K-Ar dating of volcanic rocks younger than 0.1 m.y.

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In this research project it was very important to measure Ar isotopic ratios of rock samples precisely. To set up the mass spectrometer we at first made a computer program to simulate ion trajectory. This computer program draws ion trajectory for any magnet shape and ion source position, which is very useful to design a mass spectrometer. We gathered all parts of the mass spectrometer during the first year and set up the machine. Basic and necessary data were collected for ion source, collecter, Ta furnace, and gas purification system. We noticed that the discrimination factor of isotopic ratios were largely influenced by the high voltage of the electron multiplier. Complete automotion system, using a computer and A-D converter, was also developed to get the age data directly.Sensivity of the mass spectrometer have been stable and the discrimination factor was very large but was very much constant, which lead us to do age determination of actual rock samples.The age determinations were performed for two samples which ages were already known and younger than 0.1 m.y. If errors were assigned only from the variation of the discrimination factor, the obtained ages were 0.056 <-!+> 0.005 m.y. for Korean trachyte and 0.028 <-!+> 0.017 m.y. for andesite, respectively. The errors from the hot blank correction gave the relative error of 50% for trachyte and 100% for andesite. We also measured three rock smaples which ages were about 3 m.y. The ages were precisely determined for these samples if air contaminations were low.