Hydrothermal scavenging on the Juan de Fuca Ridge: ²³⁰Th_xs, ¹⁰Be, 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, ∼8km west of the "MegaPlume" area previously identified near 45°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 ²³⁰Th_xs activities down-core are consistent with continuous slow sediment accumulation rates of 0.54 cm/ky for ∼200 ky since the deposition of the deeper Fe-rich horizon. ¹⁰Be₍₀₎ and ⁹Be isotope concentrations also co-vary with hydrothermal Fe down-core and exhibit ¹⁰Be₍₀₎/⁹Be ratios which approach that of Pacific Ocean deep water, indicative of a seawater-scavenging source. ¹⁰Be₍₀₎/ ²³⁰Th_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 ¹⁰Be into ridge-flank sediments.