Physical properties measurements taken on core recovered from Site 1188, also known as Snowcap hydrothermal site, include magnetic susceptibility, natural gamma radiation, compressional wave velocity, thermal conductivity, and standard index properties. In most cases, measurements were made at least once per lithologic section. In areas of large-scale heterogeneity and when recovery allowed, sample density was increased. The magnetic susceptibility meter and natural gamma radiation device were run on Cores 193-1188A-10R through 21R, using the multisensor track (MST). Because the remaining cores from this hole were particularly fragmented, incomplete, or disturbed, they did not lend themselves to the continuous automated measurements of the MST. The cores from Hole 1188F were collected using the ADCB system, so they were too large to run through the MST. Additionally, because in most cases the cores were extremely fragmented, they were inappropriate for MST measurements and so no magnetic susceptibility or natural gamma radiation data were obtained. In both Holes 1188A and 1188F, compressional wave velocity measurements were made on discrete samples where possible in one direction and at ambient pressure. Thermal conductivity was measured on almost every lithologic unit from both holes, except where recovery was too low or rock pieces were too small for the measurement procedure (<5 cm). Index properties for both holes, also measured in every lithologic unit when recovery allowed, were measured on minicores, rock fragments, or both.
Figure F118 shows the magnetic susceptibility profile of Hole 1188A from 70 to 190 mbsf. Magnetic susceptibility varies greatly over the length of recovered core, ranging from -2.0 × 10-5 to 9736.0 × 10-5 SI, with an average value of ~575 × 10-5 SI. Generally, magnetic susceptibility increases toward the bottom of the hole. The interval from 85 to 130 mbsf has a relatively low magnetic susceptibility, ranging from -1.9 × 10-5 to 154.1 × 10-5 SI. Although Core 193-1188A-10R (77-87 mbsf) has a small magnetic susceptibility peak, the majority of the highly magnetic cores are below 135 mbsf, at and below Core 193-1188A-16R. The magnetic peak at Core 193-1188A-10R corresponds to an interval with small amounts of disseminated magnetite. The increasing magnetic susceptibility toward the bottom of the hole also coincides with an increasing amount of magnetite in the core. Varying amounts of sulfide minerals may affect the magnetic susceptibility as well, but more detailed measurements and analyses are necessary to discriminate between minerals, and will be discussed elsewhere (see "Rock Magnetism").
Natural gamma radiation (NGR) records are summarized in plots of total counts per second (cps) vs. curated depth (see Fig. F119). Values range from 0 to 21.5 cps, averaging 8 cps. The data from each core cover a broad range of values. Much of this variation is most likely caused by errors in the measurement process. All cores, at least over certain intervals, did not fill the core liner completely. Because NGR measurements depend on a fixed volume that is equal to that of a full core liner, the intervals where the recovered material was less than the full volume of the liner have lower than expected values. Elsewhere in the same core, the core liner may have been more completely filled, giving a higher NGR reading. Therefore, apparent trends in the data may be more indicative of recovery than of any fluctuation in radioactive elements.
Regardless of these recovery-related uncertainties, when coupled with the geochemistry data from Snowcap hydrothermal site, some information can still be extracted from the NGR values. K is the major element that produces a radioactive signal in the fresh dacites of the PACMANUS hydrothermal field (1.7%-1.8% K2O) (see "Geochemistry"). As increased bleaching, alteration, and silicification of the rocks occur, the potassium is commonly leached from the rock, producing lower NGR values. Similarly, higher values may be indicative of less or a different style of alteration. As many of the cores are heterogeneous, some of the scatter in the data from each core likely represents variations in the amount or style of alteration between the individual measurement locations. Although illite is common throughout the hole as an alteration product, the region from ~125 to 155 mbsf has a relatively high NGR count and corresponds to the largest amounts of illite identified by XRD. The core with one of the lowest NGR measurements (Core 193-1188A-14R; 116-125 mbsf) has no illite present in XRD scans. However, when compared to the overall silicification and alteration trends, the peaks in NGR data do not consistently correspond to the amounts of silicification or other types of alteration. Therefore, variations measured by the NGR device may be a result of measurement error as well as a result of heterogeneity within lithologic units.
Compressional wave velocities are shown in Table T21 and plotted in Figure F120. Values range from ~3.5 to 6.3 km/s, and average 4.6 km/s. Velocity values tend to increase with depth. Lithologic variations, vesicularity, amount of alteration, and structural features may account for some of the variance in velocity values. In many cases, the more massive volcanic rocks have higher compressional wave velocities than the brecciated and flow-banded rocks.
