Yasutaka Terakado

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.

Nd and Sr isotopic variations in acidic rocks formed under a peculiar tectonic environment in Miocene Southwest Japan

Initial Nd and Sr isotopic ratios were obtained for middle Miocene igneous rocks as well as for related rocks from the Outer Zone of Southwest Japan to investigate the petrogenesis of acidic magmas and their relation to a peculiar tectonic environment bearing on the back-arc spreading of the Japan Sea. On the ɛNd-ɛSr diagram, data points for the acidic rocks fall in the −ɛNd, +ɛSr quadrant occupying different positions from those for sedimentary and old crustal rocks, and seem to define several subparallel lines which extend towards the lower-righthand sedimentary field. The S-type acidic rocks occupy an intermediate position between I-type rocks and sedimentary ones, a fact suggesting mixing of an igneous component and a sedimentary one. The linear mixing trend observed on the ɛNd-ɛSr diagram can be attained in the restricted case that the igneous component has similar Sr/Nd concentration ratios to that of the sedimentary one, which implies an intermediate to acidic composition for the igneous component. Inconsistency between the elemental and isotopic variations observed may be reconciled by considering that mixing, probably in the relatively deep part of the crust, might have occured prior to chemical differentiation processes. The episodic igneous activity and the high heat energy required to melt such materials involving sedimentary rocks may be explained by a model in which a hot mantle region probably corresponding to the rising part of the mantle convection supplied the heating energy to the Outer Zone of Southwest Japan when passing beneath Southwest Japan in the course of movement of the hot rising part from the Shikoku basin areas to the Japan Sea area.

Nd and Sr isotopic variations in acidic rocks from Japan: significance of upper-mantle heterogeneity

Initial Nd and Sr isotopic ratios have been measured for Cretaceous acidic and related intermediate rocks (24 volcanic and two plutonic rocks) from the Inner Zone of Southwest Japan (IZSWJ) to investigate the genesis of acidic magmas. The initial Nd and Sr isotopic ratios for these rocks show three interesting features: (1) εNd values for acidic rocks (+2 to −9) are negatively correlated with εSr values (+10 to +90) together with those for intermediate rocks (εNd=+3 to -8; εSr=0 to +65). (2) The εNd values for silica rich rocks (>60% SiO2) correlate with the longitude of the sample locality, decreasing from west to east in a stepwise fashion: Four areas characterized by uniform εNd values are discriminated. (3) Low silica rocks (<60% SiO2) in a certain area have distinctly different εNd values from those of the high silica rocks in the same area.These results as well as those deduced from the additional samples collected, for comparison, from other provinces in Japan suggest that the acidic rocks can be formed neither by fractional crystallization processes from more basic magmas nor by crustal assimilation processes. The isotopic variations of the acidic rocks may reflect regional isotopic heterogeneity in the lower crust, and this heterogeneity may ultimately be attributed to the regional heterogeneity of the uppermost-mantle beneath the Japanese Islands.

RbSr dating of acidic rocks from the middle part of the Inner Zone of southwest Japan: tectonic implications for the migration of the Cretaceous to Paleogene igneous activity☆

Abstract RbSr age determinations by mineral and whole-rock isochron methods have been performed on Cretaceous to Paleogene granitic and rhyolitic rocks from the middle part of the Inner Zone of southwest Japan in order to examine some chronological problems and tectonic models for the migration of igneous activity. Our chronological results include: (1) isotopic homogenization of granitic and rhyolitic rocks in the Miyazu-Izushi area was disturbed by mixing of acidic and basic magmas; (2) the Rokko granite was emplaced at 77.8 Ma and reheated at 72 Ma; (3) mineral isochron ages obtained for the acidic volcanic rocks range from 62.6 to 85.8 Ma and are considered to date the time of eruption. Igneous activity migrated eastward from ∼ 95 Ma to ∼ 70 Ma, which may be related to a similar eastward migration of the development of sedimentary basins in fore-arc regions. The observed eastward migration has been discussed with relevance to a ridge subduction model in which the heated position by subducted ridge moved from west to east. Although the cause for the migration of the igneous activity is poorly understood, several difficulties with the simple ridge subduction model are pointed out.

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.

Experimental study on the sorption of rare-earth elements and other trace elements during rhyolite-hydrothermal water interactions

Abstract The behaviour of the rare-earth elements (REE), Sc and Ba during rhyolite-hydrothermal water interactions has been investigated experimentally. Both leaching and sorption experiments were performed using various solutions (i.e., pure water, dilute HCl solution, the solution doped with REE, Sc and Ba). Most runs were at 250°C and 1 kbar. The mobility of these elements in our hydrothermal reactions was found to be dependent on the concentration of HCl (or pH) in the solution. In near-neutral solutions, REE showed considerable mobility but no relative fractionation, whereas fractionation did occur between REE and Sc. In dilute HCl solution REE behaviour is quite complex: in sorption experiments, REE remained in 2 N HCl solution, whereas in 0.1–0.6 N HCl solutions REE were readily incorporated into the solid. In contrast, Ba showed strong affinity for the dilute HCl solution. A large fractionation between light and heavy REE was observed in an intermediate case between high and low HCl concentrations. The mechanisms of these trace-element behaviours are not clearly understood, but ion exchange properties of a thin surface layer may be important.