Hydrothermal scavenging on the Juan de Fuca Ridge: Th-230(xs), Be-10, and REEs in ridge-flank sediments

We have investigated the geochemistry of a hydrothermally enriched sediment core recovered from the western flank of the N.Cleft Segment, Juan de Fuca Ridge, similar to 8km west of the ''MegaPlume'' area previously identified near 45 degrees N. The core contains varying biogenic, lithogenic, and hydrothermal components, as reflected in CaCO3, Al, and Fe contents, respectively. Horizons of pronounced hydrothermal input, in core-top sediments and at depth, exhibit increased concentrations of Fe, Cu, Zn, and Pb and shifts in Pb isotopic compositions toward nonradiogenic (MORB/hydrothermal) values. REE concentrations co-vary with hydrothermal Fe down-core, and shale-normalised REE distribution patterns exhibit both negative Ce-anomalies and positive Eu-anomalies, indicative of input from plume-particle fall-out. Unsupported Th-230(xs), activities down-core are consistent with continuous slow sediment accumulation rates of 0.54 cm/ky for similar to 200 ky since the deposition of the deeper Fe-rich horizon. Be-10((0)) and Be-9 isotope concentrations also co-vary with hydrothermal Fe down-core and exhibit Be-10((0))/Be-9 ratios which approach that of Pacific Ocean deep water, indicative of a seawater-scavenging source. Be-10((0))/Th-230(xs(0)) ratios throughout most of Core GC88-6 are greater than mean Pacific Ocean values, indicating that hydrothermal scavenging can lead to significant net removal of dissolved Be-10 into ridge-flank sediments. Copyright (C) 1997 Elsevier Science Ltd.