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), 4067–4078, doi: 10.1016/S0016-7037(97)00230-5 (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.