RARE-EARTH ELEMENT FRACTIONATION BY SELECTIVE SEDIMENT DISPERSAL IN SURFACE SEDIMENTS - THE SKAGERRAK

Neutron activation analysis was carried out to study the abundance, di stribution and fractionation of REE (Rare Earth Elements) in surface s ediment samples of 28 box cores collected along transects across the S kagerrak. Results show that the clays have 3-5 times higher contents o f K, Sc, Cr, Fe, Co, Rb, K, Cs, Ta and Th, LREE (La, and Ce) and MREE (Sm, Eu, and Tb), compared to sands and silts. The HREE (Yb and Lu) ar e only up to two times enriched in the clays and silts, compared with the values measured in sands. Shale-normalized REE patterns vary from flat for the sands towards enrichment (1.3-1.5 times) of LREE relative to HREE in the clays. This is consistent with published REE data for oceanic and riverine sediments and evidences fractionation. Hf and Zr are up to thee times enriched in sands and silts compared to clays. Sa mples with highest contents of Hf show HREE enrichment in shale-normal ized REE pattems. Simple model calculations show that on the average z ircon grains in Skagerrak silts and sands contain 800-1200 ppm Yb. Ass uming that this value is also valid for the zircon grains in the shale s, this indicates that lower HREE contents observed in Atlantic ocean shelf and oceanic sediments compared to shales cannot be related to a greater abundance of zircon in shales.