= bulk density.All whole-round core samples from Hole 1200A longer than 40 cm were run through the multisensor track (MST). RCB drilling in Hole 1200A recovered mainly hard rock sections with a diameter of ~5.75 cm, smaller than the nominal 6.6-cm core diameter. The reduced core diameter required corrections of the values measured by the MST. The values in the database do not reflect these corrections, but the data in the figures presented below have been corrected.
The natural gamma ray (NGR) count rates were corrected for the reduced sample volume using the following equation:
NGRcorr = [(6.6)2/(5.75)2] x NGRmeas.Gamma ray attenuation (GRA) bulk density values were corrected for the smaller core diameter using the following equation:
corr
= (6.6/5.75) x meas,
where
= bulk density.
Magnetic susceptibility (MSL) measurements were made on the MST using the archive half of all of the split core sections from Hole 1200A at 2-cm sample intervals. The following equation is used to convert measured values from the MSL into SI volume susceptibility; a correction factor of 0.46, provided by the instrument manufacturer (Bartington), is used to account for the reduced core volume. Bartington provides correction factors that assume the sample is in the form of a cylinder. A Bartington correction factor of 0.46 assumes that the split core section has the same magnetic susceptibility as a cylinder with half the cross-sectional area of a full core with a diameter of 5.75 cm:
Mcorr = (Mmeas/0.46) x 10-5,where M = magnetic susceptibility.
The P-wave velocity logger (PWL) provided no sensible data in Hole 1200A because of the lack of acoustic coupling between the core and the liner.
Results from the corrected MST runs are presented in Figure F56. Index and physical properties measurements, made using discrete samples of clasts and, where possible, matrix material, are presented in Tables T12 and T13.
Volume magnetic susceptibility values derived from the MSL (Fig. F56A) show wide scatter, >1000 x 10-5 (SI units) along the length of most core sections. The scatter in susceptibility values could be the result of gaps in the core, which are not corrected for in our determination of volume magnetic susceptibility. These gaps will reduce the volume magnetic susceptibility. The scatter is significantly lower in measurements from Cores 195-1200A-6R (41.40-51.10 mbsf), 7R (51.10-60.70 mbsf), and 16R (137.60-147.20 mbsf), which exhibit lower susceptibility values than other cores.
The mean bulk density of the clasts from Hole 1200A, derived from index properties measurements, is 2.52 g/cm3. Density measurements made with the GRA are consistently less than or equal to index properties bulk density measurements from the same depth interval (Fig. F56B). The discrepancy between these two measurements is likely caused by the presence of fractures in the core. The index properties samples were taken from unfractured sections of clast material. Fractures present in cores from Hole 1200A result in a reduced apparent bulk density. Density measurements from the GRA of <2.0 g/cm3 have been deleted from Figure F56B because it is unlikely that any igneous or metamorphic rock has a density lower than this value (Telford et al., 1976). The index properties bulk density values represent the true bulk densities and represent the maximum values that the GRA would measure on complete, unfractured whole-round core.
P-wave velocity was measured on discrete samples of clast material, cut into ~2-cm cubes with faces oriented perpendicular to the x-, y-, and z-axis of the core (Fig. F57). Measured velocities range from 3.80 to 5.48 km/s (mean = 4.89 km/s). For each sample, with the exception of Sample 195-1200A-11R-1, 57 cm (89.97 mbsf), the difference in the velocity measured along each axis is considered below the error level. Sample 195-1200A-11R-1, 57 cm (89.97 mbsf), exhibits a >10% variation in velocity between the velocities measured parallel to each axis. This sample, which exhibited a distinct fabric, was identified as dunite and was the only sample of this lithology to be tested.
Figure F58 shows a plot of P-wave velocity vs. density for clasts from Hole 1200A. As shown by many investigations (e.g., Miller and Christensen, 1997), P-wave velocity tends to increase with density.
Thermal conductivity measurements were made on every clast piece in every core section and, where possible, in the matrix (Fig. F59A). Thermal conductivity values from the clasts vary between 1.66 and 2.85 W/(m·K) (mean = 2.21 W/[m·K]) (Fig. F59). Only a single thermal conductivity measurement could be made in the matrix material from Hole 1200A, and this gave a value of 0.37 W/(m·K) at a depth of 127.9 mbsf. Thermal conductivity values for the clast material from Hole 1200A are, on average, slightly lower than those obtained at ODP Site 780 (Shipboard Scientific Party, 1990a).
