Geochemical investigation of five lunar meteorites : Implications for the composition, origin and evolution of the lunar crust
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概要
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The total number of lunar meteorites is now eleven, and the number of distinct finds now stands at eight. We report new data for the bulk compositions of five lunar meteorites, including Y-793274,a unique regolith breccia composed of 2/3 mare material of VLT (very-low-Ti) affinity and 1/3 highlands material compositionally similar to Apollo 16 regolith. Mixing-deconvolution of the bulk-rock composition shows that for any reasonable assumed highlands-component composition, the TiO_2 content of the mare component must be <1.2 wt%. A unique clast from the Y-791197 highlands regolith breccia is also probably of VLT-mare affinity, based on its REE pattern, Sc/Sm, Mn/Sm, and Eu/Al ratios, and the low Ti/(Ti+Cr) ratio of its pyroxene. VLT affinities have previously been inferred for numerous mare clasts observed in thin section studies of highlands meteorites, and three other recently-discovered lunar meteorites have either VLT or borderline VLT/"normal" mare compositions. Apparently, the abundance of VLT-basaltic matter in the lunar crust is greater than previously supposed; and the fundamental dichotomy of lunar magmatism into distinct nonmare and mare styles may have been less abrupt than commonly envisaged. Alternatively, the prevalence of VLT varieties of mare basalt among the lunar meteorites might be a sign of source-crater pairing. However, it seems probable that at least three, and more likely five or more, separate craters are represented; and at least one of the sources is probably on the farside. In terms of major-element composition, the four highlands regolith breccia meteorites resemble the Apollo 16 regolith. The Si content and Ca/Al ratio of the upper lunar crust appear to be monotonously close to 210mg/g and 0.53 (molar), respectively. However, concentrations of incompatible elements, including K, Th, and U, are far lower in the highlands meteorites than in regolith samples from the central nearside. This trend implies that inferred "lower limits" on the bulk-Moon content of U (and associated refractory lithophile elements) should be relaxed.
- 国立極地研究所の論文
著者
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Warren Paul
Institute Of Geophysics And Planetary Physics University Of Califonia
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Kallemeyn Gregory
Institute of Geophysics and Planetary Physics, University of California
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Kallemeyn Gregory
Institute Of Geophysics And Planetary Physics University Of Califonia
関連論文
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