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

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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.