U-Pb isotopic characteristics of lunar meteorites Yamato-793274 and Yamato-86032

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U-Pb data from lunar meteorites Yamato (Y)-86032 and Y-793274 confirm that they are of lunar origin, but also indicate that they are not from the same source region on the Moon and experienced different events while residing as lunar regolith. The Pb of both clast and matrix from Y-86032 is the least radiogenic among lunar meteorites thus far analyzed, similar to that of lunar anorthosite 60025,and indicates derivation from a low-^<238>U/^<204>Pb (μ) source, assuming initial lunar Pb compositions were essentially primitive in nature. Whereas the data from Y-86032 plot slightly above the geochron on a ^<206>Pb/^<204>Pb vs. ^<207>Pb/^<204>Pb diagram, data from 60025 and Y-82192 (considered a pare of Y-86032) plot precisely on the geochron. Very small amounts of U, Th, and Pb were leached from Y-86032 using 0.1N HBr; the Pb isotopic compositions of the leaches are essentially modern terrestrial values, indicating that this rock probably did not reside long at the lunar surface and was not exposed to volatile-rich gases expelled during impact event (s). In contrast, large amounts of the three elements (up to five times those of Y-86032) were removed from Y-793274 during leaching, and the Pb isotopic compositions of these leaches are very radiogenic and similar to those of mare basalt and some lunar soils, indicating that this rock was subjected to lunar impactrelated, volatile-rich gases containing radiogenic ^<206>Pb-rich Pb, very similar to typical lunar soils and some breccias containing significant amounts of marederived material. This is the first occurrence of leaches that are more ^<206>Pb-rich than their corresponding residues and indicate that the meteorite may have originated from near a mare region. To compare, the Pb from leaches of Y-791197 clasts (N. NAKAMURA et al.; Mem. Natl Inst. Polar Res., Spec. Issue, 41,106,1986a) was ^<206>Pb-poor, indicating the meteorite may have originated from the lunar highlands. The Pb in residues of Y-793274 is more radiogenic than that of Y-86032 and Y-82192,similar to that in ALHA81005 and lunar anorthositic gabbros 78155 and 15418,and indicate a lunar source region with high-μ (>100 and possibly as high as 300). Corrected U-Pb data indicate a formation age of ∿ 4.4 Ga and a disturbance to the system at ∿ 4.0 Ga.



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