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Site U13831

Expedition 336 Scientists2

Site summary

Integrated Ocean Drilling Program (IODP) Expedition 336 Site U1383 (prospectus Site NP-2) is located 5.9 km northeast of IODP Site U1382 in an area of elevated conductive heat flow in 4414 m water depth. The primary objective at Site U1383 was to install a multilevel subseafloor borehole observatory (CORK) for long-term coupled microbiological, biogeochemical, and hydrological experiments in uppermost basaltic crust. Basement coring, downhole logging, and hydrologic experiments were also planned.

Hole U1383B (22°48.1328′N, 46°03.1556′W) was prepared for drilling 500 m into basement by installing a reentry cone with 20 inch casing extending to 35 meters below seafloor (mbsf). We then prepared the hole for 16 inch casing by drilling an 18½ inch hole to 68 mbsf; the sediment/​basement interface is at 53 mbsf. After installing and cementing the 16 inch casing to 54 mbsf, we started to prepare the hole for 10¾ inch casing by drilling a hole into basement with a 14¾ inch tricone bit. We decided to abandon Hole U1383B after this tricone bit failed at 89.8 mbsf, resulting in large parts of the bit being left in the hole. Although we did not choose to deepen this hole, it remains a viable CORK hole because it has a completely functional reentry cone and casing system with ~35 m of accessible basement. A remotely operated vehicle (ROV) landing platform was therefore installed in the reentry cone to facilitate future ROV operations, which will include installation of an instrumented plug in Hole U1383B.

Hole U1383C (22°48.1241′N, 46°03.1662′W) was started at the site of the original jet-in test, 25 m southwest of Hole U1383B. The primary objective was installing a multilevel CORK in the uppermost ~300 m of basement. The ultimate configuration of the CORK in Hole U1383C was to be determined by the depth of basement penetration and the downhole logging results. A reentry cone with 16 inch casing was installed to 34.8 mbsf, and a 14¾ inch hole was drilled to 69.5 mbsf for the 10¾ inch casing string. The sediment/​basement interface was encountered at 38.3 mbsf. After cementing the 10¾ inch casing at 60.4 mbsf, drilling in Hole U1383C proceeded with a rotary core barrel (RCB) bit from 69.5 to 331.5 mbsf. From this 262 m long interval, 50.3 m of core was recovered (19.2%). Rocks are glassy to fine-grained basalts with variable phenocryst (plagioclase and olivine) contents. Three major lithologic units were distinguished on the basis of primary texture and phenocryst abundance. From 69.5 to 127 mbsf, the core consists of microcrystalline to fine-grained, sparsely plagioclase-phyric basalt with abundant glassy margins and numerous intervals of hard interflow limestone. From 127 to 164 mbsf, massive plagioclase-olivine-phyric basalts occur, occasionally hosting limestone (with and without basalt clasts) as fracture fill. Below 164 mbsf, glassy to variolitic to cryptocrystalline basalts (most likely pillow flows) predominate, and limestone is largely missing. Each of these three main lithologic units is divided into numerous subunits on the basis of hyaloclastite layers and rare tectonic breccias. The overall abundance of glass is noticeably greater than that in Hole U1382A, and the extent of palagonitization ranges from weak to moderate. Basalts are avesicular to sparsely vesicular and show vesicle fills of clay, zeolite (mainly phillipsite), calcium carbonate, and Fe oxyhydroxide. Brownish alteration halos commonly track veins filled with clay or carbonate and zeolite. Within Unit 3, a gradational change from glassy, to variolitic with abundant hyaloclastite layers, to more massive microcrystalline, to fine-grained basalt with rare glassy margins can be observed.

Although the hyaloclastites are noticeably palagonitized throughout the hole, the extent of background alteration appears to decrease downsection. Vein densities average 33 veins/m and increase somewhat downsection to 50 veins/m. Zeolite veins are abundant in the upper section of the drilled interval, whereas carbonate veins predominate in the lowermost part. Sparse vesicles are filled with zeolite and clay.

Physical properties measurements reveal tight correlations between sonic velocity, density, and porosity of the basalt. These correlations reflect changes in physical properties as a result of low-temperature alteration. In basalts with up to 40% alteration, P-wave velocities and bulk densities as low as 4750 m/s and 2.43 g/cm3 were recorded. These most altered basalts also show exceptionally high porosities (up to 16.6%). Despite the locally high alteration intensity, natural gamma radiation (NGR) core scanning revealed fairly low average potassium and uranium concentrations (e.g., 0.19 ± 0.05 wt% K). Fresh basalts show physical properties and K concentrations typical for mid-ocean-ridge basalt.

