At the Costa Rican convergent margin, coring during Ocean Drilling Program (ODP) Legs 170 and 205 and associated postcruise studies identified the occurrence of three hydrological systems, one of which is acting within the uppermost part of the incoming oceanic crust. The existence of active fluid flow is substantiated by particularly low heat flow and by the pore fluid chemistry of basal sediments, which show similar characteristics to modern seawater (Kimura, Silver, Blum, et al., 1997). Sediment samples were collected from the sediment section of Subunit U3C overlying the incoming igneous section during Leg 205 on the Costa Rica convergent margin. This study was undertaken to determine whether low-temperature hydrothermal activity is recorded by sediment and to identify the geochemical features of low-temperature hydrothermal activity.
Leg 205 Site 1253 is located ~1 km away from Leg 170 Site 1039 and ~200 m seaward of the deformation front in the deepest part of the Middle America Trench (Morris, Villinger, Klaus, et al., 2003). Three sedimentary units and one gabbro sill were recognized at Site 1039. Unit U1 is composed of dark green diatomaceous ooze with ash layers and is underlain by Unit U2, which contains essentially clays. Basal Unit U3, which is divided into Subunits U3A, U3B, and U3C, exhibits a dramatic increase in biogenic sedimentation (nannofossil ooze) with minor ash layers. Sediment coring began at Site 1253 at 370 meters below seafloor (mbsf), within Subunit U3C. A mixture of nannofossil chalks with varying subordinate proportions of clay dominates the 70 m of recovered sediments (370–440 mbsf). A gabbro sill, Subunit U4A, intrudes the sediment Subunit U3C between 400 and 430 mbsf. The igneous intrusion modified the structure of the upper 3 m above it by driving strong diagenesis, lithification, and lamination. Below the gabbro sill, less lithified nannofossil chalks were recovered, but they are identified as the same lithologic unit as above the sill. We will refer in this study to sediment samples from Subunit U3C located below and above Subunit U4A but all located above Subunit U4B (i.e., the lower igneous unit). The dominant sediment is biogenic carbonate with <5% dark green clay layers. The sediment section has preserved a lamination fabric acquired during deposition under the influence of current activity. In addition, volcanic ash layers are an important minor lithology but do not represent more than 1% of the total stratigraphy. Sediment with the lowest proportion of detrital input exhibits a light color, so we collected a suite of samples at regular intervals showing the lowest proportion of dark green clay. Based on paleomagnetic analyses and biostratigraphy, the sediment section overlying the igneous section sampled for this study is between 15.6 and 18.21 Ma (Morris, Villinger, Klaus, et al., 2003; A. Strasser, pers. comm., 2004). Sediments sampled here are distinct from the basal carbonates at Leg 170 Site 1039, which show pronounced liesegang structures and other visual indicators of hydrothermal activity (Kimura, Silver, Blum, et al., 1997).
We present chemical analyses for major elements and carbonate contents along with trace element concentrations (transition metals and large ion lithophiles) for 27 sediment samples. Fourteen samples were selected above the igneous Subunit U4A between 370 and 400 mbsf (Cores 205-1253A-1R through 5R) and thirteen samples below at a depth interval of 430–440 mbsf (Cores 205-1253A-10R through 11R).