ABSTRACT

REE data for a GSJ reference carbonate sample (JDo-1) by ICP-AES, when normalized by reliable seawater REE data, exhibit fairly smooth abundance patterns except for Ce. This allows us to estimate the monoisotopic REE (Pr, Tb, Ho, and Tm) concentrations compatible with the widely accepted ID-MS data for seawater REE. For the same purpose, reported seawater REE data by ICP-MS have also been used. The monoisotopic REE concentrations estimated by the two methods are in good agreement. Eventually, the seawater REE patterns normalized by chondrite or average shale show obvious concave tetrad effects of W-type. There is no room for doubt as to the seawater tetrad effect, but its origin is open to debate. We proposed here that the tetrad effect originates from REE partitioning described by the ligand-exchange reactions of REE³⁺ between particulate REE and REE(CO₃)₂^-(aq) in seawater. The particulate REE must be REE(OH)₃-like ones rather than REEO_1.5. The refined spin-pairing energy theory explains that the logarithmic equilibrium constants for the reactions exhibit a tetrad effect when Racah parameters for 4f electron repulsion in REE³⁺ ions are different between the pair of REE(III) species. We inferred that Racah parameters of REE(OH)₃ and REEO_1.5 are similar but they are commonly much smaller than those parameters of REE(CO₃)₂^-(aq). When the reactions occur in the particulate-seawater system with high particulate concentration such as coastal seawater and estuarine water, the signature of concave tetrad effect is inevitably given to the seawater solution. Subsequent removal of excessive particulate matter from the system leaves the detectable tetrad effect in the seawater. Shale-normalized seawater REE patterns correspond roughly to the logarithmic equilibrium concentration ratios by the ligand-exchange reactions. Our proposal gives a consistent explanation for the seawater-like tetrad effects in REE patterns of marine carbonate rocks and the absence of such tetrad effects in those of deep-sea nodules. This is also related to a theoretical interpretation of the Masuda-Coryell plot in REE geochemistry.