Whole-rock geochemical data show that basalts from Units 1 and 2 systematically differ in Zr/Y and Zr/Ti ratios from basalts from Unit 3. The compositional variability in the different units is primarily due to fractionation of olivine, but some trends (gain of K, loss of Mg) are also related to increased alteration intensity. The porphyritic basalts do not show the distinct plagioclase accumulation signature revealed in similar basalts from Hole U1382A and Deep Sea Drilling Project (DSDP) Hole 395A. Correlations between Site U1383 and Sites U1382 and 395 based on geochemical composition cannot be made for individual flow units, indicating that the sites belong to different volcanic centers that were fed by mantle sources with variable compositions.

Hard rock samples for microbiological analysis were collected from every RCB core from Hole U1383C. Roughly 12% (6.11 m total) of core recovered from Hole U1383C was taken as whole-round samples from the core splitting room and dedicated to microbiological analysis. The 79 hard rock microbiology samples span a range of lithologic units, alteration states, and presence of chilled margins, and some contain at least one vein or fracture. Additionally, a few background contamination samples were collected for shore-based DNA analysis, including recovered plastic bags that held the fluorescent microsphere solutions in the core catcher.

Samples recovered were preserved for shore-based DNA and RNA analysis, shore-based fluorescence in situ hybridization (FISH) and cell counting, ship-based culturing and enrichments, shore-based isotopic analysis, and ship-based fluorescent microsphere analysis. Microspheres were used during all coring operations to help in evaluating core contamination. The enrichment and cultivation experiments initiated include carbon fixation incubations; carbon and nitrogen cycling experiments; enrichments for methanogens, sulfate reducers, sulfide oxidizers, and nitrate-reducing iron-oxidizing bacteria; and enrichments for heterotrophic metabolisms. Deep ultraviolet (UV) (<250 nm) scanning of hard rock materials was used to identify sample regions with concentrations of biomass and organic material.

Wireline logging data include natural total and spectral gamma radiation, density, compressional velocity, electrical images, and deep UV–induced fluorescence (acquired with the new Deep Exploration Biosphere Investigative tool [DEBI-t]) of a 274.5 m section of open hole. Lithologic Unit 1 is characterized by variable caliper, density, and sonic velocity values. Gamma ray intensities are generally low but increase in the bottom part of the unit. Lithologic Unit 2 has a uniform caliper and high densities and apparent sonic velocities and shows high-resistivity massive flows with fractures in the Formation MicroScanner (FMS) images. The upper section of lithologic Unit 3 (153–166 mbsf) is characterized by decreases in density, apparent resistivity, and velocity and an increase in gamma ray intensity. This interval corresponds to thin flows with interpillow/​flow sediments and tectonic breccia. From 166 mbsf to the bottom of the hole, the logging data reveal fairly uniform values for density, velocity, and apparent resistivity. Areas with peaks in gamma ray intensity correspond to intervals with abundant hyaloclastite in the recovered core (in particular around 175 mbsf and from 220 to 250 mbsf).

Drill string packer experiments were attempted in Hole U1383C to assess the transmissivity and average permeability of open-hole zones bounded by the bottom of the hole at 331.5 mbsf and three different packer inflation seats at 53, 141, and 197 mbsf. The packer experiments were not successful.

In preparation for the CORK observatory, Hole U1383C was cased through the 38.3 m thick sediment section with 10¾ inch casing to 60.4 mbsf and through a 14¾ inch rathole to 69.5 mbsf. After the hole was RCB cored to 331.5 mbsf and cleaned in five wiper trips, there was no noticeable fill, which was verified during logging. The CORK screens and downhole instrument string targeted three zones, selected mainly on the basis of recovered core and the caliper log. An upper zone extends from the combination packer and landing seat at 58.4 mbsf in the casing to the first open-hole packer and landing seat at 145.7 mbsf. Within this section the miniscreens are centered at 100 mbsf, with the slotted portion of the casing extending from 76 to 129 mbsf. The middle zone extends to an open-hole packer and landing seat at 199.9 mbsf. Within this section the screens are centered at 162 mbsf, with the slotted casing extending from 146 to 181 mbsf. The deep zone reaches to the bottom of the hole (331.5 mbsf) with miniscreens centered at 203 mbsf. The miniscreens are connected to the wellhead by five umbilicals with stainless steel or Tefzel internal tubing that are strapped to the outside of the casing. The downhole tool string consists of six different OsmoSampler packages, two dissolved oxygen sensors and recorders (one in the shallow zone and one in the middle zone), two miniature temperature recorders, sinker bars, sealing plugs, and interspersed sections of ⅜ inch (0.95 cm) Spectra line. The wellhead is instrumented with a pressure logger monitoring each of the three horizons and bottom seawater and a fast-flow OsmoSampler with both standard and microbiological sampling packages. The CORK extends to 247.6 mbsf, yet leaves the bottom portion of the hole open for future logging and access (247.6–331.5 mbsf).

1 Expedition 336 Scientists, 2012. Site U1383. In Edwards, K.J., Bach, W., Klaus, A., and the Expedition 336 Scientists, Proc. IODP, 336: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/​iodp.proc.336.105.2012

2 Expedition 336 Scientists’ addresses.

Publication: 16 November 2012
MS 336-105