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The refined spin-pairing energy theory (RSPET) has been improved in order to understand quan titatively the tetrad or double-double effects recognized in the Ln3+ ionic radii. Since the ionic radii have been determined from the lattice constants and structural parameters of Ln01.5 and LnF3, the lattice energies of the compounds and the enthalpy difference of d Hf (LnF3)-A Hf (LnO1,5) have been exam ined by the improved RSPET. The RSPET parameters for the lowest levels of 4fq electronic configura tions strongly depend upon the effective nuclear charge (Z*). Such effects due to Z* have been taken into account. This made it possible to separate the variations in the lattice energies and the enthalpy difference across the Ln3+ series into the following two parts: (1) the large variation as a smooth func tion of q (the lanthanide contraction trend), and (2) the small zig-zag variation referred to the tetrad or double-double effect. The lattice energy of Ln01.5 and A Hf (LnF3)-A Hf (Ln01.5) exhibit upward con cave tetrad curves in their plots against q of Ln3+. The tetrad effect in the lattice energy of LnF3 is less conspicuous. This means that the Racah parameters for Ln3+ decrease very slightly in going from the gaseous free Ln3+ to LnF3, and then decrease greatly to Ln01.5, in accordance with the nephelauxetic series. The differences in Racah parameters between LnF3 and Ln01,5 have been estimated from A Hf(LnF3) -A Hf(Ln01.5) by means of an inversion technique based on the improved RSPET. The RSPET results for the thermochemical data are consistent with the careful spectroscopic determinations of Racah parameters for NdF3 and Nd01.5. Both the tetrad effect and the smooth lanthanide contraction seen in the Ln3+ ionic radii can be interpreted in terms of the quantum mechanical energetics of 4f elec trons.

In our earlier work, we determined relative REE partition coefficients between calcite and aqueous (CaCl 2 + 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 CO 2 + N 2 gas. Absolute values of REE partition coefficients,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.Keywords: rare earth element, partition coefficient, carbonate complex, marine limestone, tetrad effect experiments between calcite and CaCl 2 -NaCl solution. Relative partition coefficients were determined successfully, but absolute values could not be obtained because of a difficulty in evaluating the amount of calcite overgrowths. In this study, we have conducted new series of experimental runs using the same experimental system as in our earlier work, and have acquired quantitative estimates of the amount of calcite overgrowths.The purposes of this study are, (i) to determine the absolute values of REE partition coefficients between calcite and aqueous solution, (ii) to estimate REE abundances in ancient seawater from a combination of the partition coefficients and REE data of limestone samples, and (iii) to discuss the early diagenetic process of marine carbonate in terms of the incorporation of seawater REEs into marine limestones.

REE and Y in limestones have been determined by ICP-AES method coupled with pre-concentra tion chemical procedures. The lanthanide tetrad effect has been clearly observed in the REE patterns of two Permian limestones. It is the W-type tetrad effect seen in seawaters. Y/Ho ratios for the limestones show large positive deviations from the chondritic ratio. Similar non-chondritic Y / Ho ratios are seen in a Precambrian limestone from South Africa, seawaters, and marine phosphorites. Their REE patterns are well characterized by the W-type tetrad effect. These facts and recent results of aquatic geochemistry of REE and Y strongly suggest that the positive Y anomalies are intimately associated with the W-type tetrad effect of REE in natural aquatic solutions including seawaters and in some hydrogenous deposits from the solutions. Y is not a pseudo lanthanide that behaves like Ho in natural conditions where the lanthanide tetrad effect is operative. This must be a thermochemical effect related to the absence of 4f electron in Y3+ and the systematic differences in 4f electronic configurations in the REE3+ series. The REE plus Y patterns with positive Y anomalies and the W-type tetrad effect are important for the gochemical studies of limestones and other hydrogeneous deposits.

The distribution coefficients of rare earth elements (REEs; all lanthanides except Pm, Y and Sc) between Fe oxyhydroxide precipitates and 0.5 M NaCl solutions with NaHCO 3 (0.0~12.0 mM) at 25°C and 1 bar have been determined. This experimental system is a simple model for REE partitioning between deep-sea ferromanganese nodules and seawater. The distribution coefficients, K d (REE: precipitate/solution), show systematic variations with increasing NaHCO 3 concentrations. We have determined REE(III)-carbonate complexation constants from the experimental distribution coefficients as a function of NaHCO 3 concentration and pH. The REE(III)-carbonate complexation constants show almost the same series variations as reported values obtained by the solvent-extraction method, although our results by the precipitation method are rather higher by 1.0~1.5 in log unit than the literature data at zero ionic strength. amine how REE(III)-carbonate complexation affects the REE partitioning process in seawater. Kawabe et al. (1999a, b) reported preliminary experimental results of REE partitioning between Fe oxyhydroxide precipitates and NaCl solutions with and without carbonate ions. They emphasized that the lanthanide tetrad effect is associated with the distribution coefficients of REEs between Fe oxyhydroxide precipitates and NaCl solutions. They analyzed their experimental results using the refined spin-pairing energy theory, RSPET, (Jørgensen, 1979;Kawabe, 1992). Although Kawabe et al. (1999b) estimated the complexation constants for REE(CO 3 ) 2 -from their experimental results, they could not estimate those for REECO 3 + from the preliminary data. REE(III)-carbonate complexation constants have been reported by Byrne and co-workers (Cantrell and