Detection of the extinct nuclides by the precise isotopic ratio measurements and chronology of the early solar system

绝灭核素是存在于太阳系早期的放射性核素，由于半衰期短而彻底衰变。然而，它们的子核素经常在原始陨石（如碳质陨石）中被检测到同位素异常，这可以用于早期太阳系的年代学。我们在Beardsley陨石中发现了Ba同位素异常，认为这是由于已灭绝的核素Cs-135（半衰期为2.3Ma）造成的。在目前的研究中，我们试图建立Cs-135的存在，并将其应用于早期太阳系的年代学。我们测量了各种陨石，特别是碳质球粒陨石的钡同位素丰度。CI、CM、CR和CV组的大部分碳质球粒陨石同时显示Ba-135和Ba-137同位素异常，表明这些异常主要是由s过程和（或）r过程核合成引起的。另一方面，热蚀CK组碳质球粒陨石和无球粒陨石未见明显异常。它们的Ba同位素丰度可以在母体中均匀化。CM球粒陨石钡同位素异常中的部分r过程成分可以得到修正，Cs-135年代学表明其形成年龄为7 (+5/-2)Ma。根据Mn-53绝灭核素年代学，这一年龄短于CI碳酸盐的形成年龄，可能表明Ba在母体中的重新分布。

Extinct nuclides are radioactive nuclides which existed in the early solar system but decayed thoroughly due to their short half lives. However, their daughter nuclides are often detected as isotopic anomalies in primitive meteorites such as carbonaceous meteorites, and this can be used for chronology of the early solar system. We have found the Ba isotopic anomaly in the Beardsley meteorite and suggested that this is due to the extinct nuclide Cs-135 (half life 2.3Ma). In the current study, we try to establish the existence of Cs-135 and apply it for chronology in the early solar systemWe measured the barium isotopic abundances in various meteorites, especially carbonaceous chondrites. Most carbonaceous chondrites of CI, CM, CR and CV groups show the Ba-135 and Ba-137 isotopic anomalies simultaneously, which imply that these anomalies are mainly due to nucleosynthesis of s-process and/or r-process. On the other hand, thermally-altered CK group carbonaceous chondrites and achondrites do not show any significant anomalies. Their Ba isotopic abundances could be homogenized at their parent bodies.Part of the r-process components in the barium isotopic anomalies of CM chondrites could be corrected and the Cs-135 chronology suggests the formation age of 7 (+5/-2) Ma. This age is shorter than the formation age of CI carbonates by Mn-53 extinct nuclide chronology and may suggest the re-distribution of Ba at the parent bodies.