ABSTRACT

Partitioning experiments of rare earth elements (REEs) between calcite and aqueous (CaCl₂ + NaCl) solution at 25°C and 1 atm have been made in order to elucidate the incorporation process of seawater REE into marine limestones. Calcite-supersaturated solution doped with REE was constantly pumped into a reactor, in which calcite seeds and the solution were mixed and CO₂ + N₂ gas was bubbled to allow calcite overgrowths. Absolute values of REE partition coefficients, K_d(REE) = (X_REE/X_Ca)_calcite/([REE]_total/[Ca])_solution, could not be determined because of very small calcite overgrowths and difficulty in evaluating X_Ca. The relative values of K_d(REE) like K_d(REE)/K_d(Gd), however, were determined successfully. K_d(REE) given by [REE]_total depends on REE(III)-carbonate complexation in solution, and does not correspond to a reaction incorporating a specific dissolved REE species into calcite. Hence, here we used K_d(REE)_ω = (X_REE/X_Ca)_calcite/([REECO₃⁺]/[Ca])_solution, which is given by K_d(REE) and REE(III)-carbonate complexation constants. K_d(REE)_ω relates directly to ΔG_r. for the reaction incorporating REECO₃⁺_(aq) into calcite. The series variation of experimental logK_d(REE)_ω shows a convex tetrad effect, suggesting larger Racah parameters of REECO₃⁺_(aq) relative to REE(III) in calcite. The variation also shows a kink at around Pm, Sm and Eu, possibly reflecting a coordination change of REE(III) in calcite across the series. Yttrium is definitely enriched in solution relative to heavy REEs. Using the relative values of experimental logK_d(REE)_ω and REE analyses of upper Paleozoic Japanese Ishimaki and Tahara limestones, REE abundance patterns for seawater coexistent with the marine limestones have been calculated provided that pH, ΣCO₂ and salinity are the same in the ancient and present times. The calculated patterns are quite similar to those for relatively deep waters at around 400 to 1000 m in the modern Pacific. Ishimaki and Tahara limestones are of the seamount-type. Hence, the calculated seawater REE patterns with large negative Ce anomalies suggest that the incorporation of seawater REE into the limestones occurred in moderately deep water, probably because of subsidence of volcanic seamounts that were capped by Ishimaki and Tahara limestones.