As noted, a high priority for Leg 205 was the installation of modified CORKs (CORK-II; for details see Morris, Villinger, Klaus, et al., 2003, and Jannasch et al., 2003) into the fluid flow systems of the Costa Rica margin, successfully accomplished at Site 1253 on the incoming plate and at Site 1255 in the prism. In both cases, drilled holes were cased with cement, with 4.5-in casing run in the hole to house sampling and pressure screens and the downhole instrumentation (see Morris, Villinger, Klaus, et al., 2003, for operational details). Postinstallation, a packer was inflated to isolate the horizon of interest from overlying levels. Installations at both sites monitor P and T of formation fluids at selected intervals while simultaneously collecting a time series of inflowing fluids for subsequent analysis of dissolved and gaseous species (see Jannasch et al., 2003). Pressure monitoring follows the methods described in Davis et al. (2000), and methods of temperature monitoring are described in Heesemann et al. (this volume). The fluid samplers (OsmoSamplers) use osmotic pumps to suck fluid from the inlet ports and pump it into long, clean Teflon coils or copper coils (for later analysis of gaseous species), thus creating a time series of stored fluid. Redundant OsmoSamplers were deployed at every level instrumented.
Schematics in Figure F8 show the design of the CORK at Site 1253, which is monitoring and sampling within a fractured region of the upper igneous section. At this site, a screen for pressure monitoring and fluid collection is located at a depth of 497–504 mbsf with a dangling OsmoSampler deployed at 512–519 mbsf. Miniature temperature loggers (Pfender and Villinger, 2002) are deployed with the OsmoSamplers at both levels. Both OsmoSamplers are within the open hole beneath a packer centered at 473 mbsf. Another screen for pressure monitoring is located just above the packer.
Several attempts to install a CORK-II with instrumentation in the décollement zone failed at Site 1254 (Morris, Villinger, Klaus, et al., 2003) but were successful at Site 1255. As shown in Figure F9, the sampling screen and one pressure screen are within the décollement, centered at a depth of 140.2 mbsf. The packer is located at 129.3 mbsf, and the hole ends at 153 mbsf, creating a very limited horizon around the décollement zone for fluid sampling. A second screen for monitoring pressure is again located just above the packer, and miniature temperature loggers are again deployed with the OsmoSamplers. Downhole instrumentation at Site 1255 also included an OsmoFlowmeter experiment (Jannasch et al., 2003). Located below the OsmoSamplers, the flow meters use osmotic pumps to inject a tracer fluid into the formation at a constant rate. Four sampling ports on the same horizontal plane as the injection port sample the formation fluid that has mixed with the injected tracer fluid. Dilution of the tracer is proportional to rate of fluid flow; azimuthal variation in the flow rate recorded in the four sampling ports speaks to anisotropy in the fluid flow. Unfortunately, the operations required to install the CORK and its instruments make it impossible to link flow directions as sampled by the OsmoFlowmeters to a geographic reference frame.
Despite the emphasis on downhole instrumentation, 370–600 mbsf was cored during Leg 205 at Site 1253 (equivalent to Leg 170 Site 1039) on the incoming plate. In addition to the sill and lower igneous complex discussed above, Leg 205 coring recovered, for the first time, sediments from the 30-m interval between the two igneous subunits. In the sedimentary wedge, Leg 205 cored from 150 to 223 mbsf and 300 to 368 mbsf in Hole 1254A, equivalent to Hole 1040C. Coring focused on an upper fault zone and the décollement zone itself, with penetration to the uppermost part of the underthrust sediment section. At Site 1255, equivalent to Site 1043, coring was limited to the interval about the décollement, 123–157 mbsf. Despite the limited coring, a significant amount of work is emerging from the papers in this Scientific Results volume, combining results for samples from Legs 170 and 205.