ABSTRACT

In our earlier work, we determined relative REE partition coefficients between calcite and aqueous (CaCl₂ + NaCl) solution, but absolute values could not be obtained because of small overgrowths of calcite. In this study, we have made similar experiments in order to determine the absolute REE partition coefficients between calcite and aqueous solution, and have evaluated partition coefficients between calcite and seawater. Calcite-supersaturated solution doped with REEs was constantly pumped into a reactor to form calcite overgrowths on the seeds under CO₂ + N₂ gas. Absolute values of REE partition coefficients, K_d(REE)=(X_REE/X_Ca)_calcite/([REE,_tot.]/[Ca])_solution, have been obtained. The patterns of K_d(REE) show convex tetrad effects and Y/Ho fractionation, which is consistent with our earlier work. We have calculated REE abundances in Permian seawater using the partition coefficients corrected to seawater compositions and REE analyses of Permian seamount-type limestones. All the estimated REE abundances in Permian seawater indicate concave tetrad effects, large Y/Ho fractionation and negative Ce and Eu anomalies in their chondrite-normalized patterns, which are quite similar to those for present-day seawater. The estimated REE abundances in Permian seawater, however, are lower than those in present-day seawater by approximately one order of magnitude. This suggests that significant parts of marine limestones could not incorporate seawater REEs after the permeability and porosity of the original biogenic carbonate sediments with low-REE contents had been reduced to certain critical levels.