Texas A&M University, United States

Bulk geochemical analyses were carried out on 27 sediment samples located just above the igneous section of the incoming plate along the Costa Rican convergent margin. This pilot study was intended to determine whether hydrothermal activity within the sediment section could be detected within the bulk sediment. The metaliferous index (100 X Al/ [Al + Fe + Mn]) values on bulk sediment (24.3 < X < 43.3) are all much lower than the value for deep pelagic sediment (50.8) but higher than the value for the hydrothermal sulfide from the East Pacific Rise (1). The high Zn, Cu, and Co concentrations associated with high Ba content for a constant detrital input (molar Ti/Al = 0.121 + or - 0.026) indicate that the transition metal enrichment is not related to volcanic ash but rather to Fe-Mn oxides, sulfides, and barite. These observations testify to a record of hydrothermal activity within the 70 m of sediment overlying the basement.