U-Th-Pb isotopic systematics of lunar meteorite Asuka-31

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U-Th-Pb isotopic systematics indicate that Asuka-31 is of lunar origin and was formed 3940±8Ma. The Pb isotopic composition is extremely nonradiogenic compared with those of typical Apollo mare basalts. The Pb-Pb, U-Pb, and Th-Pb ages are concordant at 3.94Ga. The U-Pb data from maskelynitized plagioclase does not plot on the internal isochrons defined by other mineral separates, indicating that it was disturbed by a later shock event(s). The U-Th-Pb systematics of Asuka-31,combined with previous results, suggest a scenario for the early Pb isotope evolution of the Moon. According to the currently popular hypothesis for lunar origin, following a collision between the Earth and another large planetesimal, the Moon was formed mainly from the Earth's mantle and partially from the other impactor. The primary ^<238>U/^<204>Pb (μ) value of the Moon had been increased four to five times that of the Earth's mantle value (∿8) by volatilization of Pb during the impact. Further depletion of Pb within the Moon's mantle is believed to have occurred during lunar core formation. The chalcophile behavior of Pb and large partition coefficient of Pb in silicate minerals compared to those of U and Th helped to decrease μ values of early cumulates that formed from the magma ocean and settled in the deep lunar mantle. The μ values of later cumulates gradually increased as a result of extensive fractionation. We suggest that Asuka-31 originated from partial melting of early cumulates enriched with sulfides.
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U-Th-Pb isotopic systematics of lunar meteorite Asuka-31

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