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doi:10.2204/iodp.pr.327.2010

Site summaries

Site U1362

Hole U1362A was drilled and cored to 528 meters below seafloor (mbsf; 292 msb; Fig. F7), subjected to geophysical logging and hydrologic testing, and instrumented with a multilevel CORK observatory (Tables T1, T2, T3). Hole U1362B was drilled to 359 mbsf (117 msb; Fig. F8), subjected to a 24 h pumping and tracer injection experiment, and instrumented with a single-level CORK observatory.

Basement in Hole U1362A was cored from 346.0 to 496.0 mbsf (110–260 msb) with 44.4 m of core recovered. The recovered core consisted of (1) aphyric to moderately phyric pillow basalts, (2) aphyric to sparsely phyric sheet flows, and (3) sparsely to highly phyric basalt flows (Fig. F9). The above lithologies were divided into eight units based on changes in lava morphology, rock texture, and phenocryst occurrence. Pillow lava units (Units 1, 3, and 5) were subdivided according to changes in phenocryst abundance and mineralogy. Sheet flow units (Units 4, 6, and 8) were subdivided based on the presence of chilled margins and variations in phenocryst mineralogy. No breccia units were recovered (only two centimeter-sized breccia pieces were recovered in total).

Pillow basalt is the most abundant flow morphology of Hole U1362A, with Units 1, 3, and 5 accounting for 78.85 m of the stratigraphy. Pillow basalt was primarily identified by the occurrence of curved glassy chilled margins with perpendicular radial cooling cracks. The basalt is sparsely to highly phyric with olivine, clinopyroxene, and plagioclase phenocrysts having spherulitic, hyalophitic, intersertal, and glomeroporphyritic textures. Pillow basalts range from sparsely to moderately vesicular, with a range of secondary minerals filling the vesicles. Alteration in the pillows is variable and ranges from slight to highly altered.

Sheet flows are the second most common lava morphology in Hole U1362A, and these were classified on the basis of continuous sections of the same lithology that increase in grain size downward through the unit. Curated core recovery in these units averaged 43% and was as high as 119% in Core 327-U1362A-17R. Two near-continuous sheet flows were recovered in Cores 17R and 18R and were divided into two subunits on the basis of a change in phenocryst mineralogy. The primary mineralogy of the sheet flows is very similar to that of the pillow basalts, comprising a range of aphyric to moderately phyric basalt with olivine, clinopyroxene, and plagioclase phenocrysts. Grain size within the sheet flows ranges from cryptocrystalline to fine grained, and textures vary from intersertal to intergranular. The sheet flows are nonvesicular to highly vesicular, with some flows exhibiting a similar abundance throughout and others having high variability within the flow. Alteration within the sheet flows varies from slight to complete. Fracture and vein intensity within the sheets flows is lower than in the pillow basalts and resulted in improved core recovery and larger individual pieces.

The third lithologic type of basalt flow was classified based on the absence of definitive morphological features associated with either pillow lavas or sheet flows, allowing only a general "basalt flow" interpretation to be made. These units are aphyric to moderately phyric crypto- to microcrystalline basalt with the same primary mineralogy as all basalts from Hole U1362A. They are generally sparsely vesicular with secondary minerals filling vesicles, and textures vary from hyalophitic to variolitic. Alteration is moderate to high and is present as groundmass replacement (mesostasis and phenocrysts), vesicle fill, vein formation, and halos.

Two individual pieces of breccia were recovered, a hyaloclastite sample in Core 327-U1362A-13R and a hydrothermal breccia vein in Core 327-U1362A-9R. The hyaloclastite is characterized by moderately to highly altered angular clasts in a saponite and altered glass matrix. The hydrothermal breccia vein is formed of subangular clasts with moderate alteration similar to the host rock and exhibits evidence of clast rotation and separation with a matrix of highly altered, ground basalt.

Geochemical analyses of basalt samples indicate that they are all normal depleted mid-ocean-ridge basalt (N-MORB), and based on cross-plots of TiO₂ vs. Zr they are inferred to all have the same magmatic source. Hydrothermal alteration of the basement was observed in all basalts from Hole U1362A, varying from slight to completely altered, with the majority moderately altered. Alteration of the rocks manifests in four ways: (1) replacement of phenocrysts, (2) replacement of groundmass (mostly mesostasis), (3) filling of veins and adjacent alteration halos, and (4) lining and filling of vesicles. In thin section, alteration was observed to range from 8% to 91%. Away from vesicles and veins, background alteration is generally moderate to high in pillow lavas and predominantly moderate in sheet and basalt flows and is dominated by saponitic background alteration. Olivine is present only as completely replaced pseudomorphs.

