Ilmenite Pb-Pb age of lunar basalt, Asuka-881757

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  • 月玄武岩隕石Asuka-881757のイルメナイトPb-Pb年代

Abstract

<p>Asuka-881757 is an unbrecciated, coarse-grained lunar basalt meteorite with pyroxene (59 %), plagioclase (30 %), and ilmenite (6 %) as major constituent minerals. In this study, we focused on ilmenite (FeTiO3), which is the most radiogenic phase in this meteorite, and developed a new mineral decomposition method composed of HNO3 leaching and HCl dissolution to minimize the effects of secondary Pb contamination. Four ilmenite fractions yielded Pb-Pb ages of 3910.66 ± 3.10 [Ma; MSWD = 1.3] (2σ error). In particular, the Pb isotopes after acid washing formed an isochron connecting the radiogenic and initial Pb (lunar model initial at 3910 Ma) components, suggesting that the effects of secondary Pb contamination were sufficiently removed by this method. The obtained age is one order of magnitude more precise than the previously reported formation ages from the lunar basalt, and is the most precise age ever obtained from lunar basalts. </p>

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