Thermal conductivity data for Holes 1188A and 1188F are shown in Figure F121. Values for Hole 1188A range from 1.05 to 4.44 W/(m·K). Most of the values are between 1.5 and 2.5 W/(m·K), with an average of 2.22 W/(m·K). One sample lies outside of this range, at ~125 mbsf, with a value of 4.44 W/(m·K). This sample was taken from a wide anhydrite vein, and thus the thermal conductivity value is closer to that of pure anhydrite (5.61 W/[m·K]) (Clark, 1966). The remaining data have a slight trend, with low values from ~50 to 85 mbsf and from ~135 to 180 mbsf, and high values in between, from ~90 to 130 mbsf. However, the trend variations are relatively small compared to the overall scatter in the data. The values seem to follow the amount of GSC alteration (see "Hydrothermal Alteration"); as the alteration increases the thermal conductivity decreases and vice versa.
Thermal conductivity values in Hole 1188A generally seem to be higher for brecciated volcanics than for fresh and altered dacites. All of the brecciated pieces from this hole have thermal conductivity values >2.0 W/(m·K). The altered dacites have values that range from 1.0 to 2.6 W/(m·K), but within this group the majority of rocks with values >2.0 W/(m·K) have anhydrite veins or pyrite-filled vesicles, which may account for the increased thermal conductivity values.
Thermal conductivity values for Hole 1188F range from 2.01 to 4.53 W/(m·K) and average 2.82 W/(m·K), which is significantly higher than for Hole 1188A. Throughout Hole 1188F, most of the vesicles are filled with quartz ± anhydrite ± pyrite, whereas in Hole 1188A most vesicles are unfilled. This should contribute to the higher thermal conductivity values in Hole 1188F, particularly in the altered dacites where most vesicles are found. This can be seen in the data by the lack of any thermal conductivity values below 2.0 W/(m·K) in Hole 1188F. The wide variety of thermal conductivity values at each depth reflects the heterogeneity of the core.
The data for water content, bulk density, dry density, grain density, porosity, and void ratio are displayed in Table T22. Grain densities of powder samples prepared for ICP-AES are given in Table T23. Figures F122 and F123 show grain density and porosity values for both holes, respectively. In Hole 1188A, grain density values range from 1.98 to 2.80 g/cm3, with an average of 2.65 g/cm3. In cases where index properties were measured on both rock fragments and minicores, values were consistent for both types of samples. Similarly, when grain density was measured on both ICP-AES powders and whole samples, values determined were also consistent. The low density value of 1.98 g/cm3 from Section 193-1188A-5R-1 may be due to the presence of opaline silica, as identified by XRD. For Hole 1188F, grain density values rang e from 2.47 to 3.59 g/cm3 and average 2.82 g/cm3. Agreement between measurements on rock fragments and minicores was not as good in Hole 1188F as it was in Hole 1188A. This could be caused by a sampling bias toward vein material over surrounding rock in rock fragment samples. Vein material, which is generally denser than surrounding material because of the enrichment in anhydrite and pyrite, tends to fracture into small pieces more easily, which is more suitable for rock-fragment sampling. Over the interval spanned by the two holes, grain density increases with depth until it reaches its peak between ~230 and 290 mbsf. Below 280 mbsf, grain density decreases slightly to a constant value between 2.7 and 2.8 g/cm3. The region between 230 and 290 mbsf is an area of increased anhydrite and pyrite content. The increase in these two minerals contributes to the higher grain density values. Additionally, because the majority of samples in this region are rock fragments, the sampling bias toward vein material in rock fragments may be partially responsible for the increase in grain density. Toward the bottom of the hole, both anhydrite content and grain density decrease again. Density values from ~290 to 400 mbsf are still higher than in Hole 1188A, possibly due to an increase in chlorite.
Porosity values for Hole 1188A are quite variable over the length of the hole, scattered from 0.4% to 44.7%. The average porosity for the hole is 25%. Hole 1188F has a smaller range in porosity, with values between 11.7% and 27.8% and an average of 18%. There is an overall trend of decreasing porosity with depth. This correlates with an increase in alteration and filling of vesicles with depth. Although the amount of silica stays relatively constant throughout Holes 1188A and 1188F, much of the silica in the upper part of Hole 1188A is in the form of cristobalite as opposed to quartz. Generally, cristobalite only coats vesicle walls, whereas quartz tends to fill the vesicles completely. As the quartz content increases with depth, more vesicle openings may be filled resulting in a decrease in porosity.
For both holes, the calculated sample porosities are higher than the vesicle space estimates made from visual core inspections. This may mean that small pore spaces in the rock framework make up a large portion of the porosity, which would possibly allow penetrative fluid flow throughout the rock.