A low U/Pb souce in the Moon: U-Th-Pb systematics of lunar meteorite Yamato-793169
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U-Th-Pb isotopic data were determined on mineral separates of the diabasic lunar meteorite, Yamato-793169,in order to define its age and characterize its magma source. Such results have been helpful toward our understanding of the magmatic/chemical evolution of the Moon. Analysis of HBr leaches revealed that the meteorite was heavily contaminated with terrestrial Pb during its residence in Antarctic ice. The three most radiogenic residues indicate a Pb-Pb age of 3916±90Ma. However, this age determination is uncertain because of possible modern terrestrial Pb contamination. Residues of leached pyroxene and plagioclase separates exhibited the most radiogenic Pb compositions (^<206>Pb/^<204>Pb values up to 125) and the U-Pb data from these fractions plot on the lunar cataclysm array. This observation lead us to recalculate or correct all the U-Pb data for terrestrial Pb contamination in order to cause the other fractions to lie on the lunar cataclysm array as well. The contamination-corrected data and residues of pyroxene and plagioclase separates define U-Pb, Th-Pb, and Pb-Pb ages that are within error of each other. The best estimate for the Pb-Pb age of Yamato-793169 using the corrected data is 3930±240Ma. The initial Pb isotopic composition obtained from U-Pb and Th-Pb isochrons is ^<206>Pb/^<204>Pb=11.2±0.8,^<207>Pb/^<204>Pb=13.1±4.3,and ^<208>Pb/^<204>Pb=33±11. Another approach to the data combines U and Th abundances removed during leaching (indigenous) with those in the residues, but disregards the Pb in the leaches (terrestrial contamination). Using this approach, the U-Pb data of the fractions plot inside the concordia curve along a discordia line, and pyroxene separates define intercept ages of 3.92 Ga with source ^<238>U/^<204>Pb(μ) of 10 between 4.56 and 3.92 Ga. Although we cannot define the age of this diabase exactly, these calculations indicate that the age is around 3.9 Ga and not 3.3-3.6 Ga as observed for most LT and VLT mare basalts. Furthermore, the calculated initial Pb isotopic composition for this meteorite is slightly higher than the Canyon Diablo troilite Pb composition, indicating derivation from a source with a low but measurable μ value at ∿3.9Ga. Assuming a single-stage Pb isotopic evolution model between 4.56 to 3.9Ga, the source μ value is estimated to be about 10. This value is much lower than those obtained for many Apollo mare basalts, suggesting that the deep lunar mantle may have had magma reservoirs with μ values as low as that of the bulk Earth mantle.
- Proceedings of the NIPR Symposium on Antarctic Meteorites
Proceedings of the NIPR Symposium on Antarctic Meteorites 6, 58-75, 1993-05
National Institute of Polar Research