 mbsf (Fig. F57A, F57B). Porosity values appear to show greater scatter, including some lower values (range from ~62% to 74%) immediately above the upper igneous unit between 394 and 396 mbsf and within sediments between the two igneous units (432-451 mbsf). Data coverage in these intervals is limited by poor recovery and also reflect that obviously baked sediments were not sampled for MAD because of their importance for other analyses. More data would be needed to verify whether these subtle differences in porosity reflect systematic changes in sediment properties. Porosity within the igneous units is significantly lower than that of sediments, ranging from 2% to 8%, and bulk densities are 2.77-2.97 g/cm3. Porosity within the upper igneous unit decreases from 8% near its upper boundary to ~2%-4% at its base (Fig. F57C). Bulk density within the upper igneous unit does not vary systematically with depth and ranges from 2.83 to 2.88 g/cm3. Within the lower igneous unit, porosity decreases systematically with depth, from ~6%-8% near its upper boundary to 1% at its base (Fig. F57C). This decrease in porosity coincides with a systematic increase in bulk density with depth from 2.77 g/cm3 at 451 mbsf to 2.97 g/cm3 at 598 mbsf (Fig. F57D).
GRA densities range from ~1.2 to 1.7 g/cm3 in sediments and from ~2.3 to 2.8 g/cm3 in igneous units (Fig. F58). The GRA densities show considerable scatter, mainly because of varying RCB core diameter and voids and fractures in the cores. In general, there is less scatter in the data from the igneous units because voids were easier to identify for these cores, in which the liner was split before runs, than for sediment samples. The calculated values of GRA densities assume a core diameter of 6.6 cm, whereas RCB cores have a smaller diameter. Therefore, GRA densities were corrected assuming a constant core diameter of 5.6 cm by multiplying the reported bulk densities by a factor of 6.6/5.6. Corrected GRA densities within the upper and lower igneous units follow the same trends as densities and porosities determined using the MAD measurements (Fig. F58).
Grain densities within the sediments, determined from dry mass and volume measurements, range from 2.45 to 2.75 g/cm3 and exhibit significant scatter both above the upper igneous unit and in the zone between the two igneous units (Fig. F59). Grain densities immediately above the upper igneous unit (394-396 mbsf) are 2.75-2.8 g/cm3, corresponding to a zone of slightly lower porosity. Grain densities within the sediments between the two igneous units are also slightly higher than those in the uppermost sediments (370-394 mbsf), ranging from 2.59 to 2.78 g/cm3. These high grain densities may represent recrystallization as a result of intrusion of the upper igneous unit, although denser sampling would be needed to verify this trend. Grain densities within the igneous units exhibit less scatter and are uniformly higher than in the sediments, ranging from 2.85 to 3.0 g/cm3 in the upper igneous unit and 2.88-3.01 g/cm3 in the lower unit (Fig. F59). Grain densities are uniform throughout the upper igneous unit, whereas in the lower unit there is a systematic increase in grain density with depth from 2.88-2.96 g/cm3 near its top to 2.98-3.01 g/cm3 at its base. This trend, combined with decreasing porosity with depth in this interval, results in a trend of increasing bulk density within the lower igneous unit.
On the first three cores (370-385 mbsf), thermal conductivity was determined using the full-space needle probe method. Erroneous values of thermal conductivity were measured when the liner was not completely filled with sediments and the needle did not fully penetrate in the sediments or if the insertion of the needle produced cracks in the sediments that could not be seen through the core liner. Moreover, with increasing depth it became increasingly difficult to insert the needle into the stiff sediments. The half-space method was used for all cores below 385 mbsf to ensure consistency with half-space measurements made in the igneous units. The half-space method was also used to remeasure all points in Cores 205-1253A-1R through 3R for this reason.
Comparison between full-space and half-space measurements shows that the half-space method systematically yields higher thermal conductivity values than the full-space method. The deviations range from 0.1 to 0.2 W/(m·K). This could be explained by air in the liner or by cracks induced by penetration of the needle, both of which would produce a lower conductivity reading than that of the undisturbed sediment. In Cores 205-1253A-1R and 2R, thermal conductivities are ~0.9-1.0 W/(m·K) (Fig. F60). Core 205-1253A-3R was too disturbed to measure with the half-space method, so the full-space method value is shown in the plot at 385 mbsf. The thermal conductivity of 0.87 W/(m·K) measured in this core would probably be ~0.1-0.2 W/(m·K) higher if determined by the half-space method.
