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Petrography and chronology of lunar meteorite Northwest Africa 6950

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Abstract

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, En46–52Fs12–16Wo32–41 and En58–68Fs19–28Wo7–20. 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 (Ab4–9Or0.6–3.7). The weighted mean 207Pb/206Pb 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 clan. We 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.

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Acknowledgements

This work was financially supported by National Nature Science Foundation of China (Grant Nos. 41842060, 41803027), China National Space Administration (Grant No. D020205), and China Geological Survey Research Fund (Grant Nos. JYYWF20180104, JYYWF20181702). We thank members of the Beijing SHRIMP Center for their help with SHRIMP analysis and sample imaging. We are also grateful to Prof. John W. VALLEY for his constructive comments for the paper.

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Authors and Affiliations

  1. Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China

    Zemin Bao, Yuruo Shi, Peizhi Wang, Xiaochao Che, Yuelan Kang, Huiyi Sun & Chen Wang

  2. Department of Earth and Environment, Boston University, Boston, MA, 02215, USA

    J. Lawford Anderson

  3. John de Laeter Centre, Curtin University, Perth, WA, 6845, Australia

    Allen Kennedy

  4. School of Geosciences and Infophysics, Central South University, Changsha, 410083, China

    Zuokai Ke & Xiangping Gu

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  1. Zemin Bao
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Correspondence to Yuruo Shi.

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Bao, Z., Shi, Y., Anderson, J.L. et al. Petrography and chronology of lunar meteorite Northwest Africa 6950. Sci. China Inf. Sci. 63, 140902 (2020). https://doi.org/10.1007/s11432-019-2809-3

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  • DOI: https://doi.org/10.1007/s11432-019-2809-3

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