During Leg 205, we returned to near the Leg 170 drill sites. The planned Leg 205 sites were 1253, 1254, and 1255. Science objectives for Leg 205 had two primary foci. The first was the igneous and alteration history of the basement at reference Site 1253 on the incoming plate. The second was the three hydrological systems: in basement at Site 1253, along the décollement at Sites 1254 and 1255, and in the uppermost section of the subducting sediment section at Site 1254. In the prism sites, coring in the fault zones provides an opportunity to integrate structure, fluid flow, and fluid chemistry. These goals were accomplished as described in detail below by limited coring of selected intervals, downhole temperature measurements, logging at Site 1253, an extensive shipboard and postcruise analytical plan, the installation of long-term observatories to monitor temperature and pressure, and sampling fluids and gases at key hydrological intervals.

During Leg 205, coring and sampling began at Site 1253 within the carbonates above the sill encountered during Leg 170 and continued through the sill and the previously undrilled sediments below and ~100 m into the lower igneous unit. The scientific objectives to be addressed through coring, sample analysis, and logging at Site 1253 were as follows:

1. Calculate subduction fluxes to the trench. Leg 170 data provide information for the bulk of the sediment section, whereas Site 1253 coring and shipboard and postcruise analysis will add data for the lowermost sediment section, the sill, and the lower igneous unit. Emphasis will be placed on elements and isotopes of particular interest for evaluating sediment dynamics through the forearc, fluid and element fluxes out of the downgoing plate as it passes through the seismogenic zone, subduction and recycling behavior of carbonate and CO₂ from trench to arc, composition of the subducting igneous basement (fresh and locally altered), and the trench-arc recycling of key tracers for quantifying sediment and oceanic crust contributions to the arc lavas. Leg 205 data will be combined with results from the 600- to 800-m penetration of EPR oceanic crust during Leg 206 to provide a more complete picture of the downgoing oceanic crust.
2. Investigate the extent of sill emplacement, their origin and tectonic implications thereof, and their contribution to the bulk composition of the subducting igneous crust. Leg 170 encountered a gabbro sill, with trace element characteristics consistent with an origin related to Galapagos activity.
3. Determine the igneous and alteration mineralogy, petrology, and geochemistry in the uppermost 140 m of igneous section and characterize the original igneous structure therein, using coring and logging data. Use the geochemical data to calculate subduction fluxes. Attention will be paid to low-temperature alteration features that may result from near-trench fluid flow as well as that deriving from ridge-crest and near-off-axis hydrothermal circulation.
4. Collect microbiological samples within the more fractured and veined parts of the oceanic section for shipboard cell counts and preliminary DNA extractions and postcruise analysis. Contamination testing using perfluorocarbon tracers (PFTs) and microspheres will be carried out.
5. In conjunction with Leg 170, construct an ash stratigraphy that maps Galapagos and Central American ashes as the plate moves from near ridge to near trench.
6. Determine physical properties in the core and borehole that may affect estimations of igneous composition and lithologic variation or that relate to fluid flow and deformation such as porosity, density, fracture distribution and orientation, and strength. Identify regions of higher fracture density as possible sites for osmotic samplers.

A long-term borehole observatory (i.e., a modified CORK-II) (Fig. F12) was installed at Site 1253 to sample fluids and to monitor temperature and pressure within the uppermost permeable basement. Osmotic samplers were installed at two different levels within the section. The science objectives for the CORK-II installation were to

1. Use pressure, temperature, fluid, and gas compositions together with downhole measurements to characterize the fluid and heat fluxes responsible for the abnormally low heat flow in the vicinity of this site caused by seawater incursion inferred to be to basement. Monitoring and sampling will run for 1–2 years, after which pressure data will be downloaded and the temperature loggers and osmotic samplers will be replaced with instruments designed to operate for 4 years before recovery.
2. Evaluate the thermal, hydrological, and chemical implications of this extensive fluid circulation for the thermal structure of the uppermost part of the subducting plate, the hydrological pathways available in the shallow subduction zone and beneath the overlying prism, and global element fluxes.

Limited coring focused on an upper fault zone and the décollement; these two fault zones have pore fluid chemical anomalies indicative of advective flow of deeply sourced fluids. Two penetrations of the décollement and good recovery therein allowed more detailed structural analysis at several sites (in conjunction with Leg 170), higher resolution chemical sampling, and integration of structural characteristics with indicators of fluid flow. Coring results were used to guide installation of a CORK-II to monitor and sample within the area of maximum flow of deeply sourced fluids in the décollement. Together, these approaches plus shipboard and postcruise work will be used to address the following scientific objectives:

1. Make detailed observations of the structural development of the upper fault (one site during Leg 170 and one during Leg 205) and décollement (two sites during Leg 170 and two more during Leg 205) that will contribute to a better understanding of the structural development of the décollement zone as a function of position along and within the plate boundary fault zone, lithologic properties, and fluid flow characteristics.
2. Determine physical properties, including permeabilities, of the décollement horizon from further structural experiments on whole-round samples to constrain hydrological modeling and permit integration of fluid flow and deformation models.
3. Determine heavy hydrocarbon chemistry within the fault and décollement zone sediments and major and minor cation and anion concentrations in pore fluid profiles from fault zone and décollement whole rounds. The advective spikes and diffusive gradients derived therefrom will be evaluated against the structural features in the cores and also compared to profiles measured during Leg 170 to evaluate possible heterogeneity.
4. Collect whole-round samples from the upper fault zone and décollement under appropriate conditions for postcruise microbiological investigations to determine the resident microbial ecology of the zone for comparison to eventual microbial experiments on fluids collected from the décollement. Contamination tests will be carried out using microspheres only to eliminate any possible chemical contamination of pore fluids by the PFT solution.
5. Install a CORK-II at one site into the zone of maximum advective flow to determine pressure, temperature, and composition of fluids and gases along the décollement and evaluate any possible changes through time for hydrologic modeling. OsmoFlow meters will be deployed at these sites in an attempt to constrain fluid flow rates through the sampler screen. This same data set will constrain the flux of elements out of the downgoing sediment section along the décollement to evaluate the role of fluid egress on element fluxes to the ocean and its corollary, changing composition of the residual slab because of fluid loss.
6. Use selected elements, element ratios, and isotopic compositions in the fluids from the décollement, both from whole rounds and ultimately CORK-II osmotic samplers, in an attempt to constrain dehydration reactions at the updip and, perhaps, downdip limits of the seismogenic zone.