YTTRIUM AND HOLMIUM IN SOUTH-PACIFIC SEAWATER - VERTICAL-DISTRIBUTIONAND POSSIBLE FRACTIONATION MECHANISMS

Concentrations (determined by ICP-MS) of dissolved Y abundances in a s eawater profile above the East Pacific Rise at 21-degrees-S, 114-degre es-W range from 50 pmol/kg in surface water to a maximum of 205 pmol/k g at 2250 m depth. Y depletion in deeper water (e.g., 166 pmol/kg at 2 500 m) appears to be a local phenomenon, probably related to the prese nce of Fe-Mn-oxihydroxide particles of hydrothermal origin. In general , the nutrient-like vertikal distribution of dissolved Y is rather sim ilar to the distribution of heavy rare-earth elements and suggests tha t Y takes part in vertical biogeochemical cycling. Comparison to deter mined Ho concentrations reveals that despite similar valencies and ion ic radii, Y and Ho may be fractionated in aqueous solution. Y/Ho molar ratios of seawater cluster between 90 and 110 and are well above Y/Ho molar ratios of both chondrites and shales which are almost-equal-to 52. Ho apparently shows higher affinity to particle surfaces than does Y. Though to some extend Y-Ho fractionation occurs in seawater itself , non-chondritic Y/Ho ratios observed all along the seawater profile s uggest that the most prominent fractionation step may occur elsewhere. Fractionation of Y and Ho may be a consequence of more stable surface complexes or less stable solution complexes of Ho compared to those o f Y, or may result from higher solubility of Y phosphate relative to H o phosphate; of course, combinations may be possible. At this state, t horough evaluation of the relative importance of these mechanisms is i mpossible due to lack of relevant thermodynamic data for Y which could be compared to existing data for Ho or its neighbours in the rare-ear th element series.