MST measurements were performed on all whole-round core samples from Holes 1200D, 1200E, and 1200F. The results are presented in Figures F60, F61, and F62. Index and physical properties are presented in Tables T14, T15, and T16. APC core samples were affected by gas expansion within the core liner, forming gas voids up to 2 cm wide. The voids reduce magnetic susceptibility values by a small amount at a distance of ~10 cm on either side of the void. Removal of the effect of the voids proved difficult and largely unnecessary, and attempts to do so were abandoned. GRA density values are reduced almost to zero by the presence of a void, but only at a distance of up to ~2 cm from the void. The locations of wide (>0.5 cm) voids, visible through the core liner, were recorded and used to edit the data. Anomalously low density values (<1.0 g/cm3 or obviously outside the local trend) were eliminated from the GRA data set by hand.
Although the core liners were mostly full, the PWL was unable to obtain P-wave velocity measurements. Discrete P-wave velocity measurements using insertion transducers (PWS1 and PWS2) on split core samples were also unsuccessful. This is attributed to high signal attenuation, probably caused by the occurrence of small gas-filled fractures.
The volume magnetic susceptibility in Holes 1200D, 1200E, and 1200F generally increases with depth, from ~100 x 10-5 to ~1000 x 10-5 (mean = 779 x 10-5). However, there are distinct drops in susceptibility in Holes 1200E and 1200F at ~11 mbsf. The drop in susceptibility coincides with an increase in gamma ray counts in Hole 1200E. A similar anomaly is not observed at the same depth in Hole 1200F.
In addition to the MSL measurements, discrete volume magnetic susceptibility measurements were made on the same clast and matrix samples used for index properties measurements (Tables T14, T15). The volume magnetic susceptibility for discrete samples is calculated using the grain volume. A Bartington MS1B meter, calibrated to measure volume susceptibility for a 10-cm3 sample, was used to obtain three measurements for each sample, from which the mean was calculated. The susceptibility values are corrected for the dry sample volume (Vsamp) using the following equation:
Mcorr = (10/Vsamp) x Mmeas,where M = volume magnetic susceptibility.
The volume magnetic susceptibility of discrete matrix samples varies between 255 x 10-5 and 2541 x 10-5 (mean = 1347 x 10-5). The clast material exhibits a wider range of magnetic susceptibilities than the matrix, between 30 x 10-5 and 13,738 x 10-5 (mean = 2220 x 10-5). The magnetic susceptibility of discrete matrix samples is higher than the whole-core susceptibility. Whole-core measurements include a water volume of ~27%, which contributes little to the magnetic susceptibility; the MST volume magnetic susceptibility values should be increased by nearly 40% to be comparable to the values obtained for discrete matrix samples that contain no water.
Although there may be a systematic relationship between magnetic susceptibility and clast lithology (Table T14), the number of samples for many lithologies is too small for meaningful statistical analysis. However, the distribution of matrix and clast susceptibilities (Fig. F63) shows that, with the exception of a few clasts with extremely high magnetic susceptibilities, the dominant carrier minerals of the magnetic susceptibility are concentrated in the matrix material of the serpentine muds. This is consistent with shipboard observations of increased magnetite along fragile serpentine-filled veins in the clasts from Site 1200 (see "Structural Geology").
Bulk density measured by the GRA densitometer increases from ~1.75 at the seafloor to ~2 g/cm3 at 50-60 mbsf. The mean bulk density derived from index properties measurements on matrix material is 1.87 g/cm3, consistent with the whole-core bulk density measurements. The mean bulk density of clast material is 2.45 g/cm3, significantly higher than the matrix density. The grain density derived from index properties samples is similar for both clast and matrix samples (Fig. F64). The mean grain density for both the clast and matrix material is the same (2.64 g/cm3), which is consistent with the matrix being formed by the mechanical deformation of clast material.
Natural gamma ray emissions are nearly constant for all holes at ~15 counts per second. However, there is one notable gamma ray anomaly around 11-17 mbsf in Hole 1200E that coincides with a carbonate interval (see "Lithostratigraphy"); the top of this interval coincides with a drop in magnetic susceptibility in Holes 1200E (Fig. F61) and 1200F (Fig. F62).
The shear strength of the matrix material was measured once per section on the working half within 30 min of splitting the core. The measured peak shear strengths exhibit wide scatter (Fig. F65). A weak zone at a depth of 11-12 mbsf coincides with the low magnetic susceptibility and high gamma ray count shown in Figure F61. A mean value of 52.5 kPa is determined from all measurements. Disturbance from XCB coring and core splitting may explain the anomalously low (<30 kPa) strength measured in the sample at 52 mbsf. Water used to wash the split core faces was observed in the core liner during these measurements.
Thermal conductivity measurements range from 1.04 to 1.54 W/(m·K) (mean = 1.32 W/[m·K]) and show little scatter (Fig. F66). There appears to be a small increase in thermal conductivity with depth, but it is poorly constrained because of the limited number of measurements.
The formation factor was determined from resistivity measurements made using a Wenner four-probe array with a 13.3-mm probe spacing. The probes were inserted to their full 15-mm length into the split core surface to provide consistent apparent resistivity measurements but not absolute values. Pore water was assumed to have the same electrolytic properties as surface seawater.
