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Site U13621

Expedition 327 Scientists2

Site summary

Integrated Ocean Drilling Program (IODP) Hole U1362A was drilled and cored to 528 meters below seafloor (mbsf; 292 meters subbasement [msb]), geophysically logged and hydrologically tested, and instrumented with a multilevel subseafloor borehole observatory (“CORK”) (see Tables T1, T2, T3 in the “Expedition 327 summary” chapter). Hole U1362B was drilled to 359 mbsf (117 msb), tested with 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 (30% recovery). The recovered core consisted of (1) aphyric to moderately phyric pillow basalt, (2) aphyric to sparsely phyric sheet flows, and (3) sparsely to highly phyric basalt flows. The above lithologies were divided into eight units on the basis of changes in lava morphology, rock texture, and phenocryst occurrence. Pillow lava units (Units 1, 3, and 5) were divided according to changes in phenocryst abundance and mineralogy. Sheet flow units (Units 4, 6, and 8) were divided on the basis of 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 basalt ranges 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 high.

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 downhole through the unit. Curated core recovery in these units averaged 43% and was as high as 112% in Core 327-U1362A-17R (hard rock recovery >100% occurs in part because of tides, as discussed in text and in Fig. F9 in the “Expedition 327 summary” chapter). Two near-continuous sheet flows recovered in Cores 17R and 18R 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 basalt, 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 a single flow. Alteration within the sheet flows varies from slight to complete. Fracture and vein intensity within the sheet flows is lower than in the pillow basalt and resulted in improved core recovery and larger individual pieces.

The third lithologic type of basalt flow was classified on the basis of the absence of definitive morphological features associated with either pillow lava 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 basalt from Hole U1362A. The basalt is 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 cataclastic zone in Core 9R. The hyaloclastite is characterized by moderately to highly altered angular clasts in a saponite and altered glass matrix. The cataclastic zone 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.

The lithostratigraphy developed for Hole U1362A is somewhat different from that developed for Hole U1301B, located only 800 m to the south (see Fig. F10 in the “Expedition 327 summary” chapter). Compared with Hole U1301B, core from Hole U1362A contains considerably greater fractions of sheet flows and basalt flows (called “massive basalt” and “basalt lava,” respectively, in Hole U1301B) and evidence for more extensive and higher temperature hydrothermal alteration. Hole U1362A also contains less hyaloclastite breccia; however, only five coherent pieces of this rock type were recovered from Hole U1301B, and it seems likely that much more of this fragile rock type was present but not recovered from the formation in both locations.

Geochemical analyses of basalt samples indicate that they are all normal depleted mid-ocean-ridge basalt (N-MORB) and are inferred to all have the same magmatic source on the basis of cross-plots of TiO2 vs. Zr. Hydrothermal alteration of basement varying from slight to complete was observed in all basalt from Hole U1362A, 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 lava 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 types 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) 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 massive lava and in some pillow lava pieces. No faults or shear veins with any evidence of displacement were found.

In addition to cores recovered from Hole U1362A, small millimeter- to centimeter-sized chips were recovered from the drill bit in Holes U1362A and U1362B. In both cases the drill bit penetrated only a few meters below the sediment/basalt interface before being pulled to the rig floor, where the drill cuttings were removed. The source of the drill cuttings can be constrained to a short interval at this interface and represents the only recovery of basement material at the sediment/basalt interface at Site U1362. Basalt exhibiting a wide variety of hydrothermal alteration was recovered in these chips, and similar compositions were recovered in both Holes U1362A and U1362B. Alteration types included pervasive green and red alteration, iron oxyhydroxides, pale gray sulfide-bearing mud, and basalt chips with epidote crystals. The occurrence of epidote with pyrite at the seafloor, combined with anhydrite at depth, may indicate that Hole U1362A was formerly a location of hydrothermal upflow.

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 higher than 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 lava and basalt flows, generally yielding much lower values.

