The refined spin-pairing energy theory (RSPET) has been improved in order to understand quantitatively 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 LnO_1.5 and LnF₃, the lattice energies of the compounds and the enthalpy difference of ΔH_f^o(LnF₃)-ΔH_f^o(LnO_1.5) have been examined by the improved RSPET. The RSPET parameters for the lowest levels of 4f^q electronic configurations 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 function 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 LnO_1.5 and ΔH_f^o(LnF₃) − ΔH_f^o(LnO_1.5) exhibit upward concave 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 LnO_1.5, in accordance with the nephelauxetic series. The differences in Racah parameters between LnF₃ and LnO_1.5 have been estimated from ΔH_f^o(LnF₃) − ΔH_f^o(LnO_1.5) 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 NdO_1.5. 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 electrons.

抄録全体を表示

The abundances of fourteen rare earth elements (REE) were precisely determined for hydrothermal and sedimentary uraninite (UO₂) ore samples, including those from the Oklo natural nuclear reactor. A lanthanide tetrad effect is recognized from the REE patterns of all uraninite samples, irrespective of their origin of occurrence and uranium contents. The REE patterns of Oklo uraninites are characterized by the shape of W-type tetrad effect, distinct from the patterns of hydrothermal uraninites with M-type effects. One uraninite ore in sandstone from the Orphan deposit, Arizona, U.S.A., shows M-type tetrad effect, indicating that the uraninite may be detrital, and not precipitated during or shortly after the deposition of the enclosing sediment. These results suggest that lanthanide tetrad effect in uraninites closely relates to conditions and processes of their formation.

抄録全体を表示

We examined the possibility of diffusion-controlled fractionation of the rare earth elements during igneous processes on the basis of lattice diffusion, by using available sets of diffusivity data on olivine and melilite. During partial melting of olivine, it is shown with a simple model that Fe and Mn are more enriched in the early stage liquid phase, while Ca, with lower diffusivity, is less enriched in the liquid. This implies that, during partial melting of olivine, the heavy rare earth elements with ionic radius similar to that of Mn can be more enriched in the early stage liquid phase relative to the light rare earth elements with ionic radius similar to that of Ca, contrary to what would be expected from equilibrium partition. Low inter-diffusivity of Mg + Si and Al + Al pairs in åkermanitic melilite suggests that diffusivities of the rare earth elements in olivine are low, and that the effect of diffusion would continue for a longer period of time for trivalent rare earth elements than for divalent cations. Eu²⁺ also possibly behaves quite differently from trivalent rare earth elements during diffusion, and thus may produce Eu anomalies that are not expected by equilibrium partition. Diffusion can produce widely different distributions of the rare earth and other trace elements even in a single system depending on the time scale of the diffusion process. However, since natural systems are much more complicated than models, it is not clear whether the calculated results have practical significance in explaining the distribution of trace elements in nature.