Formation factors range from 2.68 to 7.14 (mean = 3.97) (Fig. F67A), which is relatively high for sediments. The trend of the data shows that the formation factor increases slightly with depth, consistent with the decrease in porosity with depth (Fig. F67B). The increase in porosity at a depth of 11-12 mbsf is yet another indicator of the change in material properties that occurs at that depth.
Hydraulic conductivity (K) and specific storage (Ss) were determined for cylindrical samples of matrix material extracted from the whole-round core. We determine hydraulic conductivities between 1.1 x 10-11 and 2.8 x 10-10 m/s and specific storage values between 6.0 x 10-5 and 5.8 x 10-4 m-1 (Table T17), consistent with measurements using sediments with a similar grain size and distribution (Mitchell, 1976). Tests for hydraulic conductivity anisotropy were made by making measurements on samples extracted parallel and perpendicular to the core axis. A value of Kxx/Kzz = 1.15 suggests that any hydraulic conductivity anisotropy is relatively small and may not be significant given the small number of samples tested.
To test the effect of different axial loads on these flow parameters, two subsamples from Sample 195-1200D-1H-4, 120-130 cm, were tested with axial loads of 6.3 and 62.8 MPa. The tenfold increase in load decreased the hydraulic conductivity by more than one-third and decreased storage by more than one-fifth, as a result of the material becoming more consolidated.
It is important to note that in this type of test, the pore pressure gradients are very large and the sample passes through a large range of consolidation states. For example, in the case of Sample 195-1200D-1H-4, 120-130 cm, the initial void ratio of 1.4 is reduced to 0.66 at the end of the loading test. Our measurements of hydraulic conductivity and specific storage represent weighted average values obtained over a range of consolidation states.
In situ temperature measurements were made in Hole 1200E using the DVTP and in Hole 1200F using the Adara temperature probe. The temperature records are shown in Figure F68. The typical temperature sequence first shows the temperature decrease as the temperature tool is lowered in the water column. The tool is lowered to the seafloor to the mudline, where it is allowed to reach equilibrium for >10 min. The temperature tool is then pushed into the sediments and held at a constant depth for another 10 min. Here, the temperature quickly increases in response to frictional heating and subsequently decays. This middle series is the temperature decay curve used to find the sediment temperature. Finally, the tool is removed from the sediments to the mudline, where it remains for 10 min, after which it is returned to the surface. This portion of the temperature sequence also shows a step increase in temperature, followed by a fast decrease and then slow increase as the tool is raised to the surface. Shown on the figures are the seafloor or mudline temperature (Tml) and the estimated sediment temperature at depth (Tsed).
The sediment temperatures at depth were determined using the curve fitting programs CONEFIT for data collected with the DVTP and TFIT for data collected with the Adara temperature probe. The estimates of sediment temperatures can vary by several degrees, depending on the value of the thermal conductivity of the sediment and the region of the decay curve chosen for the estimation. The sediment temperatures shown in Figure F68 were found using only the latter portions of the temperature decay curves.
Using the values of the mudline temperatures, the sediment temperatures at depth, and the depth of the measurement, the thermal gradient was calculated. Those values are shown in Table T18. A weighted bulk thermal conductivity = 1.38 W/(m·K) was calculated from the mean values of the clast and matrix thermal conductivities (2.21 and 1.32 W/[m·K], respectively) and their relative abundance (7% clast and 93% matrix), as estimated from the recovery data. Because there was negligible variation of the thermal conductivity with depth (see Fig. F66), the integrated thermal resistivity was not calculated. Instead, the heat flow was calculated directly using the following equation:
q = -K x (
T/z),
where
T/z
= thermal gradient.
It should be noted that these values of heat flow assume that all heat flow is due to conduction.
When compared to the global average of ~50 mW/m2 (Garland, 1979), the heat flow in Hole 1200E is lower (q = 13 mW/m2), whereas that in Hole 1200F is higher (q = 99 mW/m2). Hole 1200E is located within several meters of fissures in the mud volcano. Fluid flow is likely occurring in these fissures, and the thermal gradient in Hole 1200E might be suppressed by advective transport of heat by the fluid. Hole 1200F is located farther from the fissures where the thermal gradient might not be suppressed. However, this interpretation is complicated by the absolute temperature values in Holes 1200A and 1200E. Higher temperatures at both the mudline and within the sediment would be expected if heat were being transported advectively. This is not the case. The mudline temperature in Hole 1200F (1.78°C) is larger than that in Hole 1200E (1.67°C). This difference may be caused by use of the Adara temperature probe in Hole 1200F, whereas the DVTP was used in Hole 1200E, and so direct comparisons may not be valid.
