Resolution and Reliability of the .ALPHA.-spectrometric 230Th/234U Method for Age Determination.

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  • Omura Akio
    Department of Earth Sciences, Faculty of Science, Kanazawa University
  • Ise Akihiro
    Department of Earth Sciences, Faculty of Science, Kanazawa University
  • Sasaki Keiichi
    Department of Earth Sciences, Faculty of Science, Kanazawa University
  • Shinsaka Takashi
    Department of Earth Sciences, Faculty of Science, Kanazawa University
  • Hasebe Yumiko
    Department of Earth Sciences, Faculty of Science, Kanazawa University

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  • αスペクトル｀２３０´Ｔｈ／｀２３４´Ｕ年代測定法の分解能と信頼性
  • アルファ スペクトル 230Th 234U ネンダイ ソクテイホウ ノ ブンカ

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Abstract

The performance of the 230Th/234U method of dating has been advanced by the introduction of the TIMS (Thermal Ionization Mass Spectrometry) method to analyze the uranium and thorium isotopes (e. g., Edwards et al., 1987). It seems to maximize the potential of the traditional method, so that highly precise α-spectrometric 230Th/234U dates have recently appeared (e. g., Ku et al., 1990). For example, corals corresponding to the isotope stage 5e are dated at 125.5±1.3ka by the TIMS method (Edwards et al., 1987) or at 125.6±5.2ka by the α-spectrometric method (Omura et al., 1994). The errors, 2σ standard deviation, meaning a probability of 95.5%, are lower than those reported previously. The 230Th/234U method has thus attained a high level of accuracy, as expected by Quaternary geoscientists, in consequence of efforts to minimize the statistical error. It does not, however, follow from the reduction of error that every new 230Th/234U date is fully reliable, because all corals have not necessarily been preserved as a closed system for uranium and thorium isotopes.<br>In this study, the following criteria were used to evaluate the suitability of a given coral sample for reliable age determination.<br>1. The sample shows no evidence of recrystallization of the original aragonitic skeleton or cementation with the secondary calcite or aragonite.<br>2. Skeletal textures have not changed in the form of decalcification and/or dissolution.<br>3. The sample should be free of the initial 230Th, as indicated by the presence of 232Th.<br>4. The uranium concentration should be the same as in the present-day counterpart grown in the same area, i. e., 2 to 4ppm for corals in the Ryukyu region.<br>5. The initial 234U/238U should be within the range of 1.13 to 1.16, [1.13<R0<1.16], consistent with that in the present sea water (1.144±0.002; Chen et al., 1986).<br>6. The 231Pa/235U ratio should be consistent with the 230Th/234U age of the sample. The [230Th/234U]-[234U/238U] and [230Th/234U]-[231Pa/235U] concordias are very useful to discuss whether a given coral has been closed or opened for uranium and thorium isotopes through its diagenetic history.<br>The α-spectrometric and TIMS dates of the same sample, evaluated as reliable using those criteria, were consistent with each other. This fact suggests that the above criteria are quite effective to evaluate the reliability of a 230Th/234U coral date. Fossil corals that satisfy all of them are a rarity. In the case of Pleistocene corals on Yonaguni Island, for instance, fewer than 0.44% of those that occurred were estimated to be useful for 230Th/234U dating.<br>For the 230Th/234U method of dating, the most important subject for a future study is to develop a scheme for reliable dating of organic remains other than scleractinian corals, such as mollusks, echinoderms and so on. The two sorts of concordias mentioned above might be practical and useful for the purpose.

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