Sediments were obtained from parts of Holes 1224A, 1224B, 1224C, and 1224E. The sediments consist mostly of abyssal clays of varying color. Occasional coarser horizons are present as are horizons with varying densities of microfossils, both siliceous (radiolarians and sponge spicules) and calcareous (coccoliths and discoasters) (Fig. F8).
Core recovery from Holes 1224A and 1224B was not significant enough to characterize the sediments. One significant discovery, however, was the recovery of light-colored, noneffervescing granules and pebbles from a depth of between 6 and 15.6 m from Hole 1224A. These are currently thought to be fossil worm burrows or hydrothermal deposits.
The total sediment depth at Site 1224 is 28 to 30 m. The top 6.53 m, as characterized by a single piston core in Hole 1224C, is massive brown clay that gradually changes color to very dark brown. Radiolarian spicules are present throughout the section, but they increase with depth and are common at the bottom of the unit. Sponge spicules are not found near the top of the section but are common below 4.50 m.
In Hole 1224E, we recovered 10.52 m of clay in the interval from 8.0 to 27.1 mbsf, in which two punch cores were collected with the RCB coring system by pushing through the sediment without rotating. The clay varies in color between dark brown, very dark brown, black, and dark yellowish brown. The high disturbance due to the punch coring process causes the colors to be streaked and mottled throughout the hole. Most color changes are gradual. Light-colored granules and pebbles are found in the top few centimeters of Core 200-1224E-1R (~8 mbsf). Like the burrows in Hole 1224A, they do not effervesce and are thought to be infilled burrows. These sediments also contain early stage manganese nodules. They are up to 2 mm in width and may be irregular or elongated in shape. Coccoliths and discoasters are present below 17.5 m.
The basalt stratigraphy at the site is summarized in Figure F9. In this figure, the depth for the top of each core, except for the topmost cores into basement, is taken as the top of the cored interval, as is the ODP convention. The cored interval is determined from the drill string length, which is entered into the ODP database. The top cores in all four holes, however, assume that the top of basaltic basement lies at a constant depth of 28 mbsf, which was our best estimate based on all drill holes and jet-in tests. For these cores only, the recovered basalt is placed below this fixed depth, rather than at the top of the cored intervals. The basement depth of 28 mbsf is probably uncertain by about a meter, and there may be some slight relief to the top of basalts as well. This approach avoids assigning basalt recovery to depths that actually are above the point where basement was touched by the drill string.
In Hole 1224A, we recovered 0.37 m of fine-grained, aphyric basalt from near the basement/sediment contact. A handful of broken basalt pebbles suggests that the core bit skittered over the basement surface before spudding securely.
In Hole 1224D, we penetrated 39.5 m of basalt recovering 15.67 m (46.5% recovery). The basalt is aphyric and consists of two flows that are finer grained near the flow tops and bases and coarser grained in the middle. Glass was recovered only at the top of the first flow. The rock is very little altered, but alteration is more intense near fairly widely spaced fractures that are lined with green clays, calcite, and pyrite. Flow tops are more fractured than flow interiors, and flow-top fractures are lined with Fe oxyhydroxides and calcite. Alteration halos are 12 cm wide near these veins. The same basalt unit was cored again in Hole 1224E in a single core that penetrated 9.6 m into basement, with 4.39 m of recovery. The first three cores from Hole 1224F again cored the upper basaltic basement. Thus, all four basement holes, which lie within 20 m of each other, cored immediately into the same fairly massive basalt flows.