抄録全体を表示

We have determined experimentally the partition coefficients of elements between silicate minerals and coexisting melts at 16 GPa and 2, 000°C in chondritic and CaO-rich silicate systems. Majorite garnet + melt ± olivine forms in the MgO-rich chondritic system, while in the CaO-rich system, merwinite coexists with melt ± clinopyroxene. The partition coefficients (D-values) of La, Ce, Sm, Yb, Sc, Zr, Hf and other minor or major elements for majorite garnet, merwinite, and clinopyroxene/melt pairs have been determined through EPMA analysis. D-values of a limited number of elements were also determined for the olivine/melt pair with EPMA. The D(HREE and Sc) for majorite/melt are close to unity. These values are much lower than reported values for pyrope/melt pairs at 2–3 GPa and 1, 200–1, 500°C and for pyrope megacryst/host rock matrix pairs. The pressure-induced coordination change in Al³⁺ at higher pressures above 10 GPa is responsible for the enormous difference in the D values as well as the contrasting temperature conditions: The coupled substitution of (^VIIIM²⁺, ^IVSi⁴⁺) = (^VIIIHREE³⁺, ^IVAl³⁺), important for accommodating HREE³⁺ into garnets, is severely limited because ^IVAl³⁺ is unlikely to exist in the majorite/melt system. The mechanism to reduce D(REE and Sc) with increasing pressure may also be operative between the clinopyroxene/melt pair. Systematic comparison of Onuma diagrams for ^VIIIM²⁺ and ^VIIIM³⁺ in almandine, pyrope, and majorite garnet/melt pairs clearly indicates that high enrichments of HREE in almandine and pyrope garnets are related to charge-balanced substitution and the presence of more or less polymerizing silicate melts with ^IVAl³⁺. These Onuma diagrams also suggest that the partitioning behaviors of Fe²⁺ and Co²⁺ between garnet and melt are anomalous. This may reflect the tendency for Fe²⁺ and Co²⁺ to prefer six coordinated sites in melts to cubic sites in garnets due to the crystal field effect. The D(REE and Sc) value pattern for the merwinite/Ca-rich melt pair has been found to be quite similar to those for CaTiO₃/melt and CaSiO₃ (perovskite)/melt pairs. This is interesting in view of the substructure of merwinite that mixed Ca²⁺ and O^2- layers comprise a pseudo-hexagonal dense packing analogous to the perovskite structure.

抄録全体を表示

The intra-grain distribution of rare earth elements (REEs) in coexisting apatite, monazite, xenotime and zircon was determined on an electron microprobe for a sample of Cretaceous Busetsu Granite, Okazaki, Japan. The REE distribution suggests that these accessory minerals did not originate as a restite component, but crystallized successively from magmatic melt despite of their possible cotectic relationships. The crystallization sequence is early zircon and euhedral apatite, followed by monazite, xenotime and anhedral apatite. Euhedral apatite shows a core to rim increase in light REEs and a decrease in middle to heavy REEs. Monazite inclusions within biotite show a core to rim decrease in La and Ce with a counterbalancing increase in intermediate REEs and Y. Some large monazite grains in the intergranular space of major minerals display multiple zonation patterns. Zircon shows a core to rim increase in Y content and Hf/(Zr + Hf) ratio, and xenotime shows a core to rim decrease in Zr. Anhedral apatite exhibits an extreme core to rim increase in light and heavy REEs. The REE distribution in these accessory minerals is closely related to the sequence of their crystallization. The successive appearance and disappearance of accessory minerals in the crystallization sequence are explained in terms of deviation from cotectic saturation levels owing to kinetic crystallization from the supersaturated melt.

抄録全体を表示

Rare earth element (REE) concentrations were precisely determined by inductively coupled argon plasma/atomic emission spectrometry (ICP) for 9 tholeiitic and 13 associated calc-alkaline rocks from Adatara volcano, located at the volcanic front of Northeast Japan arc. The tholeiitic samples commonly display nearly flat REE patterns, whereas the calc-alkaline samples characteristically show concave light REE-enriched patterns. In both suites, the REEs, with the exception of Eu, are positively correlated to varying extents with K₂O and other incompatible elements. Eu concentrations in the calc-alkaline suite do not vary significantly, resulting in significant negative Eu anomalies. In the tholeiitic suite, chondrite-normalized (Ce/Yb)_CN ratios are low and show little variation (1.54–1.80) over a significant range in SiO₂ content (52–62 wt%). In contrast, (Ce/Yb)_CN in the calc-alkaline suite show much greater variation (2.17 to 2.94) over a smaller range in SiO₂ content (56 to 62 wt%) and increase systematically with SiO₂. REE data for the tholeiitic suite are consistent with a crystal fractionation model that successfully reproduces the observed major and trace-element variations. A similar fractionation model is compatible with major-element and some trace-element variations for the calc-alkaline suite, but results in excessively high heavy REE contents, and fails to reproduce the elevated (Ce/Yb)_CN ratios for the most acidic samples. The preferential enrichment of light REEs may reflect additional processes such as: (1) assimilation, (2) incorporation of a differentiated liquid (or evolved magma) that has undergone fractionation of heavy REE-rich minerals, and (3) mixing of primary magma derived by a small degree of partial melting from a common (amphibolitic or hornblende gabbroic) source. Although the tholeiitic and calc-alkaline suites possess distinct ⁸⁷Sr/⁸⁶Sr, Rb/Ba and Zr/Nb ratios, different magma sources are not required to produce the distinct REE patterns of the coexisting suites. An equilibrium batch partial melting model can reproduce both the flat (tholeiitic) and concave (calc-alkaline) REE patterns from a common peridotitic source by larger (10–15%) and smaller (5%) degrees of partial melting, respectively. Therefore, both tholeiitic and calc-alkaline suites could have been derived from a common lherzolitic magma source, provided that the calc-alkaline suite underwent incorporation (assimilation) of isotopically distinct (lower crust-derived) materials prior to evolution.

