INTRODUCTION

Ocean Drilling Program (ODP) Leg 205 returned to the Middle America Trench to install long-term observatories in order to investigate the active fluid flow across the Costa Rica margin and its implications for the seismogenic zone and subduction factory. This was achieved by installing circulation obviation retrofit kit (CORK)-IIs at two sites across the Middle America Trench offshore Costa Rica, previously drilled during ODP Leg 170 (Kimura, Silver, Blum, et al., 1997; Morris, Villinger, Klaus, et al., 2003) that were instrumented for long-term monitoring of pressure and temperature and for collecting time series of fluid and gas samples for subsequent chemical analysis (Jannasch et al., 2003). Sediment coring, therefore, focused on the décollement zones at the prism sites (1254 and 1255, corresponding to Leg 170 Sites 1040 and 1043, respectively) and basement drilling at the incoming plate Site 1253 (Leg 170 Site 1039). Because of this, few new pore fluid and sediment samples were obtained during Leg 205. The distinct pore fluid chemical signals across the décollement zones observed during Leg 170 (Kimura, Silver, Blum, et al., 1997) were used to identify the décollement zones at the adjacent sites drilled during Leg 205. At the incoming plate, Site 1253, pore fluids and sediment samples were obtained only from the basal sediment section. Diagenesis was observed in the overlying sediments and between sills, and correspondingly, the pore fluids of the most altered sediments were modified as well.

Tracers of interest to geochemists in constraining the extent of smectite-illite reaction in the updip fluid source region and deciphering the bulk composition and temperature of the fluid source region include concentration ratios such as K/Li or K/Rb, F/Cl, and B and Cl isotope ratios. Halogen concentration ratios are presented below; however, because of mass spectrometer instrument problems, Cl and B isotope ratio data sets are incomplete. Preliminary results of the pore fluid Cl isotopic compositions indicate that at Sites 1040 and 1254, the ³⁷Cl values in décollement and fracture zone pore fluids are more negative than the values in prism and underthrust sediments. Tracers of interest to geochemists for tracking element recycling in volcanic arcs by subduction (e.g., Rb, Cs, Sr, and Ba concentrations and Sr and Li isotope ratios) were analyzed in the fluids and are reported in Tables T1 and T2. When flow rates are established using data generated by ongoing monitoring of deep-sourced fluids, flux rates for elements leaving the subducting plate in fluid advected from deeper sources will be calculated. Such data will be useful for evaluating the impact of shallow slab dewatering on ocean chemistry and on composition of the residual slab subducted deeper (ultimately to depths of magma generation).

The tracers chosen for pore fluid analysis from the Costa Rica subduction zone have two important characteristics:

1. Some tracers identify fluid-rock reactions (i.e., Li, B, Sr, and Ba), or specifically fluid-hydrous mineral reactions and mobility in the subduction zone (F, Cl, and Br).
2. Other tracers provide information on the approximate temperature of reaction (Li, K, Rb, and Cs) or on the sources and mixing of fluids involved in different fluid-rock reactions (Li, Sr, Cl, and O isotope ratios). Some of these are particularly important because they also are not involved in key retrograde reactions during migration (i.e., Cl, Br, and, possibly, F), thus, they maintain their identity even when mixed with diagenetic fluids formed in situ along the flow lines.

Because the geochemical behavior of halogens (F, Cl, and Br) is dominated by strong partitioning into the fluid phase, they are excellent tracers of fluids during subduction. Elemental and isotopic halogen systematics can thus supply critical information not available from other measurements about sources of fluids, flow paths, and reaction conditions. Cl, F, and Br have long been identified as excess volatiles that are thought to have accumulated in seawater, their principal exogenic reservoir, by outgassing of the mantle or from a late accretion of volatile-rich material (Rubey, 1951; Schilling et al., 1978; Jambon, 1994). Despite their abundances and widespread distribution, the geochemical behavior and cycles of F, Cl, and Br are, as yet, not well documented (Jambon, 1994). Rb and Cs, along with K, Ba, and Sr (also La, Th, and U, for which data are not presented) are enriched in arc volcanics worldwide. They are large ion lithophile elements. Rb and Cs concentrations are not routinely measured in pore fluids from ODP legs; however, there have been several recent hydrothermal experiments that investigated their mobility as a function of temperature using representative sediments and seawater (James et al., 2003; W. Wei, pers. comm., 2005). Rb and Cs behave similar to K at low temperatures and are typically partitioned into solids; however, Rb and possibly Cs concentrations increase in the fluid as a result of ion exchange with NH₄ in the sulfate reduction zone. Each of the alkali metals (Li, K, Rb, and Cs) has a characteristic behavior with respect to partitioning into the fluid phase as a function of temperature (W. Wei, pers. comm., 2005). In general, at moderate to high temperatures (>60°C) the alkali metals partition into the fluid phase, with Cs partitioning earlier than Rb.

Strong evidence for vigorous, shallow flow of cool fluids in the oceanic section of the subducting plate at Site 1039 that may affect the updip limit of seismicity was obtained during Leg 170 and is supported by Leg 205 shipboard data. At the locations of 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 taken during recent cruises show that seamounts are sites of fluid discharge and recharge (Fisher et al., 2003b), and modeling suggests that lateral flow rates of 3–30 m/yr in zones within the upper 600 m of high-permeability basement (10^–10 to 10^–8 m²) are required to match the low heat flow on East Pacific Rise (EPR)-generated crust (Fisher et al., 2003a). Chemical data also suggest vigorous and recent/contemporaneous fluid flow. The active flow system consists of slightly modified modern seawater within the upper oceanic crust (Silver et al., 2000; Kastner et al., 2000). For example, Sr chemical and isotopic values in basal sediments are distinct from those appropriate for seawater of the sediment age or for pore fluid compositions modified by ash weathering, as seen higher in the sediment column (Silver et al., 2000). Simple modeling suggests that the gradients, also observed for Li, Ca, and SO₄, would diffuse away in ~15 k.y., unless supported.

At the décollement sites (1040, 1043, 1254, and 1255), pore fluid samples from the plate boundary and fault zone above it show strong, narrow anomalies in the 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 and fault zones that transports species generated at temperatures of ~150°C (i.e., at or near temperatures thought to occur at the updip limit of the seismogenic zone). The persistence of local compositional anomalies suggests transient flow.