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.

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Ilmenite occurs as an intercumulus phase in most gabbroic rocks and some plagioclase wehrlites of the Asama igneous complex, an ultramafic-mafic layered intrusion in the Mikabu greenstone belt, central Japan. The Mg/(Mg + Fe²⁺) ratio of ilmenite reaches 0.35, but generally ranges from 0.01 to 0.22; the Fe³⁺ content varies from 0.04 to 0.26 pfu (per formula unit calculated on the basis of O = 3). Ilmenite crystallized together with intercumulus clinopyroxene in plagioclase wehrlite, but most of the calculated clinopyroxene-ilmenite temperatures are much lower than the magmatic temperature. The subsolidus Mg-Fe²⁺ exchange probably occurred between ilmenite and surrounding olivine and pyroxene during the cooling stage of the igneous body. Ilmenite, entirely surrounded by plagioclase, occurs in plagioclase wehrlite of a lower portion of the exposed layered sequence; it contains about 9 wt.% MgO, and is the most magnesian in the complex. The ilmenite shows clinopyroxene-ilmenite equilibration temperatures of 980-1100°C, and seems to preserve the magmatic composition. The TiO₂ solubility of melt and the MgO distribution coefficient ilmenite/melt suggest that the Asama magma crystallizing the ilmenite contained 3-5 wt.% TiO₂ and 6-7.5 wt.% MgO. The Asama magma was apparently similar to Hawaiian tholeiites, and different from mid-oceanic ridge basalts or island-arc tholeiites. The Asama complex is considered to have formed in an oceanic island.

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Fast neutron flux gradient in the hydraulic rabbit irradiation facility of the JMTR reactor (core portion of the reactor) was determined using a laser-heating ⁴⁰Ar/³⁹Ar dating system. The gradient was obtained as a J value gradient determined by measuring age-known standard minerals closely spaced in an irradiation capsule. A vertical flux gradient was 1.5-1.7%/cm and relatively small compared to other reactors. On the other hand, a horizontal flux gradient was much larger than the vertical gradient being up to 13%/cm. This gradient can cause as much as 10.5% variation in J values across a single sample packet (8 mm × 8 mm in size) at a given vertical position in an irradiation capsule. This possible dispersion far exceeds the analytical error for each isotope analysis and causes large error and poor accuracy of obtained ages without correction for horizontal flux gradient. This study indicates that both unknown samples and monitor minerals should be irradiated at the same horizontal position, or relative positions among the unknown samples and monitor minerals should be measured in both vertical and horizontal directions and the 3D variation of fast neutron flux should be determined.

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δD and δ¹⁸O values of various minerals and whole-rocks from late Cretaceous granitic rocks in the southern part of the Kyeongsang Basin, Korea have been determined. The granitic rocks are subdivided into seven units in four plutons on the basis of petrological features. The analytical results indicate the isotopic exchange occurred extensively between the ancient meteoric water and the granitic rocks. The isotopic compositions of fresh hornblende and quartz suggest that the initial magmatic δD and δ¹⁸O values are -62 to -69‰ and +8.3 to +8.9‰, respectively. Based on the empirical fractionation of δD (about -30 to -40‰) between chlorite-rich mica and water, the δD and δ¹⁸O values of meteoric water are calculated to be -50 to -60% and -8 to -9‰, respectively. The largest Masan pluton seems to have experienced the water/rock interaction under almost closed system. The maximum water/rock ratio is calculated to be about 0.2. The isotopic exchange between meteoric water and Kimhae-Busan plutons is thought to have extensively occurred under open system. Whether the isotopic exchanges have occurred under closed or open systems in the study area seems to be closely related to the fracture systems developed around the plutons, the pluton sizes and the amounts of circulating meteoric water.

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Ikaite (CaCO₃·6H₂O) transforms to calcite, aragonaite and/or vaterite, which are the polymorphs of calcium carbonate, at normal temperature and pressure. The presence of water promotes the transformation of natural ikaite to calcite in a room but the presence of magnesium ion in a solution reduces the transformation rate. The formation of calcium carbonate polymorphs through the transformation is influenced by temperature and magnesium ion concentration in a solution which contacts with ikaite, being analogous to those on the formation of calcium carbonate polymorphs from calcium bicarbonate solution. Rapid transformation of ikaite is favorable to the formation of vaterite, a metastable phase.

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Under the highly stagnant condition at the Cariaco Trench, the lanthanide tetrad effect takes place abruptly within an extremely narrow depth range. The lanthanide tetrad effect maximum at the Cariaco Trench is observed at 256 m coinciding with its oxic/anoxic interface at 280 m. It is also interesting that the maximum value of the lanthanide tetrad effect at this trench is substantially the same as that in ordinary open ocean.

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