Very high-K KREEP-rich clasts in the impact melt breccia of the lunar meteorite SaU 169: New constraints on the last residue of the Lunar Magma Ocean

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• 作者：Y. Lin^a ; ^{linyt@mail.igcas.ac.cn} ; W. Shen^a ; Y. Liu^b ; L. Xu^c ; B.A. Hofmann^d ; Q. Mao^b ; G.Q. Tang^b ; F. Wu^a ; X.H. Li^b
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2012
• 期刊代码：27_00167037
• 类别：ear
• 出版时间：15 May, 2012
• 卷：85
• 期：Complete
• 页码：19-40
• 文件大小：2170 K

摘要

In the impact melt breccia (IMB) of Sayh al Uhaymir (SaU) 169, the most KREEP-rich lunar meteorite to date (), clasts of a new type of lithologies were discovered, consisting of Ca-poor and Ca-rich pyroxenes (60.8 vol.%), Ba-rich K-feldspar (27.9 vol.%), phosphates (5.6 vol.%), Nb-rich ilmenite (4.0 vol.%), zircon (1.2 vol.%) and minor sulfide (0.6 vol.%). These mafic lithic clasts are more enriched in KREEP component (鈭?500 脳 CI) than the host meteorite and are highly enriched in potassium. They are referred to as very high-K (VHK) KREEP lithology, and probably most close to the last residual liquid of the Lunar Magma Ocean without significant dilution by other Mg-rich magmas. The fine-grained matrix of the SaU 169 IMB has very similar mineral chemistry to the VHK KREEP lithology, but contains abundant plagioclase with trace K-feldspar. The matrix shows decoupling of K from the REEP-like component; however, it cannot be simply interpreted by mixing the VHK KREEP lithology with anorthosites, which should have diluted the REEP-like component with the same proportion.

SIMS Pb-Pb dating was conducted on zircons in various petrographic settings and with different crystal habits. All analyses show a main age peak at 3921 卤 3 Ma and a smaller one at 4016 卤 6 Ma. The main age peak is identical to the previous Pb-Pb age by and U-Pb age by , dating the catastrophic shock event contributed to the formation of SaU 169 IMB. The older ages are consistent with the previous report of an older bulk U-Pb age by , suggestive of presence of relict crystals in a few large zircon grains. The VHK KREEP clasts predated the fine-grained matrix, but have the same zircon Pb-Pb ages as the latter within the analytical uncertainties. Plagioclase was converted to maskelynite whereas zircon was shocked to diaplectic glass, probably by a second event at 鈭?.8 Ga. However, the identical zircon Pb-Pb ages of the amorphous parts and the remained crystalline areas indicate no resetting of Pb-Pb isotopes by the later shock metamorphism, or there was another severe impact event postdated solidification of the fine-grained matrix within a few million years.