Crystal structure control of the dissolution of rare earth elements in water-mineral interactions

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1 Geochemical Journal, Vol. 40, pp. 437 to 446, 2006 Crystal structure control of the dissolution of rare earth elements in water-mineral interactions SHIN-NOSUKE SHIBATA,* TSUYOSHI TANAKA and KOSHI YAMAMOTO Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya , Japan (Received October 22, 2004; Accepted February 3, 2006) Leaching experiments with pyroxene and plagioclase minerals and whole-rock basalt and distilled water were performed to clarify rare earth element (REE) behavior during water-rock interactions. A system was developed to recover the leachate quickly under nearly neutral conditions (ph ~ 6) with little re-precipitation or adsorption. The release ratios (amount of an element in the leachate from the minerals or rock relative to the amount in the original minerals or rock) of REE were comparable to those of Mg and Ca. Rare earth elements were easily leached. More heavy REE (HREE) than light REE (LREE) were leached from the plagioclase, and more LREE than HREE from the pyroxene. Very little europium was leached from the plagioclase. These release patterns show an inverse relationship with the elemental partition coefficient between the minerals and magma. The trends in the REE release ratios thus depend mostly on the compatibility of each element with the crystal structure of the mineral, rather than on chemical characteristics such as the solubility of the element itself. During the basalt-water interaction, a negative Eu anomaly in the leachate increased with time because Eu was initially leached from interstitial minor phases and later was dominantly from the plagioclase which is the major host mineral of Eu. Keywords: rare earth elements, crystal structure control, leaching continuous-flow system, water-mineral interaction, REE dissolution INTRODUCTION Studying the dissolution of elements from rocks into natural waters during water-rock interactions is important for understanding weathering at the Earth s surface. The mechanism of dissolution of major elements during weathering has been extensively studied by dissolution experiments using various kinds of solution and rocks or minerals. For example, Chou and Wollast (1984, 1985) investigated the dissolution rate of albite in various acidic and basic solutions at room temperature and atmospheric pressure and found that the relationship between the dissolution rate and ph in acidic solutions was the inverse of that in basic solutions. Blum and Lasaga (1991) compared the charge variation on the surface of albite with ph variation under the same conditions as the experiment by Chou and Wollast (1985), and established a correlation between the dissolution rate of albite and the electric charge on the mineral s surface. Oelkers and Schott (1995) calculated the dissolution rate of anorthite, and reported that the dissolution rate of the anorthite depends *Corresponding author ( sshibata@gcl.eps.nagoya-u.ac.jp) Copyright 2006 by The Geochemical Society of Japan. on the concentration of Al in solution. Most of these dissolution experiments, however, were carried out with acidic or basic solutions. To evaluate elemental behavior during the early stages of weathering, it is important to study the experimental extraction of elements from minerals under the conditions close to natural environment without extreme dissolution of the mineral itself. Dissolution experiments with feldspars (Holdren and Berner, 1979) and pyroxenes and amphiboles (Schott et al., 1981) showed that under natural weathering conditions (ph ~ 6), the release rates of major elements from these minerals decrease with time. Takagi et al. (2001) and Asahara and Tanaka (2004), moreover, found on the basis of different release trends between radiogenic and non-radiogenic 87 Sr that the crystal structure controls the dissolution rates of the elements, because radiogenic 87 Sr showed the dissolution rate same as 87 Rb, the parent species of radiogenic 87 Sr in the experiments. Trace elements, especially rare earth elements (REE), are important tracers that have been used in various geochemical studies to investigate weathering processes occurring between water and silicate materials (e.g., Minami et al., 1995; Hannigan and Sholkovitz, 2001; Aubert et al., 2002; Harlavan and Erel, 2002). Because REE are regarded in general as hardly soluble in water, few studies of the release of REE from minerals under 437