The secondary mineralogy is dominated by clay minerals that are present in all four types of alteration. Saponite is the most abundant of the clay minerals and is present as black, dark green, greenish brown, and pale blue colors and in thin section is characterized by a pale brown color and mottled or fibrous form. Celadonite is also present in all four styles of alteration but is less abundant than saponite. In thin section, celadonite is bright green, and within some vesicles the color varies in intensity, reflecting a mix of saponite and celadonite. Iron oxyhydroxide is the second most abundant secondary phase, occurring both alone as iron oxyhydroxides and mixed with saponite and other clay phases to form iddingsite. Iron oxyhydroxides are identifiable by a bright orange to red color and often stain other phases present. Zeolite phillipsite was identified by X-ray diffraction (XRD) analysis of mixed veins and altered chilled margins in addition to montmorillonite (smectite group) and sepiolite (clay) from veins. Carbonate is present as vesicle fill, in veins, and within chilled margins and predominantly occurs mixed with clays and occasionally sulfides. Anhydrite is rarely present in veins from Subunit 6B.

A total of 1230 veins were logged, with an average frequency of 27 veins per meter of recovered core. Vein width ranges from <0.1 to 4 mm, and vein morphology is variable. Saponite is the most abundant vein fill and is present in 76% of the veins, with unidentified clay minerals filling 50% of veins. The next most abundant vein fill is iron oxyhydroxides, which fill 32% of veins. Carbonate and pyrite are present in 10% of veins but are only occasionally the dominant components. Celadonite occurs in 2% of veins and is a larger component of the background alteration. Rare anhydrite veins are present within Subunit 6B. Alteration halos flank 15% of hydrothermal veins and are otherwise found flanking rock edges or apparently unassociated from structural features. Halos range from single-color black, green, or orange halos to complex multihalos with mixed colors.

The dips of 519 veins and fractures were measured, and three types of fractures were distinguished: (1) veins flanked by alteration halos (haloed veins), (2) veins not flanked by alteration halos but filled with secondary minerals (nonhaloed veins), and (3) joints sometimes flanked by alteration halos but not filled with minerals. Nonhaloed veins were the most frequently observed structures. Nonhaloed veins were identified mainly in the massive lavas and in some pillow-lava pieces. No faults or shear veins with any evidence of displacement were found.

Whole-round basalt core sections were run through the Whole-Round Multisensor Logger (WRMSL) and Natural Gamma Radiation Logger (NGRL) prior to splitting. Gamma ray attenuation (GRA) density data vary widely as a result of unfilled core liners in sections with poor recovery. Despite this, peak bulk density values are consistent at ~2.5 g/cm³ for much of the core recovered. For the more cohesive, massive sections recovered in deeper cores, GRA results are slightly >2.5 g/cm³. Magnetic susceptibility measurements also vary widely, ranging from 0 to 3300 × 10^–6 SI. Total counts from the NGRL are generally low (1–5 counts/s). For all measurements, the highest values were found in massive sections, with other lithologies, namely pillow lavas and sheet flows, generally yielding much lower values.

Thermal conductivity was tested in three samples from the uppermost section of pillow basalts, yielding values of 1.63, 1.67, and 1.72 W/(m·K) at depths of 349, 354, and 355 mbsf, respectively. These values compare well with data collected at similar depths at nearby Hole U1301B (1.70 ± 0.09 W/[m·K]). Problems with the thermal conductivity half-space system prevented additional measurements.

P-wave velocities were measured on 71 discrete samples. P-wave velocity values determined by manual picking of the first arrival range from a minimum of 4.3 km/s to a maximum of 6.0 km/s, with an average of ~5.4 km/s. The average value is greater than values obtained from Hole U1301B. The lowest velocity was measured on a heavily altered sample (327-U1362A-14R-1, 11–13 cm). A test of nearby unaltered material yielded much higher velocity, which demonstrates the influence of rock alteration on P-wave velocity. We found no statistically significant overall velocity trend with depth or overall velocity anisotropy depending on sample direction.

