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，通过这种方法，我们可以测量非常小的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。为了实现这些目标，我们将ar激光器和高灵敏度的质谱仪结合在一起开发了该系统，以便我们可以测量从小于几毫克的非常小的样品中脱气的稀有气体。由于从相关样品中脱气的稀有气体量非常低，我们试图构建一个尽可能小体积的系统，并选择脱气最少的材料以获得最小的空白。为了建立利用激光系统逐步加热的Ar-Ar定年方法，进行了大量的基础实验，并成功地获得了具有几亿年历史的样品的平台年龄。因此，可以得出结论，用这种方法可以安全地测量年龄更大的几亿年的样品。然而，对于年龄小于1000万年的年轻样本来说，精度还不够。采用激光加热的方法，对来自加拿大的年龄超过2亿年的科马铁矿和伊豆大岛火山现存的玄武岩进行了稀有气体脱气，并对其同位素进行了测定。除了放射性成因的^4He和^<40>Ar外，柯马岩中的Kr和Xe含量比玄武岩略富。由于在本研究中只处理了有限的样品，因此很难将这种差异与目前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，需要进一步获得各种年龄范围更广的样品的数据。

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.