Coring continued in Hole 1224F from the massive basalts into thinner flows and pillows (at ~65 mbsf), that are more intensely fractured and altered than the massive rock above. Somewhat thicker flows were encountered again beginning in Core 200-1224F-13R, at 133.5 mbsf, or 105.5 m below the top of basement at 28 mbsf. Recovery of basalt ended with Core 200-1224F-15R obtained between 152.4 and 161.8 mbsf. Drilling continued without recovery to 174.5 mbsf. Total penetration at Hole 1224F was 146.5 m into basement making it the deepest hole cored into the basaltic basement of "normal" Pacific crust younger than 100 Ma since Deep Sea Drilling Project (DSDP) Leg 65.Based on the subdivision into massive basalt with high recovery, thin flows and pillows with low recovery, and a return to somewhat thicker flows, and because of the general correspondence of this sequence with physical properties and downhole logs, we divide the basaltic section into three lithologic units as follows:
Thin section examination of volcanic basement at Site 1224 (Holes 1224A, 1224D, 1224E, and 1224F) evidenced a relatively homogeneous mineral paragenesis. The main phases are plagioclase, clinopyroxene, opaque minerals, and rare pigeonite; therefore, the rocks can be classified as tholeiitic basalts. Olivine is rare and only a few small iddingsitized euhedral to anhedral groundmass crystals have been found. Iddingsite is a typical alteration of olivine and is made up by a mixture of goethite and layer silicates (e.g., smectite). The majority of the basalts are holocrystalline (almost 100% crystals) to hypocrystalline (glass concentration <50%) and can be ascribed to lava flows. With increasing depth of coring, hypohyaline textures and volcanic glass contents >90% become common and indicate the presence of pillow fragments with chilled margins. The deepest samples recovered (~153 mbsf) also show textural features of holocrystalline massive lava flows. With regard to their granularity, the basalts range from aphanitic (difficult to distinguish the crystals in the groundmass with the naked eye) to aphyric (absence of phenocrysts), though rare plagioclase or plagioclase-clinopyroxene sparsely phyric basalts (phenocryst content <2%) have been also found. The relative size of the crystals in the groundmass is equigranular, and their distribution is isotropic. The groundmass is hypidiomorphic with the presence of euhedral to anhedral shaped crystals. The texture of the massive lava flow basalts is intergranular (with clinopyroxene in interstitial relationships with plagioclase) to subophitic (with plagioclase laths partially enclosed in clinopyroxene) and, more rarely, intersertal (with microcrystalline to glassy material between plagioclase). Hyalopilitic (with plagioclase laths and clinopyroxene crystals in a glassy matrix) to, more rarely, intersertal textures have been found in the pillow lavas. The grain size of the groundmass ranges from very fine grained (0.0010.5 mm) to fine grained (0.51 mm).
Inductively coupled plasmaatomic emission spectroscopy (ICP-AES) data for K2O, TiO2, MgO, Ba, and Zr were obtained on samples from Hole 1224D. The rocks recovered consist mainly of massive fresh basalt, with only widely spaced and narrow veins containing carbonate minerals, clays, and pyrite. The above elements tell virtually the entire story.
The basalts are differentiated normal mid-ocean-ridge basalt (N-MORB) with 22.7 wt% TiO2. The samples selected for analysis are scarcely altered, with loss-on-ignition (LOI) values ranging from 00.45 wt%. Concentrations of K2O (0.110.27 wt%) may be slightly elevated in three of ten samples analyzed, but most values, and all those for Ba (9 to 18 ppm) are consistently lower than in many comparably differentiated MORB glasses from the East Pacific Rise. This may indicate a greater-than-average depletion of the mantle sources of basalts from Hole 1224D. Alternatively, the rocks may have experienced a slight nonoxidative alteration in which these components were partially removed from the rock. This seems unlikely, however, given the more extensive oxidative alteration observed in rocks from Holes 1224E and 1224F, obtained only a few meters away.
Both TiO2 contents and Zr concentrations are determined precisely enough to enable their use in defining chemical stratigraphy (Fig. F11). Most of the basalts from Hole 1224D belong to one chemically uniform, extensively differentiated basalt flow more than 20 m thick. This overlies a second flow that is not quite so differentiated.
X-ray diffraction (XRD) analysis was carried out on one clayey pebble from the sediment and twenty-five vein materials within the basalt. Five distinct vein types were documented by XRD analysis: clay, carbonate, zeolite, quartz, and smectite (Fig. F12). Many vein minerals in the basement at Site 1224 are stable at low temperature and pressure (i.e., zeolite). Phillipsite, the principal zeolite present at Site 1224, is a low-temperature member of the zeolite group (Miyashiro, 1973). Smectite is also commonly found as a product of the alteration of volcanic ashes and rocks from the seafloor and occurs in most of the low-grade metamorphic terranes in the world.Four of the vein types observed at Site 1224, smectite-illite, calcite-aragonite, quartz, and zeolite, are similar to veins observed at Sites 896 and 504 near the Costa Rica Rift (Alt, Kinoshita, Stokking et al., 1993). These minerals occur in relatively lower temperature hydrothermal assemblages (probably <100°C) (Laverne et al., 1996). No truly high temperature vein assemblages, such as the actinolite and epidote veins found >2000 mbsf at Site 504, occur at Site 1224. The mineral laumontite in the illite vein indicates a higher zeolite facies (Miyashiro, 1973). Aragonite generally forms at a higher temperature than calcite. These minerals indicate the local influence of warm hydrothermal fluids.
We used progressive alternating-field (AF) demagnetization of archive-half sections, one whole-core section, one working-half section, and discrete samples to characterize the paleomagnetic signal and resolve the magnetization components recorded in the recovered core. An unambiguous magnetostratigraphy could not be obtained from the only undisturbed core (Core 200-1224C-1H) that was recovered in the sedimentary section; the other sediment cores were extremely disturbed by drilling. In addition, we only had time for a cursory interpretation of the magnetization of the basaltic units, though fairly detailed demagnetization experiments were conducted on split cores and discrete samples.
