Investigation of the Evolution of the Earth Based on ^<40>Ar-^<39>Ar Ages and Noble Gas Isotopes for Very Small Samples

在这项研究中，我们的目标是建立一种方法，用来测量非常小样本的年龄和稀有气体同位素，并应用它们来揭示地球的演化。为了实现它们，我们开发了一个高灵敏度的Ar激光和质谱计相结合的系统，这样我们就可以测量非常小的样品中不到几毫克的惰性气体脱气。由于有关样品的惰性气体脱气量很低，我们试图构建一个尽可能低体积的系统，并选择脱气最少的材料，以获得最小的空白。为了建立激光分步加热的Ar-Ar测年方法，进行了大量的基础性实验，成功地获得了几亿年样品的平台年龄。因此，用这种方法可以安全地测量出年代较长的几百万年的样品。然而，对于不到1000万年的年轻样本来说，精度是不够的。此外，对来自加拿大的科马提岩和伊豆-大岛火山现今玄武岩的惰性气体进行了激光加热脱气，并测定了它们的同位素。除放射性成因的~4He和~(40)Ar外，Kr和Xe在科马提岩中似乎比玄武岩略富集。由于目前的研究只处理了有限的样本，目前很难将这种差异与地球的演化联系起来。为了确定稀有气体特征随地球演化的特征，需要进一步获得具有更大年龄范围的各种样品的数据。

In this study, we have aimed to establish the methods by which we can measure ages and noble gas isotopes for very small samples and apply them to reveal the evolution of the Earth. To accomplish them, we have developed the system by combining an Ar-laser and a mass spectrometer with a high sensitivity so that we can measure noble gases degassed from very small samples of less than a few mg. Since the amounts of noble gases degasssed from concerned samples are very low, we tried to construct a system with low volume as much as possible and selected materials with least degassing in order to gain the minimum blank. In order to establish the Ar-Ar dating mathod with stepwise heating by using the laser system, a lot of fundamental experiments were performed and succeeded in getting plateau ages for samples with ages of a few hundred million years. Hence, it is concluded that samples with older ages of a few hudred million years can be safely measured by this method. However, precision is not sufficient enough for younger samples of less than 10 million years. Further, noble gases were degassed from a komatiite from Canada with an age of more than 2 b.y.and a present basalt of Izu-Oshima volcano by laser heating and their isotopes were measured. Except for radiogenic ^4He and ^<40>Ar, Kr and Xe contents seem to be a little enriched in the komatiite than the basalt. Since only limited samples were treated in the present study, it is difficult to relate the difference to the evolution of the Earth at present. In order to identify the characteristics of noble gas signatures with the evolution of the Earth, it is required to get data further for various kinds of samples with wider ranges of ages.