2 Table 1. Concentrations of major and rare earth elements in the original minerals and basaltic rock samples Sample Concentration (%) Concentration (ppm) Na K Ca Mg Fe La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Plagioclase Pyroxene Whole rock basalt (JB-2* 1 ) Total blank (10 10 times)* * 1 Reference data from the Geological Survey of Japan (Ando et al., 1989). * 2 Calculated concentrations in the total flowed solution during the leaching experiment. No REE were detected. nearly natural conditions have been conducted. If REE extraction is controlled by their compatibility with the crystal structure of a mineral, as proposed by Takagi et al. (2001) and Asahara and Tanaka (2004), heavy rare earth elements (HREE) should be released from plagioclases more readily than Eu and light rare earth elements (LREE), which are more compatible with their crystal structure. In contrast, the LREE should be released more readily from pyroxenes than the HREE because of their high incompatibility with the pyroxene mineral structure. A partition coefficient-ionic radius (PC-IR) diagram (Onuma et al., 1968; Higuchi and Nagasawa, 1969; Matsui et al., 1977) shows the relationship between mineral compatibility and the ionic radii of elements for a specific mineral. In leaching experiments focused on REE, the concentrations of REE in solution are very low because of their low mobility in natural solutions and low solubility of REE compounds such as hydroxides. Bach and Irber (1998) carried out a leaching experiment using a batch method with cation exchange resin, for REE released from fine grained basaltic rock into deionized water to examine the mobility of REE, so that release of REE in large quantities from rock samples was observed. In batch method leaching experiments such as theirs, however, reacted water in the reactor bottle became acidic (ph = ) with leaching time by H + from cation exchange resin in the same bottle. Therefore, the batch method in a closed system, as has been used in most previous studies (e.g., Bach and Irber, 1998; Chou and Wollast, 1984; Lasaga, 1984; Wollast and Chou, 1992; Hellmann, 1994, 1995), is inferior for quantitatively and continuously determining the amount of dissolved REE in natural conditions. REE released from minerals or rocks, moreover, tend to re-adsorb onto the mineral surface and may co-precipitate with hydroxides, making it difficult to recover the entire amount of released REE from solution. Yonezawa et al. (1996), Takagi et al. (2001) and Asahara and Tanaka (2004) refilled the reactor vessel with the equivalent amount of water each time some of the reacted water was removed. This frequent sampling and refilling of water may effectively reproduce the weathering process in an experimental system; however in such a system it is very difficult to quantify the total amount of released REE because their concentrations in solution decrease significantly with leaching time. In this study, we developed a new continuous-flow system to quantitatively determine the amounts of major elements and REE released into distilled water from plagioclase and pyroxene minerals and basaltic rock. The objective of using the new flow system for leaching experiments is to bring the experimental condition closer to the natural weathering process as much as possible, and observe the circumstances of release of elements from the same sample, including saturation of element release. 438 S. Shibata et al.

