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SCIENCE CHINA Information Sciences, Volume 63 , Issue 4 : 140902(2020) https://doi.org/10.1007/s11432-019-2809-3

Petrography and chronology of lunar meteorite Northwest Africa 6950

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  • ReceivedJun 3, 2019
  • AcceptedJan 19, 2020
  • PublishedMar 9, 2020

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, ${\rm~En}_{46\text{-}52}{\rm~Fs}_{12\text{-}16}{\rm~Wo}_{32\text{-}41}$ and ${\rm~En}_{58\text{-}68}{\rm~Fs}_{19\text{-}28}{\rm~Wo}_{7\text{-}20}$. 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 (${\rm~An}_{87\text{-}95}$) with low sodium and very low potassium (${\rm~Ab}_{4\text{-}9}{\rm~Or}_{0.6\text{-}3.7}$). The weighted mean $^{207}{\rm~Pb}/^{206}{\rm~Pb}$ ages of baddeleyite and phosphate measured using a sensitive high resolution ion micro probe (SHRIMP II) are $3119\pm16$ Ma ($n=9$, MSWD = 2.5) (meansquared weighted deviates, MSWD) and $3119\pm18$ 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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  • Figure 1

    The BSE image of Northwest Africa (NWA) 6950.

  • Figure 2

    The BSE images of representative areas in NWA 6950. (a) Local details of the rock; (b) morphology of some accessory minerals in the rock; (c) a SIMV-shock induced melt vein in the stone; (d) volcanic glass. Ol: olivine, Pgt: pigeonite, MSK: maskelynite, Apa: apatite, Bad: baddeleyite, ilm: ilmenite, Gl: glass.

  • Table 1   Compositions of major elements of phosphate from NWA 6950$^{\rm~a)}$
    Phosphate (wt.%)Pho-1Pho-2Pho-3Pho-4Pho-5Pho-6Pho-7Pho-8Pho-9Pho-10Pho-11Pho-12
    ApaApaApaApaMrlApaApaApaApaApaMrlApa
    P$_2$O$_5$42.7840.6841.8241.4344.8640.1041.1642.2640.6442.0244.9740.69
    CaO51.6754.1554.5654.2245.0753.6453.2055.4854.0354.7145.1853.75
    MgO1.910.150.060.113.500.130.420.000.120.123.510.39
    F1.083.572.584.290.103.605.803.323.013.830.323.51
    Cl0.140.140.150.18bd0.170.260.060.880.08bd0.23
    FeO1.090.600.600.360.740.350.670.190.540.650.810.79
    MnO0.050.02bdbdbd0.03bd0.080.03bd0.02bd
    Na$_2$O0.190.010.060.020.44bd0.01bd0.01bd0.43bd
    SrObd0.08bdbd0.02bdbd0.01bd0.060.070.07
    La$_2$O$_3$bdbdbd0.060.190.06bd0.02bd0.01bdbd
    Ce$_2$O$_3$0.390.270.130.290.540.310.470.040.360.120.970.28
    Total99.3099.6799.97100.9695.4698.38101.98101.4399.62101.6096.2899.71

    a) Apa: Apatite; Mrl: merrillite; bd: below detection limit.

  • Table 2   Summary of isotopic ages (in Ma) of NWA 773 and its paired lunar meteorites$^{\rm~a)}$
    SampleAge (Ma)MethodMaterialRef.
    3129$\pm$11 (95% conf.)Pb-Pbbaddeleyite[5]
    NWA 7732865$\pm$31 (2$\sigma$)Sm-NdWhole rock[18]
    2670$\pm$10 (2$\sigma$)Ar-ArWhole rock[17]
    NWA 31602650$\pm$40 (2$\sigma$)Ar-ArWhole rock[36]
    NWA 31703118$\pm$14 (95% conf.)Pb-Pbbaddeleyite[5]
    NWA 70073106$\pm$22 (95% conf.)Pb-Pbbaddeleyite[5]
    3100$\pm$16 (95% conf.)Pb-Pbbaddeleyite[4]
    NWA 69503119$\pm$18 (95% conf.)Pb-PbphosphateThis study
    3119$\pm$16 (95% conf.)Pb-PbbaddeleyiteThis study
    3123$\pm$7 (2$\sigma$)Pb-Pbbaddeleyite[25]
    NWA 29773100$\pm$50 (2$\sigma$)Sm-NdWhole rock[24]
    3290$\pm$110 (2$\sigma$)Rb-SrWhole rock[24]

    a

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