レーザー融解K-Ar法による極微量年代測定法の開発

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Laser fusion measurement for a single grain of phenocryst or of in-situ measurement of less-abundant minerals found on thin sections is established for K-Ar dating method. For such kind of samples, Ar-Ar dating is applied widely to obtain radiometric ages because the Ar-Ar method is independent of the site difference between K and Ar in the specimen. However, Ar-Ar dating raises at least two difficulties: 1) the method requires a control area to treat radioactive samples that were irradiated with neutrons in a nuclear reactor before the analysis to produce ^<39>Ar from ^<39>K; 2) quantities of nuclides produced by irradiation mask information about the original isotope ratios in rock and mineral samples. Consequently, detailed correction using the initial noble gas isotope ratio is inapplicable, which poses a serious problem, especially for recent samples or samples with low K concentrations, which are expected to include minute amounts of radiogenic Ar. In these cases, large uncertainty is brought to ages useless by the masking of the original isotope ratio. Herein, we report an un-irradiated and un-spiked laser fusion K-Ar dating method, with which we can analyze both Ar and K for the identical grains or positions on a thin section. This is mainly attributable to the following protocols: 1) K measurement following/after laser fusion Ar measurement applied to the retrieved single mineral grain itself; and 2) in-situ laser fusion Ar measurement applying to the epoxy resin mounted grain, where its K-content measured using EPMA. This method is expected to enable acquisition of precise radiometric ages of young lavas or of low K samples having a low ^<40>Ar/^<36>Ar ratio.
日本地球化学会の論文

レーザー融解K-Ar法による極微量年代測定法の開発

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