Thermal conductivity was determined in three samples from the uppermost section of pillow basalt, 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 in 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 70 discrete samples. P-wave velocity values determined by manual picking of the first arrival range from 4.5 to 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 73 discrete samples from Hole U1362A. Bulk density values range from 2.2 to 2.9 g/cm3, with an average of ~2.7 g/cm3. Grain density values range from 2.4 to 3.0 g/cm3, with a mean of ~2.9 g/cm3. Porosity values range from 2.8% to 15.0%, with a mean of 7.9%. The highest value of porosity was obtained from a highly altered sample that also had the lowest velocity. 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 DNA analysis, fluorescent in situ hybridization (FISH) and cell counting analysis, and fluorescent microsphere analysis. One sample was also collected for shore-based analysis of particulate organic carbon and nitrogen and carbon and nitrogen isotopes. Hard rock samples span a range of lithologic units, alteration states, and presence of chilled margins, and most contain at least one vein or 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 the 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 the age of the crust 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.

A single wireline logging string was deployed in Hole U1362A to resolve physical and hydrological properties and identify suitable intervals for packer installation. 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. Notable near-gauge sections were identified at 417 and 447 mbsf. Good conditions were expected in these sections on the basis of 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.

A comparison of caliper logs and apparent penetration rates from Holes U1362A, U1362B, and U1301B suggests that there is some along-strike, lateral continuity in major basement units (see Fig. F11 in the “Expedition 327 summary” chapter). The uppermost 100 m of basement in all holes at Sites U1301 and U1362 was drilled without coring using a 14¾ inch drill bit, and the lower parts of Holes U1362A and U1362B were drilled with a 9⅞ inch drill bit rather than a coring bit, so quantitative comparison of penetration rates in individual holes can be difficult. Nevertheless, the rapid penetration rate achieved in the uppermost 100 m of basement at Sites U1301 and U1362 is consistent with the rubbly and oversized character of the resulting boreholes and difficulties encountered when deploying 10¾ inch casing.

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 no meaningful images can be expected where Hole U1362A is enlarged. 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.

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. 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 used to isolate the borehole.

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 10¾ inch casing shoe, near 317 mbsf.

Packer experiments were completed in Hole U1362A to assess the permeability of the formation, with the packer set at depth in the open hole. The sealed-hole pressure baseline was recorded for 1 h, and two 1 h long injection 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 (Fisher, Cowen, et al.). OsmoSamplers and pressure gauges were deployed 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). At 1 and 20 h into the experiment, freshwater instead of seawater was pumped into the formation for 1 h. The tracers injected included SF6 gas (for ~22 h), CsCl and ErCl3 salts (at 3 h), 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 that the hole could equilibrate again. The pressure record will require considerable processing to account for tides and changes in fluid density associated with switching from freshwater to saltwater and with the injection of salts as part of the tracer experiment. Rig floor and stinger fluid samples were collected during tracer injection, and shore-based analysis will be required to develop a detailed history of injectate chemistry and particle concentration during the test. Pressure data and chemical samples will be collected from CORKs in this area in summer 2011, which will provide the first information from scientific ocean drilling on hole-to-hole solute and particle velocities.

The CORKs deployed during Expedition 327 are modified from the CORK-II design prepared for IODP 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). 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, a 200 lb sinker bar, and a plug to seal the hole near the seafloor.

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). 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 OsmoSamplers and microbial growth incubators, eight autonomous temperature probes, a 200 lb sinker bar, and a plug to seal the hole near the seafloor.

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 (“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 ROV Jason. Flow will continue for at least 1 y, allowing testing of a much larger volume of crust than has been tested during previous scientific ocean drilling experiments.

1Expedition 327 Scientists, 2011. Site U1362. In Fisher, A.T., Tsuji, T., Petronotis, K., and the Expedition 327 Scientists, Proc. IODP, 327: Tokyo (Integrated Ocean Drilling Program Management International, Inc.).

2Expedition 327 Scientists’ addresses.

Publication: 5 September 2011
MS 327-103