BACKGROUND

Incoming Plate

There is strong evidence for vigorous shallow flow of cool fluids, which may affect the updip limit of seismicity, in the oceanic section of the subducting plate at ODP Sites 1039 and 1253. East Pacific Rise (EPR)-generated oceanic crust (~24 Ma) at the drill sites is within a large regional low heat flow anomaly; at the Leg 170 and 205 sites, heat flow is ~15% of that expected for the plate age, implying significant advection of cool fluids (Langseth and Silver, 1996). Heat flow data collected during recent cruises show that seamounts are sites of fluid discharge and recharge (Fisher et al., 2003), and modeling suggests that lateral flow rates of 3–30 m/y in zones within the upper 600 m of high-permeability (10^–10 to 10^–8) basement are required to match the low heat flow on EPR-generated crust (Hutnak et al., in press). Chemical data also suggest vigorous and recent/contemporaneous fluid flow. For example, Sr isotopic compositions measured in pore fluids squeezed from sediments show a strong mixing trend toward approximate modern seawater ratios in the basal sediments. These basal sediment values are distinct from those appropriate for seawater contemporaneous with the sediment age or for pore fluid compositions modified by ash weathering, as seen higher in the sediment column (Silver et al., 2000; Chan and Kastner, 2000). Simple modeling suggests that unless supported, the gradients, also observed for Li, Ca, and SO₄, would dissipate by diffusion in ~15 k.y. Just south of the drill sites, plate reorganizations juxtapose cool EPR crust and ~22 Ma crust generated at the Cocos-Nazca spreading (CNS) center (Barckhausen et al., 2001), which is characterized by heat flow consistent with conductive lithospheric cooling models. This juxtaposition apparently corresponds to a significant change in the updip limit of the seismogenic zone. At 75 km from the trench, where cool EPR crust is subducting, this zone is at ~20 km depth; at ~60 km from the trench, where warmer CNS crust is subducting, this zone is at ~10 km depth (Newman et al., 2002). At Site 1253, the interval below 473 meters below seafloor (mbsf) is packed off and two OsmoSampler packages with temperature loggers are centered within fractured intervals at 500 and 516 mbsf to sample this subseafloor fluid flow, which cools the plate and may affect seismogenesis.

Fluids from the décollement zone can be analyzed for a variety of chemical tracers to identify fluid sources, map fluid and element transport, constrain fluid fluxes, and possibly help constrain mineralogy at the updip limit of the seismogenic zone. At the décollement sites (1040, 1043, 1254, and 1255), pore fluid analyses across the plate boundary show strong, narrow (less than the full depth of the décollement zone), anomalous abundances of thermogenic hydrocarbons through C₆ and other tracers (e.g., Ca, K, and Li). Taken together, the compositional anomalies indicate vigorous advection within the décollement transporting species generated at temperatures >150°C (i.e., at or near temperatures thought to exist at the updip limit of the seismogenic zone). The persistence of local compositional anomalies suggests recent flow. The OsmoSampler and OsmoFlowmeter are located within the décollement at Site 1255. Postcruise analysis of tracers such as K/Li ratios and B and Cl isotopes in the fluids may constrain the extent of smectite-illite reaction in the fluid source region; adding O and Sr isotope ratios should further constrain bulk composition and temperature of the fluid source region. Tracers of interest to geochemists investigating element recycling in volcanic arcs via subduction (e.g., U, Pb, Rb, Sr, Ba, Cs, As, B, and Li) will also be analyzed in sampled fluids. Pumping at a constant rate, the OsmoFlowmeters inject density-compensated artificial seawater tagged with iodate and high concentrations of Rb and Cs into the borehole below the OsmoSampler. Four sampling ports on a plane with the injection port collect and archive a time series of tagged fluids for subsequent recovery and analysis. Dilution of these tracers will constrain fluxes and, possibly, anisotropy (although not directionality in a geographic sense because the installed orientation is unknown) of fluid flow. A more complete description of the OsmoSamplers and Flowmeters is given in Jannasch et al. (2003). Flux rates of elements from the subducting plate carried in fluids advected from the deeper source will be useful for investigating methane fluxes and the impact of shallow slab dewatering on ocean chemistry and on the composition of the residual subducted slab at greater depths (ultimately to depths of magma generation).

During ODP Leg 205, CORK-II observatories were installed at two sites across the Middle America Trench off the Nicoya Peninsula, Costa Rica (Fig. F1), which had been drilled previously during Legs 170 and 205 (Kimura, Silver, Blum, et al., 1997; Morris, Villinger, Klaus, et al., 2003). The observatories are designed to monitor pressure and temperature changes through time in a horizon of subseafloor fluid flow and to collect a time series of fluid and gas samples for subsequent chemical analysis (Jannasch et al., 2003). One observatory was installed at Site 1253 on the incoming oceanic plate with instruments located within the fractured igneous section at 494–504 and 512–520 mbsf. Another CORK-II was installed at Site 1255, 0.4 km arcward of the deformation front, to monitor and sample the region of maximum fluid advection within the décollement at 136–144 mbsf. Downhole instrumentation in the décollement includes OsmoFlowmeters that continuously inject tracers and monitor their dilution in the natural fluid flux at four horizontally spaced sampling ports, allowing identification of anisotropy in fluid flow.

