Petrogenesis of the Northwest Africa 4734 basaltic lunar meteorite

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• 作者：Ying Wang ; Weibiao Hsu ; Yunbin Guan ; Xianhua Li ; Qiuli Li ; Yu Liu ; Guoqiang Tang
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2012
• 出版时间：1 September, 2012
• 年：2012
• 卷：92
• 期：Complete
• 页码：329-344
• 全文大小：1313 K

文摘

We report the petrography, mineralogy, trace element abundance geochemistry, and Pb-Pb geochronology of the lunar meteorite Northwest Africa (NWA) 4734 and make a comparison with the LaPaz Icefield (LAP) 02205/02224 low-Ti lunar basaltic meteorites. NWA 4734 is an unbrecciated low-Ti mare basalt composed mainly of subophitic-textured pyroxene (60 vol % ) and plagioclase (30 % ). Pyroxene, plagioclase, and olivine exhibit large compositional variations and intra-grain chemical zoning. Pyroxene and plagioclase in NWA 4734 have rare earth element (REE) concentrations and patterns similar to those of the LAPs. The crystallization age of NWA 4734, determined in situ in baddeleyite, is 3073 ¡À 15 Ma (2¦Ò), nearly identical to that of the LAPs (3039 ¡À 12 Ma). NWA 4734 and the LAPs have similar textures, modal abundances, mineral chemistry, and crystallization ages, and are most likely source-crater paired on the Moon.

One baddeleyite grain in LAP 02224 displays distinctively older and spatially variable ages, from 3349 ¡À 62 to 3611 ¡À 62 Ma (2¦Ò), similar to another baddeleyite grain (3109 ¡À 29 to 3547 ¡À 21 Ma) reported by for the same meteorite. Raman spectra, cathodoluminescence, and stoichiometric studies of the baddeleyite suggest that the two older grains were not endogenic but were trapped by the parental magma. Equilibrium partition calculation shows that the parental melt from which the NWA 4734 plagioclase crystallized has much lower REE contents than its whole rock, indicating an open system during magma evolution. NWA 4734 could have originated from a parental melt with REE concentrations similar to that of the Apollo 12 olivine basalt. The magma likely assimilated a small amount (¡«4 wt % ) of KREEP-rich material during its ascent through the lunar crust.