Uptake and fractionation of rare earth elements on hydrothermal plume particles at 9 degrees 45 ' N, East Pacific Rise

Particulate samples (>0.45 mu m) from a neutrally buoyant hydrothermal plum e at 9 degrees 45'N on the northern East Pacific Rise were collected using large volume in situ filtration and analyzed for Fe, Al, Mn, Ni, and fourte en rare earth elements (REE). The Sm/Fe ratio (a proxy for overall REE/Fe) and Nd/Er (light/heavy REE fractionation) increased moderately with decreas ing particulate Fe. Chemically, the sense of these relationships matched th at documented previously in the TAG plume on the Mid-Atlantic Ridge (German et al., 1990), although particulate Fe was about 10 fold lower at 9 degree s 45'N. Spatial trends relative to the vent source, however, were opposite of expectation because slow Fe(II) oxidation and Fe(III) colloid aggregatio n over this interval led to increased particulate Fe (10-26 nM) with distan ce from source (Field and Sherrell, submitted). After subtraction of non-pl ume background particle composition, plume particles at 9 degrees 45'N and TAG had indistinguishable ranges of light REE-enriched fractionation relati ve to ambient seawater and had very similar Sm/Fe (therefore Kd for Fe oxyh ydroxides), demonstrating that plume particles in both oceans reflect to a first degree the local seawater REE composition. Within-plume REE variation s at 9 degrees 45'N were investigated using a simple mixing model which acc ounts for the bulk Fe-Al-Mn variations in the plume using two endmembers: f resh hydrothermal oxyhydroxide precipitates and ridge-crest background part icles (composed largely of locally resuspended sediment). Sm/Fe and Nd/Er p lot linearly with mixing ratio (R > 0.96), implying that the observed REE t rends result from mixing of these two endmembers. Extrapolation to the comp osition of pure hydrothermal precipitates suggests that Nd/Er is fractionat ed relative to seawater by a factor of 1.8 during adsorption onto fresh Fe oxyhydroxide particles. The ridge-crest background particles are 5 fold hig her in Sm/Fe and Nd/Er is 2.49 relative to seawater, partly a result of enr iched terrigenous component in the resuspended matter. A reinterpretation o f REE at TAG reveals that positive curvature in REE vs. Fe plots, argued pr eviously to reflect continuous REE uptake (i.e., increasing Kd; German et a l., 1990), may result from local depletion of the dissolved REE pool by par titioning onto Fe particles at Fe > 100 nM. Similar drawdown effects could contribute to the variable degrees of curvature observed for all seawater-s ource particle-reactive species in plumes that are sampled at high particul ate Fe concentration. In sum, REE behavior in hydrothermal plumes is more c onsistent with equilibrium adsorption and mixing of distinct particle types , than with kinetic uptake control. Precise measurements of REEs in modern ridge-crest metalliferous sediments could be compared to the endmember comp osition calculated from the plume data to evaluate long-term changes in REE of the hydrothermal component. Copyright (C) 1999 Elsevier Science Ltd.