Distribution of rare-earth elements (REE) in size fractions of recent sediments of the Indian Ocean

A sample of each of five sediment types from the Indian Ocean, all presenting a flat shale-normalized REE pattern has been divided into eight size fractions (> 35; 20–35; 5–20; 2–5; 1–2; 0.5–1; 0.2–0.5 and < 0.2 μm). Each fraction was examined for mineralogy and REE content. REE patterns of coarser fractions exhibit a negative cerium anomaly related to biogenic components, while fine fractions composed almost exclusively of smectite, show a positive Ce anomaly. Thus flat whole-sample REE patterns which are usually interpreted as indicating a ferrigenous origin can also be due to a mixture of Cedepleted and -enriched phases. The REE distribution was studied in fifteen other sediments. Thirteen fine-size fractions (< 0.5 μm) exhibit a positive Ce anomaly and two are Ce-depleted while the REE patterns of whole samples are flat, slightly Ce-depleted or -enriched. The Ce enrichment of most fine fractions could occur during submarine alteration of volcanic material forming smectite, but may also be produced or accentuated during reworking or subsequent to the deposition of sediments.