Evidence for subduction of upper plate serpentine at the western edge of the Caribbean plate

The margins community has only relatively recently begun to examine the substantially different tectonics and associated hydrologic systems of erosive convergent margins as compared with accretionary margins. One type example is the Costa Rica system, the western edge of the Caribbean plate. Such erosive margins can subduct significant amounts of upper plate material, altering the chemistry of fluids expelled at the forearc. In some cases these fluids may be diagnostic of the nature of the upper plate. Pore fluids collected from a recent mud flow at Mound 11 on the mid-slope of the forearc south of Nicoya have extremely high B/Li molar ratios while fluids expelled nearer the wedge toe in the ODP Leg 170/205 area have very low B/Li ratios. Since the formation of serpentinite is the only common subduction zone process that significantly fractionates B and Li, the fluid composition suggests that serpentinite minerals are involved in the fluid genesis. We hypothesize that the mound fluid source is just above the seismogenic zone at ~120-150°C and is composed of up to 90% altered igneous and ultramafic rock [Vannucchi et al., 2003] rich in serpentine mixed with ~10% lower plate sediment. In this zone, dehydration and illitization of smectite drives fluid production and some B and Li is released. The phase change in serpentine from low to high temperature forms releases significantly more B into the fluid, driving up the B/Li molar ratio. These deep sourced B-rich fluids pass upwards through the fractured upper plate to the slope sediment layer where they fluidize mud before extruding onto the seafloor. The deep fluid source may be unique to the Mound 11 area due to the heterogeneity of the source region but, more likely, is only evident here due to a recent eruptive cycle. Fluid sampled at other mounds may have had a longer residence time in the slope sediments where B would be adsorbed and the fluid diluted. This deep-sourced fluid only represents a minor component of fluids reaching the wedge toe via the décollement. Dilution by the more intense dewatering occurring toe-ward would likely make this deeper signal nearly undetectable. The dominant fluid source at the toe is further updip along the décollement at temperatures <100°C. Consolidation and early diagenitic reactions including the loss of interlayer water in smectite drives fluid production and releases B and Li into the fluid. This B/Li molar ratio is, however, unusually low for smectite dehydration, suggesting that some low-temperature serpentinization of upper plate materials may occur in the upper portions of the subduction channel, consuming B and leaching Li from the source rock, thus reducing the B/Li molar ratio to that seen in the Leg 170 area. Serpentinized ultramafic assemblages are known from the eastern Caribbean Large Igneous Province (CLIP) including picrites and komatiites. Kerr et al., [1998] suggest that such assemblages may be present throughout the CLIP. The results presented here support this suggestion.