αスペクトル^<230>Th/^<234>U年代測定法の分解能と信頼性 Resolution and Reliability of the α-spectrometric ^<230>Th/^<234>U Method for Age Determination

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最近のウランおよびトリウム同位体分析への質量分析計技術の導入は,<sup>230</sup>Th/<sup>234</sup>U年代測定法の精密化とともに,必要試料の大幅減量や適用範囲の<sup>14</sup>C法による測定年代域への拡張も可能にした.一方,従来のαスペクトル法でも,測定機器類の計数効率や安定性の向上と,試料の放射化学的処理法の改良によって,測定誤差がTIMS法の4~5倍にまで改善された.今では,最終間氷期最盛期相当の測定値(約125ka)の誤差が,95.5%の確率を意味する2σの統計誤差で表示しても,TIMS<sup>230</sup>Th/<sup>234</sup>U法では約1ka(1%),αスペクトル法でもおおよそ5ka(4%)と,以前に比べ格段に小さくなった.しかし,そのような年代値を,みかけ上誤差が小さいからというだけで,そのまま信用することはできない.本論では,真に信頼できる年代値を得るには,最良の試料を用いることが不可欠であるという立場から,試料が<sup>230</sup>Th/<sup>234</sup>U法固有の前提条件と必要条件を満たすことを検証するための方法を提示し,さらに<sup>230</sup>Th/<sup>234</sup>U法の信頼性を高めるための方策を論じた.

The performance of the <sup>230</sup>Th/<sup>234</sup>U method of dating has been advanced by the introduction of the TIMS (Thermal Ionization Mass Spectrometry) method to analyze the uranium and thorium isotopes (<i>e. g.</i>, Edwards <i>et al.</i>, 1987). It seems to maximize the potential of the traditional method, so that highly precise α-spectrometric <sup>230</sup>Th/<sup>234</sup>U dates have recently appeared (<i>e. g.</i>, Ku <i>et al.</i>, 1990). For example, corals corresponding to the isotope stage 5e are dated at 125.5±1.3ka by the TIMS method (Edwards <i>et al.</i>, 1987) or at 125.6±5.2ka by the α-spectrometric method (Omura <i>et al.</i>, 1994). The errors, 2σ standard deviation, meaning a probability of 95.5%, are lower than those reported previously. The <sup>230</sup>Th/<sup>234</sup>U 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 <sup>230</sup>Th/<sup>234</sup>U 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 <sup>230</sup>Th, as indicated by the presence of <sup>232</sup>Th.<br>4. The uranium concentration should be the same as in the present-day counterpart grown in the same area, <i>i. e.</i>, 2 to 4ppm for corals in the Ryukyu region.<br>5. The initial <sup>234</sup>U/<sup>238</sup>U should be within the range of 1.13 to 1.16, [1.13<R<sub>0</sub><1.16], consistent with that in the present sea water (1.144±0.002; Chen <i>et al.</i>, 1986).<br>6. The <sup>231</sup>Pa/<sup>235</sup>U ratio should be consistent with the <sup>230</sup>Th/<sup>234</sup>U age of the sample. The [<sup>230</sup>Th/<sup>234</sup>U]-[<sup>234</sup>U/<sup>238</sup>U] and [<sup>230</sup>Th/<sup>234</sup>U]-[<sup>231</sup>Pa/<sup>235</sup>U] 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 <sup>230</sup>Th/<sup>234</sup>U 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 <sup>230</sup>Th/<sup>234</sup>U dating.<br>For the <sup>230</sup>Th/<sup>234</sup>U 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.

収録刊行物

  • 第四紀研究  

    第四紀研究 34(3), 195-207, 1995-08-31 

    Japan Association for Quaternary Research

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  • NII論文ID(NAID)
    10002632969
  • NII書誌ID(NCID)
    AN0034136X
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    04182642
  • NDL 記事登録ID
    3624181
  • NDL 雑誌分類
    ZM49(科学技術--地球科学--地質)
  • NDL 請求記号
    Z15-67
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
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