3 Fig. 1. System for continuous elemental extraction and trapping with peristaltic pump and cation exchange resin. EXPERIMENTAL PROCEDURE Samples Plagioclase and pyroxene separated from Bushveld gabbro from the ultrabasic to basic complex in South Africa (Gauert et al., 1995; Zingg, 1996) were used for the leaching experiments in this study. The gabbro contains plagioclase and pyroxene crystals larger than 1 mm, which are easily separated. A tholeiitic basalt crushed into 1 2 mm sized grains was used as a whole-rock sample. This tholeiitic basalt was erupted in at Izu- Oshima and is reference rock JB-2 (Ando et al., 1989) of the Geological Survey of Japan. The mineral samples were prepared by first crushing the Bushveld gabbro into grains µm in size, and then separating the plagioclase and pyroxene crystals with a magnetic separator. Selected major element and REE compositions of the separated minerals and of the JB-2 whole-rock sample are shown in Table 1. All samples were washed twice with distilled water by ultrasonication, dried at 80 C, and used in the leaching experiments described below. Leaching experiments The leaching experiment system used in this study is shown in Fig. 1. The experiments for elemental release from the two minerals and a basaltic rock into distilled water were carried out in the same way as follows. About 20 g of sample (plagioclase, pyroxene, or basaltic rock) was placed into a reactor vessel made of Pyrex glass together with about 50 g of distilled water (ph = ). The water temperature was kept at 46 ± 4 C by warming the vessel with a ribbon heater. To stir the reacted solution, filtered air was continuously pumped into the reactor (about 60 ml/min) through a 0.45-µm membrane filter during the leaching experiment. Reacted solution was continuously extracted from the reactor by a peristaltic pump. The reacted solution was passed through a 0.20-µm membrane filter and then dripped into a cation exchange column, 1 cm in diameter and 11 cm long. The column was filled with cation exchange resin, Bio-Rad AG50W-X8, mesh The flow rate of the reacted solution was maintained at about 0.45 ml/min during the experiments. The same volume of fresh distilled water was added into the reactor to compensate for the removal of reacted solution. A small amount of distilled water was also added to the vessel to keep the water/rock ratio constant, because water in the reactor slowly evaporated during the leaching experiment. The reacted water in the reactor showed generally constant ph from 5.8 in range of 6.3 throughout each mineral leaching experiment (Table 2). The released elements in the distilled water were retained by the cation exchange column as cumulative content during each leaching period. Total leaching time was Crystal structure control of the dissolution of REE in water-mineral interactions 439

4 Table 2. Variation of ph of reacting water with leaching time Leaching time (h) 320 hours (h) for each experiment, and the column was changed at 8, 32, 104, and 320 h after the beginning of the experiment to determine the chemical variation with time. A sufficient amount of 6.0 M HCl was put into the column to recover the cations retained in the cation exchange resin during the period. The released elements for each respective period were separated into major elements and REE by cation exchange. Major elements were quantitatively analyzed by atomic absorption spectrometry, and REE were measured by inductively coupled plasma mass spectrometry. The analytical error was less than 10% for most of the elements. RESULTS AND DISCUSSION Although the REE released from plagioclase, pyroxene and basaltic rock into distilled water was very small, we could analyze it quantitatively. The results for major elements and REE in each experiment are shown in Table 3. Some HREE, Tb, Ho, Tm, and Lu, released from the minerals were below the detection limit (<about 0.1 ng) (shown by in Table 3). Values of Na, K, Ca, Mg, and Fe obtained for blanks in the leaching experiment are shown in Table 1. The release ratio (Yonezawa et al., 1996) is the ratio of the amount of an element released into solution to the amount of that element in the whole fresh mineral or rock sample (Table 3). Below, we discuss how the release ratio of each element varied with time. Plagioclase The release ratios of the major elements from plagioclase are shown in Fig. 2a. The release ratios of Na and Ca, which are major elements in plagioclase, are lower than those of other major elements, whereas the release ratio of Mg, which is incompatible with the crystal struc- ph Plagioclase Pyroxene Table 3. Cumulative release ratios of the elements by leaching period Leaching time (h) Release ratio (10 4 times) Release ratio (10 5 times)* Na K Ca Mg Fe La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Plagioclase Pyroxene Whole rock basalt *Tb, Ho, Tm, and Lu, released from the minerals were shown by, below the detection limit (<about 0.1 ng). 440 S. Shibata et al.