Moisture and density properties were determined on 71 discrete samples from Hole U1362A. Bulk density values range from 2.58 to 2.89 g/cm³, with an average of ~2.74 g/cm³ (Fig. F9). Grain density values range from 2.66 to 3.16 g/cm³, with a mean of ~2.88 g/cm³. Porosity values range from 2.8% to 14.2%, with a mean of 7.9%. The highest value of porosity was obtained from a highly altered sample that also had the lowest velocity. Overall, P-wave velocity and porosity are inversely correlated.

Twenty-five whole-round samples (4–20 cm long) were collected for microbiological analysis. Samples were preserved for shore-based deoxyribonucleic acid (DNA) analysis, shore-based fluorescent in situ hybridization (FISH) and cell counting analysis, and shipboard fluorescent microsphere analysis. One sample was also collected for shore-based analysis of particulate organic carbon and nitrogen as well as carbon and nitrogen isotopic compositions. Hard rock samples span a range of lithologic units, alteration states, and presence of chilled margins, and most contain at least one vein per fracture. Additionally, a few recovered plastic bags that held fluorescent microspheres were collected as a contamination check for DNA analysis. Colonization experiments were assembled for Hole U1362A and U1362B CORK instrument strings. Fluid samples were collected for shore-based microbiological analysis during the 24 h tracer injection experiment in Hole U1362B.

Remanent magnetization measurements were made on 79 discrete pieces and on portions of 23 core sections. Samples were demagnetized at 5 or 10 mT steps from 0 to 50 mT using the cryogenic magnetometer's inline alternating-field (AF) coils. Most samples display simple magnetization behavior. Principal component analyses were performed on select samples. The majority of samples have positive inclinations, indicating that magnetization was acquired during a normal-polarity period, consistent with crust age at this location. Some samples have steep, positive inclinations that might be influenced by a drilling overprint. A few samples have reverse magnetizations, which are most likely the result of alteration. Inclinations are scattered around 460–470 mbsf in Unit 6. The average inclination for Hole U1362A basalts is 72°, slightly higher than the expected geocentric axial dipole direction.

A single wireline logging string was deployed in Hole U1362A to identify suitable intervals for packer placement and to determine physical and hydrological properties. The logging string consisted of a qualitative spontaneous potential electrode and sensors for measuring natural spectral gamma ray, bulk density, borehole fluid temperature, tool orientation, tool motion, ultrasonic borehole images, and hole diameter. Two passes were run over the entire open hole section, and a third pass was run over two intervals of particular interest. Seven logging units were identified on the basis of petrophysical log response and borehole conditions.

Both the mechanical and ultrasonic calipers revealed a borehole that was highly enlarged over most of the open hole section (Fig. F9). Notable near-gauge sections were identified at 417 and 447 mbsf. Good conditions were expected in these sections based on rotary core barrel (RCB) recovery and by the apparent coring and drilling rates of penetration. Low recovery and higher rates of penetration correlate well with an enlarged borehole. Where the ultrasonic caliper values appear meaningful, they indicate a nearly circular borehole through the near-gauge intervals.

The ultrasonic borehole images are marred by rotational and heave-induced tool motion. In addition, the ultrasonic tool's sonde head is undersized for these borehole diameters, and, where Hole U1362A is enlarged, no meaningful images can be expected. Certain fractures and other features were observed in the 447 mbsf near-gauge section, particularly during the third imaging pass made at the highest vertical resolution.

The density readings were impaired by poor borehole conditions in many intervals. Where hole condition was good, logged density compares favorably with density measurements on discrete core samples (Fig. F9).

Gamma ray measurements in the basaltic crust are driven by potassium content and were repeatable over the three passes. Where increases in gamma ray values correspond with enlarged borehole intervals, such as the one at 470 mbsf, they may represent zones of greater alteration and may be indicative of focused hydrothermal fluid flow. A pronounced gamma ray deflection was observed at and above the 10¾ inch casing shoe, likely a measure of trace uranium and thorium in the cement.

Borehole fluid temperature data were acquired while running into the hole and during the three upward logging passes. The borehole temperature gradient increases steeply at the top of the 447 mbsf near-gauge interval, and a 0.5°C temperature anomaly was observed 8 m below the casing shoe.

