GEOCHEMICAL JOURNAL
Online ISSN : 1880-5973
Print ISSN : 0016-7002
ISSN-L : 0016-7002
34 巻 , 6 号
選択された号の論文の5件中1~5を表示しています
  • Giancarlo Cavazzini
    2000 年 34 巻 6 号 p. 395-406
    発行日: 2000/12/20
    公開日: 2008/04/08
    ジャーナル フリー
    A model is proposed to calculate the 87Sr/86Sr ratio in melts produced by ideal fractional partial melting of rock systems. In ideal fractional melting, during melt formation, diffusion (among crystals in the residue and between crystals in the residue and the melt) is absent, so that the evolution of the 87Sr/86Sr ratio in the melt is governed by mineral stabilities and by the self-production of radiogenic 87Sr due to 87Rb decay. The contribution of intra-melt production of radiogenic 87Sr has been considered in the calculations because it may be non-negligible in high Rb/Sr melts. In the model, the 87Sr/86Sr ratio in the aggregate melt is calculated considering the contribution of each mineral phase involved in melt formation as ideally separated from the others and then summing the contributions. This approach has two advantages: 1) it avoids serious uncertainties which would be introduced if composition were calculated by mixing an unknown number of unknown melts in unknown proportions; 2) equations designed to describe the behaviour of the 87Sr/86Sr ratio in the aggregate melt in modal melting processes may also be used for non-modal ones (until one of the minerals in the melting assemblages is completely consumed). Calculations indicate that Rb and Sr mineral/melt fractionation coefficients may play an important role in determining the value of the 87Sr/86Sr ratio in the aggregate melt. Different values of the fractionation coefficients may determine very different evolution paths for the isotopic ratio, in both modal and nonmodal processes and, if Sr is chemically fractionated during mineral melting, large Sr isotope differences may be generated between melts and respective source rocks, even if melting is modal.
  • Qing Chang, Takashi Mishima, Sadayo Yabuki, Yoshio Takahashi, Hiroshi ...
    2000 年 34 巻 6 号 p. 407-427
    発行日: 2000/12/20
    公開日: 2008/04/08
    ジャーナル フリー
    The first systematic data of Sr and Nd isotopic ratios, REE abundances, major element and mineral compositions are reported for the Chinese moraines from the western Kunlun Mts. and southern and northern flanks of the Tianshan Mts. and soils from the Tibet Plateau. This study was conducted in order to characterize the isotopic and geochemical features of these moraines and soils, which are considered as possible sources of the Taklimakan Desert sands. The moraines and the desert sands consist of quartz and feldspar as dominant minerals, but the former have higher clay and mica and lower carbonate mineral abundances than the latter. In spite of the wide variation, average values of most major elements of the moraines are generally similar to those of the Taklimakan Desert sands, except for CaO depletion and K2O enrichment in the moraines. Sr and Nd isotopic ratios are also highly variable for moraines, particularly from the Tianshan northern flank, while those for the desert sands show restricted values. In the Sr-Nd isotopic diagram, the Taklimakan Desert sands are plotted in the narrow field overlapped by both the moraine matrices from the western Kunlun Mts. and from the Tianshan southern flank. In addition, it is noted that average value of Nd isotopic ratios for the moraines is clearly higher than those for the desert sands. Two Tibetan soils of this study show wide variation in mineral composition and major element compositions (SiO2 = 75% and CaO = 1.5% for one sample and SiO2 = 44% and CaO = 31.8% for the other). However, Sr isotopic ratios of both soils are similar to those of the desert sands and their Nd isotopic data are slightly smaller than the desert sands. These results suggest moraine matrices from both the western Kunlun Mts. and the Tianshan southern flank and the Tibetan soils as possible sources for the Taklimakan Desert sands.
  • Kohei Matsumoto, Keita Yamada, Hiroshi Naraoka, Ryoshi Ishiwatari
    2000 年 34 巻 6 号 p. 429-438
    発行日: 2000/12/20
    公開日: 2008/04/08
    ジャーナル フリー
    Carbon isotopic composition of sterols in marine Holocene sediments, marine sinking particles, tree leaves, and soils were determined. δ13C values of algae-derived sterols such as 24-methylcholesta-5, 22-dien-3β-ol and dinosterol in the marine sediments range from −22.1 to −25.2‰, while those of 24-ethylcholest-5-en-3β-ol (24-ethylcholesterol) range from −22.6 to −24.3‰. We conclude that 24-ethylcholesterol in the marine sediments derives from marine algae, because their δ13C values are markedly different from those of the leaves of C3 (∼−29‰) and C4 (−14‰) plants and similar to those of the algal sterols.
  • Atsuyuki Ohta, Iwao Kawabe
    2000 年 34 巻 6 号 p. 439-454
    発行日: 2000/12/20
    公開日: 2008/04/08
    ジャーナル フリー
    The distribution coefficients of rare earth elements (REEs; all lanthanides except Pm, Y and Sc) between Fe oxyhydroxide precipitates and 0.5 M NaCl solutions with NaHCO3 (0.0∼12.0 mM) at 25°C and 1 bar have been determined. This experimental system is a simple model for REE partitioning between deep-sea ferromanganese nodules and seawater. The distribution coefficients, Kd(REE: precipitate/solution), show systematic variations with increasing NaHCO3 concentrations. We have determined REE(III)-carbonate complexation constants from the experimental distribution coefficients as a function of NaHCO3 concentration and pH. The REE(III)-carbonate complexation constants show almost the same series variations as reported values obtained by the solvent-extraction method, although our results by the precipitation method are rather higher by 1.0∼1.5 in log unit than the literature data at zero ionic strength.
  • Atsuyuki Ohta, Iwao Kawabe
    2000 年 34 巻 6 号 p. 455-473
    発行日: 2000/12/20
    公開日: 2008/04/08
    ジャーナル フリー
    The series variations of logarithms of apparent distribution coefficients for rare earth elements (REE), log Kd(REE), between Fe-Mn deposit and deep water have been examined theoretically based on the experimental log Kd(REE) between Fe oxyhydroxide precipitate and NaCl solution doped with NaHCO3 (Ohta and Kawabe, 2000). The experimental log Kd(REE) values are strongly affected by REE(III)-carbonate complexation. Those experimental ones in the system with the carbonate ion concentration similar to seawater reproduce the characteristics of log Kd(REE) evaluated from field data except for large positive Ce anomaly. REE speciation calculation in seawater by using our REE(III)-carbonate complexation constants indicates that the main REE species is REECO3+(aq) rather than REE(CO3)2-(aq), except for heavy REE. This is different from the result based on previous literature data for REE(III)-carbonate complexation constants. Series variations of log {m(REE(OH)3·nH2O)/[REE(CO3)2-, aq]} and log {m(REE(OH)3·nH2O)/[REECO3+, aq]} evaluated from field data have been compared with those from our experimental data. We have confirmed that our data of carbonate complexation constants are better to explain experimental and natural systems simultaneously than the previous literature data. The refined spin-pairing energy theory (RSPET) can explain the tetrad effects observed in experimental log Kd(REE) and REE(III)-carbonate complexation constants: Racah (E1 and E3) parameters decrease in the order that REE3+(aq, octahydrate) > REECO3+(aq) > REE(CO3)2-(aq) ≥ REE(OH)3·nH2O(ss). This relationship is also compatible with field data. The tetrad effects observed in log Kd(REE) between marine Fe-Mn deposit and seawater and in REE(III)-carbonate complexation constants can be explained by the systematic differences in Racah parameters among the REE(III) species.
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