ARGON DIFFUSION CHARACTERISTICS OF PYROXENES

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In the summer of 2000, Proterozoic mafic intrusives were collected from 28 sites in themiddle part of Ontario, Canada. Fresh orthopyroxene and clinopyroxene grains were separated, and irradated in a nuclear reactor to carry out 40Ar/39Ar dating.Combining fast and accurate ion current measurement with an automatic laser temperaturecontrol, we built an automatic laser step heating 40Ar/39Ar dating system, which improved the statistics of data. An automatic XYZ stage control was purchased in November, and a program to carry out continuous pin point measurements were also developed. During the measurements, a momentum from a vaporized sample released during the irradiation of a laser pulse causes a move of the thin section. This destroys the alignment of sampling points. A thin section were sandwitched with metal (titanium) washers by its weight. This new method allows to perform thin section dating with transmitted light.In 2001, several laser step heating experiments were carried out. Different age spectrawere often obtained from pyroxene grains in the same rock. A method to distinguish pyroxene samples which give plateaux in 40Ar/39Ar dating has not been established. Diffusion parameters of pyroxenes were calculated from the experiments, and estimated closure temperature was approximately 450ーC, which is higher than that of biotites and muscovites, and lower than hornblendes. This implies a significant role of pyroxene as a thermochronometer. These results will be presented in the ninth internatinal symposium on experimental mineralogy, petrology and geochemistry (Zurich, Switzerland).