Packer experiments were completed in Hole U1362A to assess the permeability of the formation. The sealed-hole pressure baseline was recorded for 1 h, and two 1 h long injections tests were conducted, each followed by 1 h to allow the pressure to recover to baseline conditions. Preliminary data analysis indicates a bulk permeability consistent with that in nearby Hole U1301B (Becker and Fisher, 2008).

A 24 h pumping and tracer injection experiment was conducted prior to the CORK deployment in Hole U1362B. Fast-sampling reverse osmosis pumps and pressure gauges were run in a specially designed stinger sub just below the casing shoe in the open hole. After waiting 1 h to allow the hole to equilibrate, seawater was pumped into the formation at a rate of 20 strokes per minute (~7 L/s) (Fig. F10). At 2 and 20 h into the experiment, freshwater was pumped into the formation for 1 h instead of seawater. The tracers injected included SF₆ gas (for ~22 h), CsCl and ErCl₃ salts (at 3 h) and CsCl and HoCl₃ salts (at 19 h), fluorescent microspheres (at 20 h), and stained bacteria (at 21 h) extracted from sea-surface water. Pumping ceased during the last hour of the experiment so the hole could equilibrate again.

The CORKs deployed during Expedition 327 are based on the CORK-II design prepared for Expedition 301. The Hole U1362A CORK monitors two basement intervals: a shallow interval extending from the base of the 10¾ inch casing to the top of the deepest set of swellable packers (307.5–417.5 mbsf) and a deeper interval extending from the base of the deepest inflatable packer to the bottom of the hole (429.2–528.0 mbsf) (Fig. F11). Pressure in both intervals is monitored through ¼ inch stainless steel tubing connected to miniscreens installed just below the inflatable packers at the top of the isolated intervals. Three ½ inch stainless steel fluid sampling lines terminated at two depths (two below packers in the upper interval and one below packers in the lower interval). A single ½ inch polytetrafluoroethylene (PTFE) microbiology sampling line ends in a titanium miniscreen that rests on perforated and coated 5½ inch casing, 7 m below the base of the deepest inflatable packer, just above the perforated collars. The downhole instrument string includes 6 OsmoSamplers and microbial growth incubators positioned within the coated perforated 5½ inch casing and collars, 11 autonomous temperature probes, and a 200 lb sinker bar.

The Hole U1362B CORK monitors a single basement interval that extends from a single set of swellable and inflatable packers positioned just inside the base of the 10¾ inch casing to the bottom of the hole (272–359 mbsf) (Fig. F11). Pressure in this interval is monitored via a ¼ inch stainless steel tube connected to a miniscreen installed just below the inflatable packers. The intakes of the three ½ inch stainless steel fluid sampling lines are located on perforated and coated 5½ inch casing, about 3 m below the packers, providing sampling redundancy. A single ½ inch PTFE microbiology sampling line ends in a titanium miniscreen that rests on perforated and coated 5½ inch casing, 7 m below the base of the deepest inflatable packer, just above the perforated collars. The downhole instrument string comprises six separate OsmoSamplers and microbial growth incubators; eight autonomous temperature probes, including two installed in OsmoSamplers suspended inside the perforated and coated drill collars at depth in the hole; and a 200 lb sinker bar.

Both CORKs include a large-diameter ball valve in the wellhead that can be opened to allow fluids to bypass the top plug through a "lateral" pipe that extends from the main CORK tubing above the seafloor seal ("L-CORK" design). Researchers will initiate a long-term flow experiment in Summer 2011 by deploying a flow meter and opening the ball valve on one of the Site U1362 CORKs using the submersible Jason. Flow will continue for at least 1 y, allowing testing of a much larger volume of crust than has been tested previously during scientific ocean drilling experiments.