5 Fig. 2. (a) Changes in the release ratios of major elements from plagioclase into distilled water with leaching time. The release ratio of Mg, which is incompatible with the crystal structure of plagioclase, is relatively higher than Ca and Na. (b) Changes in the release ratios of REE from plagioclase into distilled water with leaching time. Every HREE shows relatively high release ratios compared with those of the LREE. ture of plagioclase, is relatively high. This trend is consistent with the experimental results reported by Takagi et al. (2001) and Asahara and Tanaka (2004). The low release ratio of Fe, which is also incompatible with the crystal structure of plagioclase, can be attributed to readsorption and/or to precipitation of released Fe as hydroxide and subsequent removal of Fe hydroxide by the membrane filter (e.g., Siever and Woodford, 1979). Therefore, Fe release ratios are not referred to in the later discussion. The relationship between the release ratios from plagioclase and leaching time for some REE is shown in Fig. 2b. It is remarkable that every HREE from Dy to Yb shows relatively high release ratios compared with those of the LREE. The release ratios of Eu are lower than those of other REE, and they increase less with reaction time. This indicates that very little Eu 2+, which is compatible with the crystal structure of plagioclase, is released from plagioclase, compared with other REE, which are unstably included in the plagioclase structure. The patterns of release ratios of all REE measured are shown during the leaching period in Fig. 3. The patterns show HREE enrichment and strong negative Eu anomalies. Although LREE oxides have higher solubility in water than HREE oxides, the amounts of HREE released were larger than those of LREE, which indicates that the Fig. 3. Release ratios of REE from plagioclase by leaching period. The patterns show HREE enrichment and strong negative Eu anomalies. extraction of REE from plagioclase depends strongly on the crystal structure of the mineral as well as the chemical characteristics of the individual element, such as solubility and its complex formation constant. Crystal structure control of the dissolution of REE in water-mineral interactions 441

6 Fig. 4. (a) Changes in the release ratios of major elements from pyroxene into distilled water with leaching time. The release ratios of the major components of pyroxene as Mg and Ca are similar in magnitude to those of Na and Ca in plagioclase (Fig. 2a). (b) Changes in the release ratios of REE from pyroxene into distilled water with leaching time. The release ratios are of the same order of magnitude as those of the major elements. Pyroxene The variations in the release ratios of major elements with experimental time for pyroxene are shown in Fig. 4a. The major components of pyroxene, Mg and Ca, have release ratios (~ ) similar in magnitude to those of Na and Ca in plagioclase. Although the experiments for pyroxene show the same overall variations with the ratios of the major elements seen in plagioclase, the release ratios of the minor components, such as Na and K, are lower than those of the major elements, which is not the case for plagioclase. This is thought to reflect the structural complexity of pyroxene. Pyroxene has a more complex composition with respect to major elements than plagioclase (compare Figs. 6a and 6b). The release ratios of the REE are of the same order of magnitude as those of the major elements (Fig. 4b). The patterns of REE release ratios from pyroxene during the respective experimental periods show strong LREE enrichment and positive Eu anomalies (Fig. 5), in contrast to plagioclase (Fig. 3). Comparison of REE release ratios on the PC-IR diagram Asahara and Tanaka (2004) concluded from their leaching experiments with distilled water and biotite and plagioclase that in the case of these minerals, the release ratio of an element depends on the compatibility of the Fig. 5. Release ratios of REE from pyroxene by leaching period. The patterns show strong LREE enrichment and positive Eu anomalies in contrast to plagioclase (Fig. 3). element with the mineral structure, as displayed in the PC-IR diagram. When the PC-IR diagrams for plagioclase and augite from Taka-sima alkali-olivine basalt (Figs. 6a and 6b; 442 S. Shibata et al.