Complete information regarding these sites can be found in the site chapters in the ODP Leg 205 Initial Reports volume (Morris, Villinger, Klaus, et al., 2003).

Site 1253 is located ~200 m seaward of the deformation front in the deepest part of the Middle America Trench (Figs. F2, F3). Operationally, the primary goal for this site during Leg 205 was to recore the sediments immediately above the sill encountered during Leg 170, drill and core for the first time through the sediments below the sill, and core >100 m into the oceanic section. The other major task was to install a CORK-II observatory in the deep igneous section; coring and logging information was used to identify depths to set the packer and osmotic fluid and gas samplers.

One hole was drilled at Site 1253, which was partially cored and into which a long-term hydrologic borehole observatory was installed. After setting a reentry cone and 16¹/₂ inch casing into the seafloor, the hole was reentered with the rotary core barrel (RCB) and drilled without coring to ~370 mbsf. RCB coring below 370 mbsf penetrated 30 m of calcareous and locally clay-rich sediments with intermittent ash layers (average recovery = 75%) before encountering a gabbro sill between 400 and 431 mbsf (average recovery = 74%). Below the sill was ~30 m of partially lithified calcareous sediments with intermittent ash layers (average recovery = 20%). This interval was followed by coring ~140 m into a second igneous unit (average recovery = 75%) with local zones of 50%–55% recovery.

After coring, operations focused on preparing the hole for downhole logging and CORK-II installation. The hole was opened to 14³/₄ inches; 10³/₄ inch casing was installed to ~413 mbsf and cemented in place to inhibit communication between the borehole and the formation. After drilling out the cement shoe and drilling a rat hole with an RCB bit, the hole was logged to a depth of 530 mbsf, below which a bridge was encountered. After logging, the CORK-II components were assembled, including a 4¹/₂ inch casing screen, casing packer, and casing made up to the instrument hanger. The entire assembly was lowered into the hole and latched in to seal the borehole outside of the 4¹/₂ inch casing. The OsmoSampler package with integral temperature sensors was lowered through the center of, and latched into a seat near the bottom of the 4¹/₂ inch casing. The final operation was to inflate the packers and shift spool valves connecting the CORK-II pressure monitoring system to the formation, completely sealing the zone to be monitored. Problems with the go-devil used for this step made it difficult to determine whether the packer had inflated or the valves had turned for pressure monitoring. Alvin dives since then have confirmed that the installation is fully operational. Three absolute pressure gauges including a data logger were installed in the instrument hanger head. One sensor monitored pressure within the sealed-off fluid sampling zone at the bottom of the hole, one monitored pressure variations in the borehole above the sealed-off section, and the third sensor provided seafloor reference pressures. One additional sampling line extended from the CORK-II head down to the screened interval below the packer and is available for future pressure/fluid sampling purposes.

Details of the CORK-II installation in Hole 1253A are shown in Figure F4, and petrological and structural characteristics of key depths are shown in Figures F5 and F6. The center of the packer was set at ~473 mbsf, with the inflatable element between 471.5 and 475.5 mbsf. Cores show this to be a high-recovery interval of massive rock with relatively few fractures. The upper OsmoSampler package, inside a 7.35 m long screen, is set between 497 and 504 mbsf. A fluid sampling line runs from a 2 m pressure screen within the casing screen to the CORK-II wellhead. The lower OsmoSampler package hangs in the open hole between 512.1 and 519.5 mbsf. The placement of the osmotic samplers was determined using a combination of scientific and operational constraints. Originally, the intervals 513–521 (now OsmoSampler package 2) and 560–568 mbsf were targeted. However, logging tools encountered a bridge at 530 mbsf, restricting OsmoSampler deployment to shallower levels. The upper pressure screen above the packer was set between two igneous subunits, where sediments collapsing around the screen should make an effective seal. The final installation configuration for this modified CORK-II geochemical and hydrologic borehole observatory is shown in Figure F4.

Site 1255 is located ~0.4 km arcward of the deformation front in a water depth of 4311.6 m and close to the Site 1043 holes drilled during ODP Leg 170 (Kimura, Silver, Blum, et al., 1997). Hole 1255A is ~20 m east of Hole 1043A and ~30 m northwest of Hole 1043B (Figs. F2, F7, F8). In Hole 1043A, the complete section was cored to 282 mbsf in the underthrust sequence (Unit U3), whereas Hole 1043B was logged using logging while drilling (LWD) to 482 mbsf, the top of igneous basement. Both holes penetrated the décollement, and their results were used to plan drilling strategy and installation of the CORK-II observatory.