抄録全体を表示

We have analyzed major, minor, and rare earth elements (REE) in basalts, dolerites, gabbros and an amphibolite from a fore-arc seamount “Ogasawara Paleoland (OPL)” in the Izu-Ogasawara region, western Pacific. Barium and REE compositions of the OPL mafic rocks suggest that they were formed in a mid-ocean ridge. This interpretation is supported by the similarity of the TiO₂–Al₂O₃ systematics of the OPL mafic rocks to those of mid-ocean ridge basalts. To explain the occurrence of MORB-type mafic rocks in a fore-arc region, two possibilities are considered; (1) accreted oceanic crust of subducting slab, or (2) trapped oceanic crust remained in the fore-arc region when the subduction had initiated.

抄録全体を表示

In order to understand the metamorphic history of the parent bodies of unequilibrated ordinary chondrites (UOC's), we compare REE distributions in the HCI/HNO₃ (acid)-residues of seven Antarctic UOC' s with those of equilibrated ordinary chondrites (EOC's). REE parameters designated to characterize the abundance pattern of REE's—normalized La abundance, normalized La/Lu abundance ratio and the degree of Eu anomaly—were found to be highly variable in the acid-residues of UOC's, suggesting that REE's were redistributed on UOC parent bodies. In contrast, the REE parameters are fairly constant in the acid-residues of EOC's, suggesting that REE distribution attained equilibrium on EOC parent bodies. The REE parameters for the acid-residues of UOC's are poorly correlated with the In, Zn and Se contents in the whole rock samples analyzed in this study, but appear to correlate with carbon content. This implies that REE parameters can be used as indices for the subclassification of UOC's. From the viewpoint of REE distribution, ALH 77011 (and its paired ALH 78038) appears to be one of the least metamorphosed UOC's.

抄録全体を表示

Abundances of Na, Mg, Al, K, Ca, Sc, Cr, Fe, Co, Ni, Rb, Sr, Ba, REE, Ir, and Au were determined in a unique, noritic clast carrying a fractionated REE component from the Hedjaz (L) chondritic breccia by instrumental neutron activation analysis and isotope dilution mass spectrometry. The clast, previously described by Nakamura et al. (1990), consists of two lithologies, and is depleted in siderophile and moderately volatile lithophile elements when compared with its host meteorite. The REE abundance pattern of the clast is L-chondritic (1.4–1.6 × CI-chondrite) with a large (62%), positive Eu anomaly. Strontium is also enriched (3.2 × CI-chondrite) relative to trivalent REE. Chemical and petrologic data suggest that the elemental fractionations observed in the Hedjaz noritic clast could be accomplished in a single host-like parent body with several episodes of impact melting and heating. An impact melt enriched in feldspar components could have been generated in a neighboring region on a parent body, and then infiltrated into the precursor material of the clast. An enhancement of plagiophile elements would then be produced in the clast. Moderately volatile element and siderophile element fractionations could be related to gas/solid or gas/liquid processes and silicate/metal melt fractionations, respectively, which may have occurred during melting and formation of the clast.

抄録全体を表示