FY 1998

Hydrothermal scavenging on the Juan de Fuca Ridge: ²³⁰Th_xs, ¹⁰Be, and RREs in ridge-flank sediments

German, C.R., D.L. Bourles, E.T. Brown, T. Hergt, S. Colley, N.C. Higgs, E.M. Ludford, T.A. Nelsen, R.A. Feely, G. Raisbeck, and F. Yiou

Geochim. Cosmochim. Acta, 61(19), doi: 10.1016/S0016-7037(97)00230-5, 4067–4078 (1997)

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, ~8 km 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-normalized 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.