Site U1301

We recovered an incomplete portion of the instrument string deployed during Expedition 301, comprising five autonomous temperature loggers and part of the Spectra cable (Tables T1, T2, T3; see "Operations"). The rest of the instrumentation was left in the CORK and/or hole. Of the five autonomous temperature loggers recovered from Hole U1301B, one was able to communicate and download data during Expedition 327. All of the tools were deployed well beyond their intended 4–5 year battery life. The four nonoperational tools will be returned to the manufacturer for servicing and data download (data are stored in nonvolatile memory). The tool that did provide data showed that temperatures at depth began to rise beginning in Summer 2009, soon after cementing during Expedition 321T. In addition, the temperature log collected in the upper 364 m of the open CORK casing shows that the thermal gradient in Hole U1301B has nearly returned to a predrilling state. Collectively, these data suggest that the remedial cementing conducted in Summer 2009 during Expedition 321T was successful in sealing the hole. A short instrument string with three temperature loggers was deployed in Hole U1301B and will be useful in assessing the continued thermal rebound of the hole.

Site 1027

No scientific results were obtained at Site 1027 as a result of the failed attempt to recover the CORK installed during Leg 168 (see "Operations").

Site U1363

Five holes were cored at Site U1363, adjacent to the edge of Grizzly Bare outcrop (Tables T1, T2, T3; Fig. F12). Sediments are composed of turbidites interspersed with hemipelagic clay, consistent with core recovery during Leg 168 and Expedition 301 (Davis, Fisher, Firth et al., 1997; Fisher, Urabe, Klaus, and the Expedition 301 Scientists, 2005). Four lithologic units were distinguished (Fig. F12).

Unit 1 is composed of hemipelagic mud (clayey silt to silty clay), thin-bedded turbidites (sand-silt-clay), and thick-bedded medium sand turbidites. Unit 2 is composed of beds of silt and sandy silt intercalated with hemipelagic mud deposits (silty clay to clayey silt). Unit 3 is composed of hemipelagic carbonate-rich claystone rich in foraminifers and nannofossils. Unit 4 is represented by a few small pieces of basalt recovered from the sediment/basement interface in Holes U1363B and U1363D. The basalt is cryptocrystalline and plagioclase phyric, with glomeroporphyritic texture visible in hand specimens. Phenocrysts are large (up to 8 mm) and are anhedral to euhedral in shape. The basalt is sparsely vesicular with highly variable vesicle size and shape. Secondary minerals are present as background groundmass replacement and alteration halos as well as filling vesicles and lining hydrothermal veins.

All cores were run through the WRMSL, yielding magnetic susceptibility values from <500 × 10^–6 SI in clay sections to ~1400 × 10^–6 SI in sandy turbidites. Point magnetic susceptibility data collected with the Section Half Multisensor Logger (SHMSL) are similar, with split-core values tending to be slightly lower than whole-round values, except in the case of turbidite sequences, where SHMSL values are consistently higher. GRA density averages ~1.8 g/cm³, depending on lithology, with some compaction evident with depth in clay data. P-wave velocities measured on the WRMSL range from ~1.46 to ~1.87 km/s, excluding the erroneously low values derived from insufficient sediment filling within core liners.

Discrete measurements, including moisture and density (MAD), P-wave velocity, and thermal conductivity, were measured on most cores from Holes U1363B, U1363C, U1363D, and U1363F. Insufficient time was available to measure samples from Hole U1363G. Thermal conductivity at Site U1363 averages 1.31 ± 0.19 W/(m·K), whereas MAD bulk densities average 1.72 g/cm³, both showing bimodal distributions corresponding to clay and sand lithologies. MAD porosities range from 39% to 76%, with an average value of ~60%. P-wave velocities average 1.52 km/s, with considerable variability across lithologies. The velocities derived from discrete measurements agree with those measured on the whole-round sections with the WRMSL. Velocities also show weak anisotropy between vertical and horizontal directions. P-wave velocity increases ~50 m/s within the uppermost 50 mbsf. On the other hand, grain density is remarkably consistent regardless of depth or lithology.

Pore water samples were recovered from five holes, providing systematic trends to gauge the composition of the underlying basaltic formation fluid at these locations. Pore waters were extracted in a nitrogen atmosphere, and some analyses (alkalinity and ion chromatography) were conducted immediately to guide future drilling operations. We collected 57 pore water samples: 15 from Hole U1363G, 14 from Hole U1363F, 14 from Hole U1363B, and 14 from (adjacent) Holes U1363C/U1363D, with basement depths of 25, 33, 54, and 223 mbsf, respectively. In the upper portion of the sediment, biogenic processes release dissolved Mn and Fe near the sediment/water interface and consume sulfate with a sulfate minimum of 22 mM at 20 mbsf. There is a corresponding increase in alkalinity, phosphate, and ammonium and an initial decrease in Ca resulting from carbonate formation given the high alkalinity values. Similar trends for sulfate, Mn, and Fe exist near the sediment/basement interface. However, phosphate and ammonium are more influenced by diffusion and reaction within the upper basaltic basement. The cations Ca, Mg, and K show gradients near the sediment/basement interface, indicative of a formation fluid that is slightly altered relative to seawater. Minor and trace elements in seawater also show gradients in the basal sediment section, with greater alteration at greater distance from the outcrop.

