Akimasa Masuda

The coprecipitation of rare-earth elements with calcite and aragonite

Abstract Apparent partition coefficients of rare-earth elements (REEs) between calcium carbonates and aqueous solution were experimentally examined. Calcite or aragonite crystals were synthesized by gradual release of CO 2 gas from calcium bicarbonate solution doped with REEs at room temperature and pressure. The apparent partition coefficients for five calcite runs range from ∼ 2.5 to ∼ 10, and those for the aragonite runs fall in a range from ∼ 2.5 to ∼ 5. The mutual REE fractionation for a set of partition coefficients is relatively small. The pattern shapes of apparent partition coefficients for calcite obviously change with varying concentration level of the REEs doped in the initial solution. Such concentration dependence reveals that crystal structure-ionic radius control is not a major factor under the present experimental conditions, but other mechanisms such as the effect of REE complexation in the solution seem to be in operation.

Highly fractionated REE in the Hedjaz (L) chondrite: implications for nebular and planetary processes

Abundances of REE, Sr, Rb, K, Ca and Mg in two whole-rock fragments, two lithic fragments and three chondrules of the Hedjaz (L3) chondrite were determined by isotope dilution mass spectrometry. One whole-rock fragment shows a step pattern with the light-REE enriched, indicating that the meteorite contains components with highly fractionated REE abundances. The chondrules (PP, BO and one unknown type) show variable (0.8 ∼ 2 × CI) REE abundances and almost flat REE patterns with minor irregularities of Ce, Eu and Yb. Anomalous REE patterns were identified for two light-colored lithic fragments. A pyroxene-rich, glass-bearing subrounded clast I has low REE abundances (0.3 ∼ 0.8 × CI) together with depletion of other lithophiles (Ca, Al, Sr, Na and K). It shows a light-REE enhanced pattern with positive anomalies of Ce, Eu and Yb. This is a new REE pattern reported for lithic clasts from ordinary chondrites. It is suggested that the clast formed through melting processes (possibly under planetary conditions) from the condensates either from a later stage nebular gas or from gas vaporized from dust. The second clast II (Ca = 1.1 ∼ 1.9%) consists mainly of coarse-grained olivine, minor pyroxene and plagioclase, and trace amounts of glass; it shows an igneous texture with shock features. It has a strong positive correlation of plagiophile elements (Ca, Sr and Eu) but heterogeneous distributions of common REE from portion to portion. One of the chips indicates a remarkable REE fractionation (LREE = ∼ 40 ×,HREE = ∼ 1.7 × CI) similar to that of Group II CAIs of carbonaceous chondrites. This is the first identification of Group II REE pattern in lithic clasts from ordinary chondrites. It is suggested that the clast had formed by shock-induced melting of an inhomogeneous CV-like precursor assemblage carrying high-temperature nebular REE components on a grand-parent body and was then incorporated into the Hedjaz meteorite parent body. The presence of impact-related products with highly fractionated REE components in a chondritic breccia indicates the existence of related physico-chemical conditions in the formation of chondrules, clasts and their precursors with that of CAIs.

Trace-element variations in acidic rocks from the inner Zone of southwest Japan

Rare-earth elements (REE), Rb, Sr and Ba abundances were measured for Cretaceous to early Paleogene acidic and related intermediate rocks (49 volcanic and 7 plutonic rocks) collected from a wide range of the Inner Zone of southwest Japan. Regional variations in both north-south and west-east directions and discrepancies with respect to age are not found in most of the trace-element concentrations, but Ba and light REE (LREE) abundances are roughly correlated with the longitude of the sampling site, increasing eastwards. Despite such regional variations, most of the acidic rocks have some common features showing LREE-enriched REE patterns with large negative Eu anomalies, enrichment of Ba relative to LREE, large and variable Rb/Sr ratios, and a good correlation between Eu/Eu★ and Sr concentration. Furthermore, most of the acidic rocks (69–74% SiO2) tend to show significant trends: Eu/Eu★ values decrease with increasing Sm/Nd ratios, which are correlated with Lu concentrations. On the basis of model calculations, these trends as well as the REE patterns can be explained by a partial melting model in which source material having moderately LREE-enriched pattern melts to leave a gabbroic residue. It is also suggested that regional variations of La (LREE and Ba) may reflect regional heterogeneity of source materials.

Rare earth element distribution in the Melrose-b howardite: Pre-terrestrial negative Ce anomaly

Abstract In order to investigate the genetic nature of the polymict breccia howardite Melrose-b, a whole-rock sample was separated into four fractions after pulverization and the rare earth element (REE) abundances were precisely determined by isotopic dilution in those fractions. Such an investigation shows that the three precipitated fine-particle fractions are depleted in Ce by the same factor (0.62 ∼ 0.64) in spite of differences in mineralogy, while the corresponding factor of Ce depletion for the suspended-matter fraction is 0.462; the smaller value indicates a greater depletion in Ce. The weight fraction values of four fractions calculated from the REE abundances are in satisfactory agreement with the actual values. The concentration ratios between plagioclase and mainly pigeonite fractions accord with those observed in similar phases of the Juvinas eucrite [1]. Moreover, the bulk pattern synthesized from the three fine-particle fractions is similar in absolute level to that for the Zmenj howardite [1]. It is concluded that the negative Ce anomaly reflects the effect predating the igneous activity in the parent planetary body.