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Petrography, geochemistry, and age of a granophyre clast in the lunar meteorite DEW 12007

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Petrography, geochemistry, and age of a granophyre clast in the lunar meteorite DEW 12007
Other Titles
달운석 DEW 12007의 화강암질 암편의 암석학, 지구화학, 연대 연구
Han, Jeong
Kwon, Sung-Tack
Lee, Ki Wook
Kim, Taehoon
Lee, Mi Jung
Park, Changkun
Lee, Jong Ik
Issue Date
Han, Jeong, et al. 2015. Petrography, geochemistry, and age of a granophyre clast in the lunar meteorite DEW 12007. 78th Annual Meeting of the Meteoritical Society. 미국 UC 버클리(UC Berkeley, USA). 2015.07.27~2015.07.31.
Granitic clasts of lunar meteorites are im-portant to reveal a magmatic evolution history of the Moon. Here we present petrographic, geochemical, and geochronological in-formation for a granophyre clast (C3) in the mingled lunar brec-cia DEW 12007 [1]. A thin section (JK-1) of the DEW 12007 was used for petrography and mineral chemistry with the field-emission electron microprobe JXA-8530F at KOPRI, and zircon age da-ting with SHRIMP IIe at KBSI. Bulk composition was calculated by a modal recombination method. C3 is an oval-shaped (~4 mm) granitic clast with dense microfractures. It contains barian alkali feldspar (An1.2-11.3Ab17.4-41.1Or46.0-76.2Cn0.4-11.5; ~51 vol %) and plagioclase (An27.0-37.1Or0.7-1.7; ~3%), intergrown with silica (~40%). “Ter-nary” feldspar [2] (An11.1-39.0Ab33.6-55.7Or11.6-53.8Cn0.0-2.6) is com-pact without fractures, contrary to the surrounding plagioclase and alkali feldspar. Olivine is anhedral and chemically homoge-neous with Fa90 which is lower than Fa~97 of olivines from other lunar granophyres [3, 4]. Minor phases include needle-shaped ilmenite, tranquillityite, RE-merrillite, and apatite, and skeletal zircon. Troilite, baddelyite, and monazite are trace. No pyroxene appears. C3 has a granitic bulk composition, i.e. high SiO2 (74.01 wt %), K2O (4.81), Na2O (1.82), and BaO (1.24; the highest among lunar samples [3]), and low P2O3 (0.12), TiO2 (0.71), FeO (5.57), MgO (0.30), and CaO (0.87). SHRIMP U-Pb isotopic data (n=16) for five zircon grains give the upper intercept age of 4340.2 ± 7.5 (2σ) Ma in a concordia diagram. The weighted mean of 207Pb-206Pb ages is 4332.4 ± 1.6 (2σ) Ma. These ages belong to the oldest ones reported from zircons of lunar grano-phyres [5]. Fractional crystallization of a basaltic magma is unlikely to directly produce a granitic residual magma [3, 6]. The bulk composition with high SiO2 content of C3 is similar to those of experimentally produced silicic immiscible liquids [6]. How-ever, it is not certain that the liquid immiscibility is related to the formation of C3, due to a lack of a high-Fe fraction of immiscible liquids. Granophyric intergrowths, needle-shaped ilmenites and phosphates, and skeletal zircons represent a high degree of un-dercooling of a granitic magma. Ternary feldspar likely has formed by impact melting of plagioclase and alkali feldspar fol-lowed by rapid cooling, on the basis on the compact texture and intermediate composition.
Conference Name
78th Annual Meeting of the Meteoritical Society
Conference Place
미국 UC 버클리(UC Berkeley, USA)
Conference Date
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