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Petrography and chronology of lunar meteorite Northwest Africa 6950

Science China(Information Sciences) 2020年第4期63卷 182-194页

作者： Zemin BAO Yuruo SHI J.Lawford ANDERSON Allen KENNEDY Zuokai KE Xiangping GU Peizhi WANG Xiaochao CHE Yuelan KANG Huiyi SUN Chen WANG Beijing SHRIMP Center Institute of Geology Chinese Academy of Geological Sciences John de Laeter Centre Curtin University Department of Earth and Environment Boston University School of Geosciences and Infophysics Central South University

Northwest Africa(NWA) 6950 is an olivine-gabbro lunar meteorite that has a distinctly different petrographic texture from other lunar basalts. In this contribution, we report the petrography, mineralogy,and U-Pb geochronology of baddeleyite and phosphate in this lunar meteorite and make a comparison with other NWA 773 paired meteorites. NWA 6950 consists mainly of coarse-grained olivine, pyroxene,and plagioclase, with minor ilmenite, chromite, troilite, baddeleyite, taenite, and apatite. The abundant rounded to euhedral olivine has a limited Fo ranging from 68 to 69. Pyroxenes include both augite and pigeonite, with a narrow range of Mg#=71–80, and composition, respectively, EnFsWoand EnFsWo. Plagioclase is mainly anhedral, conforming to the crystal margins of olivine and pyroxene. The plagioclase grains are elongated and several hundreds of microns in length. Raman spectra shows that plagioclase has been transformed into maskelynite, which is very calcic(An87-95) with low sodium and very low potassium(Ab4-9 Or 0.6-3.7). The weighted mean207 Pb/206 Pb ages of baddeleyite and phosphate measured using a sensitive high resolution ion micro probe(SHRIMP II) are 3119 ± 16 Ma(n = 9, MSWD= 2.5)(mean squared weighted deviates, MSWD) and 3119 ± 18 Ma(n = 15, MSWD = 2.1), respectively,giving the crystallization age of the meteorite. NWA 6950 has similar mineral compositions, geochemical characteristics, and an almost identical age of crystallization to the magnesian gabbro of the NWA 773 *** conclude that NWA 6950 and NWA 773 are paired meteorites, which crystallize at ca. 3119 Ma, and represent magmatic activities in the KREEP(potassium(K), rare earth elements(REE) and phosphorus(P) rich lunar material) region of the Moon.

关键词： Northwest Africa 6950 lunar meteorite U-Pb chronology baddeleyite phosphate

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Petrography and mineralogy of new lunar meteorite MIL090036

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Advances in Polar Science 2014年第1期25卷 17-25页

作者： XIE Lanfang CHEN Hongyi MIAO Bingkui XIA Zhipeng YAO Jie Guangxi Scientific Experiment Center of Mining Metallurgy and EnvironmentGuilin University of Technology Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration

MIL090036 is a previously unknown meteorite （a feldspathic lunar breccia） that was discovered in Antarctica. The detailed petrography and mineralogy of this meteorite forms the subject of this paper. It has a typical clastic texture that consists of various types of rock debris （e.g. anorthosite, gabbroic anorthosite, gabbro, regolith breccia, troctolite, microporphyritic crystalline impact melt and compound clasts）, mineral crystal fragments （e.g. pyroxenes, plagioclase, olivine and ilmenite） and feldspathic glass clasts. The ifne-grained recrystallized minerals and mineral clasts are cemented together in a glassy groundmass. The anorthite content of plagioclase in the gabbro （An81-83） and anorthosite （An88-93） both have relatively low calcium content compared to those from other breccias （An90-98）. The pyroxene composition （Fs12-35 Wo3-44 En22-79） in the rock debris, crystal mineral clasts and anorthositic glass clasts are relatively iron-deifcient compared to those from gabbro debris with melt glass （Fs37-65 Wo10-29 En21-49） and groundmass （Fs18-69 Wo3-45 En14-50）. In contrast, the pyroxene grains in the gabbroic anorthosite display a narrow compositional range （Fs24-27 Wo7-14 En59-69）. Olivine grains in mineral fragments and the groundmass have a wider compositional range （Fo57-79） than those in the rock debris （Fo67-77）. The Fe/Mn ratio in olivine is in the range of 47 to 83 （average 76） and 76 to 112 （average 73） in pyroxenes, and hence classify within the lunar ifeld. The characteristics of texture, mineral assemblage and compositions suggest that MIL090036 possibly originated from a region beyond that of the Apollo and Luna samples. Further study of MIL090036 is therefore likely to lead to a better understanding of the geological processes on the Moon and the chemical composition of the lunar crust.

关键词： lunar meteorite MIL090036 lunar breccias Petrography Mineralogy

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