Given that ~15 basalt units were recovered, the magnetization of the basalts should provide a valuable paleolatitude estimate for the Pacific plate at ~45 Ma. This age corresponds to the Pacific plate's abrupt change in motion relative to the hotspots as marked by the kink in the Hawaiian-Emperor hotspot track. A cusp in the Pacific plate apparent polar wander path (APWP) may also occur at this age, marking a change in the motion of the Pacific plate relative to the spin axis. The Pacific APWP and hotspot tracks together provide key constraints on estimates of the size of motions between hotspots, ultimately extending our understanding of mantle dynamics (Acton and Gordon, 1994). Additionally, the age also lies within the period (3957 Ma) when the Hawaiian hotspot has been shown to have moved rapidly southward relative to the spin axis (Petronotis et al., 1994). If geomagnetic secular variation has been averaged by the basalt units and if secondary overprints caused by alteration do not mask the primary magnetization, then we should be able to obtain an accurate paleolatitude. Finally, rock magnetic studies of the basalts should help refine our understanding of the magnetization of the upper oceanic crust and its role in generating lineated marine magnetic anomalies.
Samples of different sediment types and from basaltic rock were collected at Site 1224 for aerobic and anaerobic cultivation, for deoxyribonucleic acid (DNA) extraction and analysis, for phylogenetic characterization, for total cell counts, and for determination of the live/dead ratio of indigenous microbial communities. Sediment suspensions and ground basalt material were used under oxygen depleted conditions in the anaerobic chamber for the establishment of enrichment cultures. Aerobic cultivation was conducted using both seawater-based media and commercial methylene blue agar (MBA). Anaerobic cultures were based on reduced mineral media.
To evaluate the microbial background at Site 1224, ambient seawater samples were collected at 1 m below sea surface upwind of the JOIDES Resolution. The microscopically enumerated total cell counts in the surface water at Site 1224 were 1.4 x 104 cells/mL.
Sediment samples from Holes 1224C, 1224D, and 1224E were obtained from different depths ranging from the near-surface layer down to 24.9 mbsf. Bacteria were present in all sediment samples taken to 24.9 mbsf.
The amount of active bacteria was assessed in two representative sediment samples taken from the near-surface layer (interval 200-1224C-1H-1, 05 cm) and from a depth of 25 mbsf (interval 200-1224E-2R-5, 143150 cm). As indicated by fluorescent signals after hybridization with the bacteria-specific probe EUB338, the amount of metabolically active bacteria ranged in these sediment layers from 62% to 41% of the total cell counts, respectively (Fig. F13).
The microscopic investigation of a thin section of basalt showed textures which resembled microbial structures. This might be a further hint for putative microbial activity in deep subsurface environments.
In Hole 1224A, P-wave velocities of aphyric basalt from Cores 200-1224A-5X and 6N are ~5900 m/s and ~5800 m/s, respectively.
In Hole 1224C, the gamma ray attenuation (GRA) densities of sediments gradually decrease with increasing depth between 0 and 6.4 mbsf, corresponding to a color change from light brown to dark brown. Similarly we observed an unusual trend for bulk and dry densities in Hole 1224C, which decreases from ~1.52 to ~1.36 gm/cm3 and from ~0.8 to 0.54 gm/cm3, respectively. Porosities in Hole 1224C gradually increase from 71% to 80%. P-wave velocities from the P-wave logger (PWL), however, show a small increase from 1460 to 1500 m/s with depth between 0 and 6.4 mbsf. P-wave velocities from PWS3 contact probe measurements from Core 200-1224C-1H to 4H (between 0 and ~5.70 mbsf) range from 1525 to 1535 m/s. The P-wave velocity in Core 200-1224C-5H is ~1555 m/s, which is greater than other sections. Grain densities in Hole 1224C show a small increase from 2.782 to 2.831 g/cm3 for depths shallower than 2 mbsf. Between 2 and 6 mbsf, grain densities remain fairly constant between ~2.70 and ~2.74.
In Hole 1224D, bulk and dry densities increase from 2.7 to 2.9 g/cm3 and 2.6 to 2.8 g/cm3, respectively, in Core 200-1224D-2R. In Core 200-1224D-3R, bulk and dry densities decrease from 2.9 to 2.8 g/cm3 and from 2.8 to 2.7 g/cm3, respectively. In Cores 200-1224D-4R and 5R, they also decrease from 2.85 to 2.80 g/cm3 and from 2.8 to 2.7 g/cm3, respectively. Porosities remain at low values ranging from 4% to 9%. PWS velocities range from 4200 to 6500 m/s. Compressional wave velocity anisotropies for each sample are around 2%10%. PWS velocities have a sinusoidal depth variation. They decrease between 25 and 35 mbsf, increase between 35 and 45 mbsf, and decrease again between 45 and 55 mbsf. This sinusoidal depth variation is also identified for Hole 1224F.
