ABSTRACT

A new experimental method was developed to understand the partitioning of rare earth elements (REEs) on carbonate phases in oceanic columns. This method comprises of a mildly-oversaturated stage (Stage 1) and an undersaturated stage (Stage 2), mimicking natural oceanic columns consisting of the upper oversaturated and deeper undersaturated layers, in which carbonate particles of different sizes partition REEs with seawater. Saturation levels were adjusted by purging N₂ gas with different CO₂ concentrations. In this method, partitioning progress and equilibrium can be monitored via two viewpoints: differences in REE/Ca concentration among differently-sized carbonate particles and ionic activity products (IAPs) of the experimental solution.

The distribution coefficients obtained by the experiments, however, varied over three orders of magnitude. By analyzing the solution and carbonate particles, the presence of Fe in the carbonate particles (Fe/Ca) was observed to influence the partitioning of REEs. Microscopic mapping of Fe and proportionality of Fe amount to surface area strongly indicate that Fe is distributed evenly on the surface of carbonate, and Fe hydroxide is considered to be responsible for the REE enrichment. Neither the presence of Mn nor type of seed crystal (calcite or aragonite) was likely to significantly influence partitioning.

When Fe/Ca of carbonate was small, the ion activity products of the solution were found to modulate the distribution coefficient, implying that the presence of REE disturbs free relocation of carbonate ions. This may lead to overestimation and underestimation of distribution coefficients in Stages 1 and 2, respectively. We conclude that the distribution coefficient of REEs on calcium carbonate is not very high, but in natural settings carbonate is still important in controlling REE distribution by hosting Fe hydroxide, which effectively partitions REEs.