7 Fig. 6. Partition coefficient-ionic radius diagram for (a) a plagioclase-groundmass system (Taka-sima alkali-olivine basalt) and (b) an augite-groundmass system (Taka-sima alkali-olivine basalt). Both diagrams are from Matsui et al. (1977). The partition coefficient of each element every of ionic valences is plotted with respect to a virtual curve, and has the tendency of opposite to that of release ratio of elements in the leaching experiments (Figs. 3 and 5). Matsui et al., 1977) are compared with the patterns of REE release ratios obtained in this study (Figs. 3 and 5), an inverse relation is observed between the partition coefficient in the PC-IR diagram and the patterns of the REE release ratios. The patterns of REE release ratios from plagioclase (Fig. 3) show HREE enrichment and strongly negative Eu anomalies, whereas the partition coefficients of LREE are larger than those of HREE and that of Eu is the largest among all REE in the PC-IR diagram (Fig. 6a). It is clear from the PC-IR diagram that the compatibility of Eu with the crystal structure of plagioclase is higher than that of La. The lower release ratio of Eu than La (Fig. 3) is quite consistent with the speculation that Eu exists mainly as Eu 2+ in the plagioclase crystal. Strong LREE enrichment and positive Eu anomalies were observed in the release ratios of REE extracted from pyroxene (Fig. 5), while the partition coefficients of LREE are smaller than those of HREE (Fig. 6b). The fact that elements with small partition coefficients as the minerals crystallize from magma have relatively high release ratios supports the inference that the release ratios of REE in the early stage of interaction between distilled water and plagioclase or pyroxene are strongly controlled by the compatibility of each element with the mineral structure. The extraction of elements from basaltic rock When the basaltic rock sample was leached with distilled water, K, Na, and Ca were more easily extracted than Mg or Fe (Fig. 7a). Potassium, Na, and Ca are enriched in some accessory minerals, glass, and/or plagioclase, whereas Mg and Fe are enriched in the mafic minerals, which are the major components of basalt. Among these components, glass is most easily weathered. The release ratios of LREE were remarkably high, close to those of the major elements K, Na, and Ca (Fig. 7b). In contrast, the release ratio of Eu, which is present mostly in plagioclase, was very low (Fig. 7b). Therefore, the release of elements from the minor minerals and glassy material in the basaltic rock should be more marked. The REE release ratios from basaltic whole rock during the respective reaction periods are shown in Fig. 8. The pattern shows LREE enrichment and negative Eu anomalies, as well as a W-type tetrad effect of total released REE in the leaching experiment. The degree of Crystal structure control of the dissolution of REE in water-mineral interactions 443

8 Fig. 7. (a) Changes in the release ratios of major elements from basalt into distilled water with leaching time. K, Na, and Ca show relatively higher release ratios than those of Mg and Fe. (b) Changes in the release ratios of REE from basalt into distilled water with leaching time. The release ratios of LREE are high, close to those of the major elements K, Na, and Ca, while Eu shows very low release ratio. the effect, for example for Dy, can be expressed as: (Dy) RR /(Dy*) RR = (Dy) RR /[{(Gd) RR + 2(Ho) RR }/3], where the subscript RR denotes the release ratio of the element. In similar way, corresponding index values were also displayed for Pr, Tb, Tm and Yb (Table 4). This effect is found in REE patterns in some groundwaters (Masuda et al., 1987; Takahashi et al., 2002), which have undergone some interaction with igneous rocks. Changes in the size of the negative Eu anomaly, presented as the value of (Eu) RR /(Eu*) RR, in the reacted waters are shown in Fig. 9. (Eu*) RR is the mean value calculated from the release ratios of Sm and Gd ((Eu*) RR = {(Sm) RR + (Gd) RR }/2), and a smaller value of (Eu) RR / (Eu*) RR means a larger Eu anomaly. The Eu anomaly is small ((Eu) RR /(Eu*) RR = 0.54) in the early stage of the water-basalt interaction, and it increases with interaction time to a final value of about 0.35 (Fig. 9). A similar negative Eu anomaly trend is observed in the interaction between distilled water and plagioclase. The magnitude of the Eu anomaly for Eu released from plagioclase is larger and changes more quickly with time ((Eu) RR /(Eu*) RR = ) compared with the experiment with basalt. The mild increase in the Eu anomaly during the waterbasalt leaching experiment indicates that the REE were Fig. 8. Release ratios of REE from basalt into distilled water by leaching period. The pattern shows LREE enrichment and negative Eu anomalies. released from many other components in addition to plagioclase, such as accessory minerals, phosphate and glass in the matrix and grain boundaries, which are more readily weathered than plagioclase. The contribution of 444 S. Shibata et al.

