ORIGIN OF ANOMALOUS RARE-EARTH ELEMENT AND YTTRIUM ENRICHMENTS IN SUBAERIALLY EXPOSED BASALTS - EVIDENCE FROM FRENCH-POLYNESIA

Basalts from French Polynesian islands occasionally display extremely high abundances and anomalous distributions of rare-earth elements (RE E) and yttrium, whereas other incompatible element concentrations and O, Sr, Nd and Pb isotopic ratios do not differ from those of ''normal' ' basalts from the same area. The REE- and Y-enriched basalts contribu te up to 15% of the sample set, suggesting that this feature is more w idespread than previously thought. REE-Y enrichment is related to the presence of rhabdophane-type REE-Y-phosphates identified through elect ron microprobe analyses in the most enriched sample and inferred from leaching experiments in the others. This phenomenon is confined to sub aerially exposed basaltic sequences, indicating a close relationship t o supergene processes. This is supported by negative Ce anomalies in t hese basalts, since decoupling of Ce from the other REE is restricted to oxidizing, low-temperature, aqueous environments. Similar Nd isotop ic ratios for enriched and normal basalts allow us to exclude the poss ibility that additional REE and Y are derived from marine sediments or guano, but rather suggest an origin from the local basalts. Moreover, light REE enrichment in the REE-Y-phosphates suggests short migration distances of the fluids, supporting the conclusion that additional RE E and Y were mobilized from weathered basalts and transported by desce nding meteoric waters.