The data clearly show a distinction between the sedimentary unit and the igneous units (Fig. F60). The sediments close to the contact with the upper igneous unit at ~400 mbsf have higher conductivity than those above this zone (values up to 1.45 W/[m·K]). Within the upper igneous unit (401-432 mbsf), thermal conductivities range from 1.68 to 1.95 W/(m·K). Because the sediments between the two igneous units (432-451 mbsf) were too disturbed and the core pieces too small, no measurements were made in this section.
Within the lower igneous unit (451-600 mbsf), a slight trend of increasing thermal conductivity with depth is observed but starts with a lower value than within the upper igneous unit. Thermal conductivities increase from 1.71-1.87 W/(m·K), near the top of this unit, to 1.83-1.93 W/(m·K) at its base. This trend correlates with the observed increases in density and P-wave velocity and the decrease in porosity with depth as determined by MST and MAD measurements. Because the conductivity of the grains is higher than that of the pore fluid, a decrease in porosity leads to an increase in thermal conductivity.
P-wave velocities in the uppermost sediments, from 370 to 394 mbsf, range from 1549 to 1577 m/s under laboratory temperatures and pressures (Fig. F61A). A small number (five) of velocities measured within sediments immediately above the upper igneous unit (394-397 mbsf) and between the two igneous units (432-451 mbsf) range from 1603 to 1914 m/s. These higher velocities correspond to zones of slightly lower porosity and higher grain density. Velocities are relatively uniform throughout the upper igneous unit, ranging from 4728 to 5345 m/s. Within the lower igneous unit, velocities increase with depth from 4552-5207 m/s near its top to 5438-5677 m/s at its base (Fig. F61A, F61B). There is a strong correlation between the decrease in porosity and the increase in velocity within this unit (Fig. F61C).
NGR results are presented in counts per second (cps) (Fig. F62). The background, produced by Compton scattering, photoelectric absorption, and pair production, was measured at the beginning and during MST runs for each core section (12.95 cps) and subtracted from the measured gamma ray values to obtain corrected counts. In addition, the NGR data were filtered after acquisition to improve data quality by discarding measurements obtained from hard rock fragments smaller than the instrument aperture of 12 cm. The filtering was conducted visually, using digital core images and manually filtering the data to exclude any NGR measurement taken from a core piece with <6 cm of unfractured material in either direction from the measurement point. The Janus database was not edited and contains all measurements.
The filtered NGR data show a clear distinction between the upper igneous unit (401-432 mbsf) and upper and lower parts of the lower igneous unit (451-600 mbsf) (Fig. F62). The upper unit is characterized by generally higher NGR emissions (7 cps) than the lower unit (1-3 cps). Within the lower unit, NGR values are systematically lower from 451 to 512 mbsf (averaging 1-2 cps, with local excursions to 5-6 cps) than from 512 to 600 mbsf (averaging 3-4 cps, with excursions to >10 cps). Increased NGR emissions typically reflect higher potassium, uranium, or thorium concentrations, suggesting that the upper and lower igneous units are chemically distinct, as are the upper 61 m and lower 88 m of the lower igneous unit.
Volumetric magnetic susceptibility measured with the MST shows a clear difference between sediments and the igneous units (Fig. F63). Within the igneous units, values are widely scattered, due at least in part to the variable diameter of RCB cores. There are no unequivocal trends in magnetic susceptibility within the sediments or the igneous units. Below 510 mbsf, magnetic susceptibility is generally slightly higher than above this depth, although the scatter throughout the igneous units is significant. This change in susceptibility occurs at the same depth as a clear shift in NGR. A second shift from high values of magnetic susceptibility to generally lower values occurs at 567 mbsf. This shift does not correspond to changes in any other measured physical properties.
Variations in physical properties correlate with major lithologic changes between sediments and igneous units. A limited number of measurements indicate decreased porosity and increased grain density and P-wave velocity within sediments immediately above and between the igneous units; these differences may reflect alteration (recrystallization) and porosity reduction caused by emplacement of the igneous units.
Perhaps the most striking trends in the physical property data are (1) the systematic increase in velocity, bulk density, and grain density and decrease in porosity within the lower igneous unit and (2) the clear shift in NGR emissions at 512 mbsf within the lower igneous unit. The cause of the trends in porosity, density, grain density, and velocity with depth in the lower igneous unit is unclear. The differences in NGR emissions suggest chemical differences between and within the igneous units, which may reflect varying degrees of alteration within igneous units that were initially chemically similar. The fact that the trends in porosity, density, and velocity are not correlated with the NGR trend suggests that the processes that control porosity, density, and P-wave velocity are separate from the chemical or lithologic processes that affect the NGR emissions.