RARE-EARTH ELEMENT ADSORPTION IN A SEAWATER PROFILE ABOVE THE EAST PACIFIC RISE

The distribution of rare-earth elements (REE) has been studied in a se awater profile above the East Pacific Rise at 21-degrees-S, 114-degree s-W. REE abundances and patterns correlate with hydrographic parameter s indicating that specific water masses display individual REE signatu res. Data are presented for the < 0.45 mum fraction (REE(diss)) as wel l as for the total non-refractory of REE (REE(tot)). REE patterns deri ved from both sample sets show (i) HREE enrichment, (ii) step-like tre nds, (iii) pronounced depletion in Ce, and (iv), excepting samples fro m below 2500 m depth. no significant Eu anomaly. HREE are enriched due to solution complexation, while Ce anomalies result from preferential sorption of Ce4+ onto particles. which is revealed by higher Sm/Yb ra tios and less negative Ce anomalies in REE(tot) compared to REE(diss) patterns, respectively. The positive Eu anomaly in waters from below 2 500 m depth results from the presence of Fe,Mn-oxyhydroxides which pre cipitated from high-temperature solutions venting at the mid-ocean rid ge. Calculation of relative surface complexation (adsorption) coeffici ents for REE in seawater reveals an increase from La to Lu by a factor of 3, with Gd showing an anomalous behaviour. The overall HREE enrich ment displays the dominance of the competing process of solution compl exation which increases from La to Lu by a factor of 20.