MODELING OF RARE-EARTH ELEMENT PARTITIONING BETWEEN PARTICLES AND SOLUTION IN AQUATIC ENVIRONMENTS

We have modeled the partitioning of rare earth elements (REEs) between O-donor surface groups and aqueous solutions with a simple thermodynamic treatment. Our model predicts that the Lanormalized ratios of adsorbed to dissolved REEs depend primarily on the first hydroxide binding constants of the REEs, their first and second carbonate complexing constants, and the ionic strength and carbonate concentration of the solution. For modern and for ancient seawater, we predict an overall light REE (LREE) enrichment of surfaces relative to coexisting aqueous solutions and a positive Eu anomaly in the adsorbed component. The REE pattern of average modem ocean water can be explained quantitatively if the oceans are equilibrated with particle surfaces having REE patterns similar to average upper continental crust. Applying our treatment to the REE patterns of sediments from the Archean Hamersley-basin iron formation, we show that the observed positive Eu anomalies and LREE depletions of these rocks can be qualitatively accounted for if a significant fraction of the REEs in these rocks were initially present as an adsorbed component equilibrated with Archean ocean water that had no Eu anomaly. This suggests that the sources of REEs in the Archean ocean could have been dominantly continental as they are today and that the observed positive Eu anomalies of banded iron formations need not be inherited from the aqueous solutions from which they were deposited and do not necessarily indicate a significant ocean hydrothermal component in these solutions.