Between 25 and 60 mbsf, PWS velocity in Holes 1224E and 1224F has a similar trend to Hole 1224D. PWS velocities have a strong depth dependence. Compressional velocities separate into seven depth zones (Fig. F14):
P-wave velocities are scattered with increasing bulk density. Compressional wave velocity vs. porosity, however, has a good inverse correlation, as P-wave velocity decreases with increasing porosity. These two relations imply that compressional velocities are not controlled by bulk densities, but are well controlled by porosities. Large porosities are associated with more fractured zones. If this is true, Zones 2 and 6 are intensively fractured.
Based on shipboard preliminary log analysis at this site during ODP Leg 200, we conclude that basement in Hole 1224F consists of at least five distinctive units (Fig. F15), with unit contacts at roughly 45, 63, 103, and 142 mbsf. These layered formations can be distinguished using the continuous electrical resistivity, density, sonic, neutron porosity, magnetic field, and possibly spectral gamma ray logs. The existence of a conduit or large-scale fracture between 138 and 142 mbsf was detected by all the log tools including the temperature tool. In addition, the temperature tool reveals that the "hot" fluid had a temperature of 4.6°C at the time of the logging. The vicinity of this conduit is much more highly altered than other rocks penetrated by the hole, as indicated by the gamma ray logs. Because of the relative position of the tools located in the tool strings, some tools can resolve the top logged intervals like gamma ray, porosity, density, and sonic logs. On the other hand, the resistivity tools and FMS placed at the bottom of the tool string can resolve the formation properties near the bottom of the hole. The values of the magnetic fields calculated from the three-component inclinometer tool are invalid near the bottom of the pipe (~35 mbsf). In the logged intervals where all the tools overlapped, they provide consistent information to support the layered structural units based on these geophysical properties.
Core lithology, physical properties, well logs, and seismic reflection data from the site were compared. Based on downhole variations observed in the data, particularly the well logs and physical properties data, we have divided the drilled interval into a sediment unit and basement into five distinct logging units (Fig. F16):
A long-standing problem in the red clay province of the eastern Pacific Ocean is to adequately resolve chert layers and basement in the presence of sediments <50 m thick. By lowering a battery-powered, free-running 3.5-kHz pinger to the seafloor on the VIT sled and recording the pulse on the ship's 3.5-kHz acquisition system, we hoped to increase the sound level incident on the seafloor, to improve the penetration into the subbottom, to reduce the footprint of the sound on the seafloor, and to increase the received signal levels. The deep-source 3.5-kHz experiment was carried out whenever the VIT camera was lowered to the seafloor either for reconnaissance surveys or reentries.
Examination of the deep-source 3.5-kHz records shows two prominent reflections at 13 and 38 ms below the seafloor. Depending on the sound velocity in the seabed, these reflectors would be 10 to 13 m and 28 to 38 m deep. The continuity of these reflectors varies with time throughout the survey, whereas the ship moves only a few meters.
Our preliminary interpretation had been that the 13-ms reflection occurs at an intermittent chert layer. The first jet-in test stopped abruptly at 13 m. Although chert layers within the sediments have been encountered at other drill sites in the eastern Pacific, nowhere at Site 1224 did we sample chert. The 13-ms reflection may correlate with a radiolarian rich layer which was cored. Basalt cores were regularly acquired at 2830 m depth, corresponding to the 38-ms reflector.
In summary, the deep-source 3.5-kHz experiment identified a previously unrecorded reflector at 38 ms below the seafloor that corresponded to basaltic basement. This reflector was not observed in the traditional 3.5-kHz survey conducted in 1997 or in the shipboard 3.5-kHz survey acquired while we came on site (Fig. F4). The 38-ms reflector, however, was observed beneath the H2O junction box.
Drilling at the H2O provides a unique opportunity to observe drilling related noise from the JOIDES Resolution on a seafloor seismometer in the frequency band 0.180 Hz. The University of Hawaii operates a Guralp CMG-3T three-component broadband seafloor seismometer and a conventional three-axis geophone at the H2O. Data are acquired continuously and are made available to scientists worldwide through the IRIS Data Management Center in Seattle. During the cruise, Jim Jolly and Fred Duennebier at the University of Hawaii relayed sample data files to the JOIDES Resolution by file transfer protocol (ftp) over marine telephone. We were then able to process data and study correlations with on-site activities and weather. The University of Hawaii also maintained a Web site showing H2O seismic data collected during the cruise (http://imina.soest.Hawaii.edu/H2O/).
Seismic activity could be associated with wind speed, sea state, shear resonance effects in the sediments, whales, water gun shooting, earthquakes, passing ships, and drilling related activities such as bit noise and running pipe (Fig. F17).
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