RARE-EARTH ELEMENTS IN OLD BIOGENIC APATITES

The REE distributions in individual Upper Devonian conodonts have been measured by ion probe. The patterns of all analyzed conodonts are enriched in middle REE (Eu-Gd) and have a weak or no Ce anomaly. Concentrations and La/Yb or La/Sm ratios vary very little within or among individuals from the same zone, which suggests that uptake of labile REE from sediments was essentially quantitative. Therefore, the REE signature of the primary carriers, probably organic and oxyhydroxides particulates from marine suspensions, was efficiently transferred to biogenic apatites and survived late diagenetic processes. REE patterns of conodonts do not resemble those of present-day seawater and post-Cretaceous biogenic phosphates, which are typically depleted in Nd and Sm with a negative Ce anomaly. Since REE distributions in the modem water column mimic those of nutrients, we assume that, in pre-Cretaceous seawater, they were not controlled by surface biological activity. We assume instead that REE in pre-Cretaceous seawater can be explained by mechanism of desorption-adsorption on particle surfaces. Progressive extraction of LREE from river water by oxyhydroxides leads to precipitates enriched in middle REE. A simple quantitative model was calculated in order to illustrate the proposed mechanism.