9 Table 4. Index value of tetrad effect for total released REE from basalt (Pr) RR /(Pr*) RR 0.81 (Tb) RR /(Tb*) RR 0.90 (Dy) RR /(Dy*) RR 0.88 (Tm) RR /(Tm*) RR 0.94 (Yb) RR /(Yb*) RR 0.87 Explanation of (Ln) RR /(Ln*) RR is described in text. Fig. 9. Variations in the Eu anomaly in solution from basalt or plagioclase with leaching time. (Eu*) is the mean value calculated virtually from the release ratios of Sm and Gd ((Eu*) = {(Sm) release ratio + (Gd) release ratio }/2). The mild increase in the Eu anomaly during the water-basalt leaching experiment compared with plagioclase indicates that the REE were released from other components in addition to plagioclase, such as from some accessory minerals was more readily weathered. REE from the plagioclase crystal structure was initially small and increased with leaching time. Rare earth elements depleted with respect to Eu were released from basalt in the early stage of the leaching experiment, as noted above, mainly from the weaker components of the basalt. The ratio of REE released to the solution from weak components of the basalt to those from plagioclase decreased with leaching time. Thus, the Eu anomaly in the solution became larger with leaching time, eventually approaching that for plagioclase. The elements which displayed relative high release ratios from the basaltic rock are the so-called incompatible elements. Most of these are included in grain boundaries and some accessory minerals. The release ratios of elements such as REE on the early stage of the leaching experiment show reverse pattern to the trend of incompatibility as well as partition coefficients in the PC-IR diagram (Fig. 6a). SUMMARY In this study, we developed a system comprising a peristaltic pump and a cation exchange column for the continuous extraction and trapping of elements. This new element-trapping system made it possible to quantify the amounts of REE released into distilled water during leaching experiments from plagioclase and pyroxene minerals and basaltic rock. Elements less compatible with the respective mineral structure showed higher release ratios, as shown by a comparison between the release ratio trends of elements and the PC-IR diagrams. Thus, REE release ratio trends depend mostly on the compatibility of each element with crystal structure of the mineral rather than on the chemical characteristics such as solubility and complex formation constants of the elements themselves. In the interaction between basalt and distilled water, the negative Eu anomaly seen in the REE pattern increased with leaching time, suggesting that the REE released in the early stage of weathering mainly originated from minor phases such as some accessory minerals and grain boundary glass rather than from the more major plagioclase phase. Acknowledgments This study was supported in part by the 21st century COE (Center of Excellence) Program G-4 Dynamics of the Sun-Earth-Life Interactive System, sponsored by the Ministry of Education, Culture, Sports, Science and Technology. We thank associate editor Yoshio Takahashi and reviewers Naotatsu Shikazono and anonymous for giving critical comments and suggestions to improve the manuscript. REFERENCES Ando, A., Kamioka, H., Terashima, S. and Ito, S. (1989) 1988 values for GSJ rock reference samples, Igneous series. Geochem. J. 23, Asahara, M. and Tanaka, T. (2004) Weathering of silicate minerals with water crystal structure control. Bunseki , (in Japanese). Aubert, D., Stille, P., Probst, A., Gauthier-lafaye, F., Pourcelot, L. and del Nero, M. (2002) Characterization and migration of atmospheric REE in soils and surface waters. Geochim. Cosmochim. Acta 66, Bach, W. and Irber, W. (1998) Rare earth element mobility in the oceanic lower sheeted dyke complex: evidence from geolochemical data and leaching experiments. Chem. Geol. 151, Blum, A. E. and Lasaga, A. C. (1991) The role of surface speciation in the dissolution of albite. Geochim. Cosmochim. Acta 55, Chou, L. and Wollast, R. (1984) Study of the weathering of Crystal structure control of the dissolution of REE in water-mineral interactions 445

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