Alkalinity shows a progression with increasing distance from the outcrop, from higher to lower values at the sediment/basement interface, and a progression of increasing values in the mid-sediment column. pH is consistently near 7.3 from the seafloor to the sediment/basement interface. In Hole U1363G, alkalinity increases from 3.5 meq/L at 1.5 mbsf to 4.6 at 10 mbsf and then decreases to 2.9 meq/L at 25 mbsf at the basement. Hole U1363F has a similar profile, with alkalinity increasing from 6.7 meq/L at 4.5 mbsf to 10.5 meq/L at 12 mbsf and then decreasing to 1.8 meq/L at the sediment/basement interface at 33 mbsf. The highest alkalinity measured was 13.9 meq/L at 16.5 mbsf in Hole U1363B, decreasing to 2.1 meq/L at the sediment/basement interface. Only the lowermost 50 m of Holes U1363C/U1363D was sampled, so an alkalinity maximum in shallower sediments was not sampled. The lowest alkalinity measured was 1.3 meq/L at 233 mbsf at the Hole U1363D sediment/basement interface. These findings are consistent with a progressively altered basement fluid with increasing distance from the Grizzly Bare outcrop.

Microbiologists collected whole-round core and pore water samples from sediments and basement pieces recovered at Site U1363. Eleven sediment intervals were targeted for microbiology sampling in Hole U1363B. Most samples were taken from hemipelagic clay layers, although some sandy turbidite layers were also sampled. The deepest sediment sample was taken from a carbonate-rich layer near the sediment/basement interface. Thirteen sediment intervals and one basement basalt interval were sampled from Holes U1363C and U1363D. Again, sediment samples were mostly from clay-rich layers, although some samples contained sand. The basement sample from Core 327-U1363D-6X was a relatively unfractured basalt with spots of light green and orange alteration crusts. Nineteen sediment and basement samples were taken from Hole U1363F. Most samples contained either clays or sandy layers, with the exception of samples from Section 327-U1363F-4H-2 and deeper, which also contained manganese crust, basalt fragments, and lighter tan-colored sediment resembling the foraminifer-rich carbonate sediments from Hole U1363B. Sixteen samples were collected from Hole U1363G. All samples were clay rich, and no hard rock samples were recovered.

At each sampling location, whole-round core samples were collected for shore-based DNA analysis, shore-based characterization of halogenated organic matter, and shore-based incubation experiments to examine dehalogenation reaction activities. Syringe samples were also collected for headspace gas analysis and microsphere contamination checks from the interior and exterior of the cores. Headspace samples were analyzed on board for safety purposes, and only a few samples contained quantifiable levels of methane or higher hydrocarbon gases. Microsphere samples will be returned to the shore-based laboratory for postcruise analysis because of time limitations at the end of the cruise. These samples will also be used for shore-based cell counting analysis and FISH analysis. A subset of samples was collected for analyses of dissolved organic carbon (DOC)/dissolved nitrogen (DN), particulate organic carbon (POC)/particulate nitrogen (PN), amino acids, low molecular weight organic acids, and lipid biomarkers.

Remanent magnetization measurements were made on two-thirds of core sections from Hole U1363B. Samples were demagnetized at 10 mT steps from 0 to 40 mT using the cryogenic magnetometer's inline AF coils. Although the majority of samples have positive inclinations, there is a large scatter of positive and negative inclinations. This is probably the result of core deformation during drilling and the alternating sequences of hemipelagic mud and turbidite deposits.

Temperature measurements were collected with the third-generation advanced piston corer temperature tool (APCT-3) and the Sediment Temperature (SET) tool in Holes U1363B–U1363E. Good measurements were obtained with both tools, and these data will be analyzed postexpedition to assess seafloor heat flow and thermal conditions in basement.

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