After setting the reentry cone in Hole 1255A, the hole was deepened to 123 mbsf with a 14³/₄ inch bit, followed by installation and cementation of 10³/₄ inch casing to 117 mbsf. Coring started at 123 mbsf, after drilling out the cement shoe, and stopped at 157 mbsf, when a sudden increase in penetration rate during cutting of the fourth core indicated that the underthrust sediments had been reached. Installation of the CORK-II was successful and was completed with deployment of the remotely operated vehicle (ROV) platform. An OsmoSampler package with a custom pressure-neutral intake probe and self-sealing seat was installed at the décollement. The observatory configuration is shown in Figure F9. The center of the packer is at 129 mbsf and the center of the screen at 140 mbsf, in the middle of the geochemical anomaly determined from data from Sites 1255 and 1043. A second pressure port inside a small screen was installed just above the upper packer. A postcruise Alvin dive showed the installation to be fully operational, and pressure data showed a return to hydrostatic conditions within the borehole.

Dive operations during the Atlantis cruise included downloading data from the multilevel CORKs at Sites 1253 (sampling/monitoring screens at two levels in uppermost igneous basement) and 1255 (screens at the décollement and in the overthrust section). Pressure variations at the seafloor are dominated by tides, response to seafloor tidal loading in the formation, and overpressures at Site 1255. Complete hydrologic sealing took several weeks; the most significant leakage in the first few weeks of monitoring is inferred to have been associated with the high-pressure polypack glands that seal the CORK liner and main casing. Once the seals seated, signals ranging in period from weeks to minutes are observed from barometric, oceanographic, and tectonic sources. Several observations, summarized in the records shown in Figure F10, are of particular interest from a hydrologic and geochemical perspective.

At Site 1253, basement is underpressured relative to the local geothermal hydrostat by ~7 kPa (Fig. F10A), from which it can be inferred that the basement is highly permeable and provides a close-to-hydrostatic drainage path to the ocean for the seaward part of the underthrust sediment section. The degree to which fluids squeezed from the subduction zone sediment complex influence basement fluid composition remains unknown, but it is clear that upper permeable basement provides a link to deep-sourced fluids.

At Site 1255, fluid pressures in the décollement and the overlying overthrust sediments are superhydrostatic, varying with time (Fig. F10B). Maximum pressures are a significant fraction of lithostatic and decline steadily over the first few months of recording. Several events of tectonic (elastic) or hydrologic (diffusional) origin are observed at both screens. One of these (labeled "first event") is seen at the upper screen roughly 2 days before the décollement screen. This precludes the possibility that the event is associated with motion of the packer and indicates a hydrologic source. Observations of fluid compositional variations will be critical for determining the cause of such events and the slow pressure decline.

Eight Alvin dives were planned for recovering and replacing the OsmoSamplers and temperature loggers at Sites 1253 and 1255 in February–March 2004. The intent was to place a winch on top of the wellhead, latch on to the instrument string with the running tool, and use the winch to break the seal and pull the OsmoSamplers package to the wellhead, where they could be floated to the surface. Replacement samplers dropped by elevator would then be guided hand-over-hand into the 4.5 inch casing and allowed to free fall to seat. The Alvin and Atlantis crews performed superbly, but we encountered several problems. During dive 1 we installed the winch. During dive 2 we were unable to latch the running tool into the sampler despite repeated attempts. Slack and additional play in the winch line suggested soft debris, possibly rust brushed from the 4.5 inch casing by passage of the tool and line atop the samplers, was occluding the latch. The running tool was recovered and additional jars added in an attempt to penetrate the debris with a heavier tool. Eventually, after overcoming several other problems, the running tool latched into the OsmoSampler package, as determined from pull on the winching motor. A design incompatibility between the winch and optimal Alvin operations resulted in the OsmoSamplers being dropped back into the hole after being winched up 70–100 m. During penultimate dive 7, we made a brief attempt to retrieve the OsmoSamplers, but time limitations made them impossible to recover, given the need to secure the sites and recover materials on bottom.

Ultimately, pressure data were downloaded at both sites; Site 1255 was left in its original condition, and Site 1253 was left with the OsmoSamplers seated at depth, the tools and ~550 m of Spectra line attached, and a ring and float attached ~20 m above the wellhead. The impact of the engineering and borehole complications were, of course, exacerbated by Alvin's limited bottom time in deep water and power. Although these factors were recognized before scheduling ship time, Jason was fully booked through and beyond the 2 y window of the OsmoSampler and temperature logger configuration. Lessons learned from this Atlantis cruise benefited final engineering design and fabrication for both IODP Expeditions 301 and 301T. Sites 1253 and 1255 were left ready for OsmoSampler recovery and replacement by the JOIDES Resolution, submersible, or ROV. Using the JOIDES Resolution to recover and replace the OsmoSamplers allowed us to install lines to the seafloor that will make future ROV/submersible recoveries feasible without the submersible winch system, which has been problematic as presently configured. Continuous operations allow for time-efficient recovery and reinstallation.