αスペクトル^<230>Th/^<234>U年代測定法の分解能と信頼性 [in Japanese] Resolution and Reliability of the α-spectrometric ^<230>Th/^<234>U Method for Age Determination [in Japanese]
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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.
- The Quat. Res.
The Quat. Res. 34(3), 195-207, 1995-08-31
Japan Association for Quaternary Research