講演情報
[PPS12-21]A two-billion-year-old KREEP-rich lunar meteorite, Northwest Africa 16895
*小池 みずほ1、酒井 亮輔1、中田 亮一2、Rider-Stokes Ben3、白石 史人1、笹木 晃平4、高畑 直人4 (1.広島大学、2.海洋研究開発機構、3.The Open University、4.東京大学)
キーワード：
月隕石、若いKREEP玄武岩、局所U–Pb年代測定、主要・微量元素分析、岩石鉱物記載
Northwest Africa (NWA) 16895 was found in Algeria in 2023 and was initially categorized as a martian augite basalt based on its petrology [1]. However, our mineralogical, geochemical, and in-situ U–Pb chronological analyses demonstrate that NWA 16895 may be a young lunar basalt [2–3]. Here we report that this meteorite represents a two-billion-year-old KREEP-rich lunar basalt, providing new insight into the late-stage lunar magmatism.
A polished section of NWA 16895 was prepared and briefly observed under a SEM-EDS. In-situ U–Pb and 207Pb–206Pb dating was conducted on apatite, merrillite, Zr-bearing phases, and surrounding silicates using a NanoSIMS 50 at AORI, UTokyo [4]. Detailed mineralogy and elemental compositions were analyzed using a FE-SEM-FIB, TEM, and EPMA at N-BARD, Hiroshima Univ. Crystal orientations and internal microstructures of phosphates were examined using a SEM-EBSD at the Open Univ. [5–6]. Trace element compositions of the bulk-rock sample were measured using an ICP-MS at Kochi, JAMSTEC, after acid digestion and column chemistry [7]. Bulk-rock major element compositions were measured using an ICP-OES at Hiroshima Univ. High-precision oxygen three-isotope analysis of another bulk-rock was performed by laser-assisted fluorination at the Open Univ [5].
NWA 16895 is dominated by zoned pyroxenes (augite–pigeonite) intergrown with acicular plagioclase and ilmenite, accompanied by Fe-rich olivine, K-feldspar, silica, troilite, apatite, merrillite, and Zr-bearing phases (Fig. 1). Fayalite-silica symplectites occur along with apatite. Minor impact melt pockets are also observed. Pyroxene exhibit Mg-rich augite core and Fe-rich pigeonite rim, with Fe/Mn ratios of ~70–80 (Fig. 2). Their Ti# and Fe# show a broad agreement with lunar low-Ti basalts (Fig. 2). Plagioclase is Ca-rich and associated with K-feldspar. Apatite is rich in F, Fe, and Si. EBSD analyses reveal minor shock-related deformation in phosphates, while plagioclase is largely transformed into maskelynite.
A 207Pb–206Pb isochron obtained from the analyses on phosphates, Zr-bearing phases, impact melt glass, silica, fayalite, and plagioclase yields an age of 2261 ± 110 Ma (MSWD = 1.1; Fig. 3). The isochron slope indicates a highly radiogenic initial Pb component, corresponding to a source µ-value of ~1500. Bulk-rock compositions are enriched in Fe, alkali elements (K, Na), Th and U, depleted in Al, and moderately low in Ti. REE abundances are ~100 × CI chondrites, with a flat pattern and a negative Eu anomaly. The oxygen isotopic composition (Δ17O) is close to 0 ‰, confirming a lunar origin.
The textural and chemical characteristic identify NWA 16895 as a moderately low-Ti, low-Al, high-alkali, and KREEP-rich lunar basalt. The largely preserved crystal orientations of phosphates support an igneous crystallization age of 2261 ± 110 Ma. This age closely matches that of a recently identified lunar meteorite, NWA 16286 (2201 ± 13 Ma; [9]), and older than the Chang’e-5 basalts collected from lunar Oceanus Procellarum (2030 ± 4 Ma; [8]), but distinctly younger than Chang’e-6 basalts from lunar far-side (2807 ± 3 Ma; [10]). The geochemical and chronological similarities suggest that NWA 16895 and NWA 16286 may be paired, although the lithologies exhibit some differences.
References:
[1] Driscoll, E. and Herd, C. D. K. (2025) 56th LPSC Abstr. 2356. [2] Koike, M. et al. (2025) 87th MetSoc Abstr. 5193. [3] Koike, M. et al. (2025) 16th Symp. Polar Sci. Abstr. OAo13. [4] Koike, M. et al. (2020) EPSL 549, 116497. [5] Rider-Stokes, B. G. et al. (2023) Nature Astron. 7, 836–842. [6] Rider-Stokes, B. G. et al. (2024) MaPS 59, 23–29. [7] Nakada, R. et al. (2019) Geochem J. 53, 293–304. [8] Li, Q-L. et al. (2021) Nature 600, 54–60. [9] Yang, M-H. et al. (2025) Science Bulletin 70, 3265–3271. [10] Zhang, W.W.L. et al. (2024) Nature 643, 356–360.
A polished section of NWA 16895 was prepared and briefly observed under a SEM-EDS. In-situ U–Pb and 207Pb–206Pb dating was conducted on apatite, merrillite, Zr-bearing phases, and surrounding silicates using a NanoSIMS 50 at AORI, UTokyo [4]. Detailed mineralogy and elemental compositions were analyzed using a FE-SEM-FIB, TEM, and EPMA at N-BARD, Hiroshima Univ. Crystal orientations and internal microstructures of phosphates were examined using a SEM-EBSD at the Open Univ. [5–6]. Trace element compositions of the bulk-rock sample were measured using an ICP-MS at Kochi, JAMSTEC, after acid digestion and column chemistry [7]. Bulk-rock major element compositions were measured using an ICP-OES at Hiroshima Univ. High-precision oxygen three-isotope analysis of another bulk-rock was performed by laser-assisted fluorination at the Open Univ [5].
NWA 16895 is dominated by zoned pyroxenes (augite–pigeonite) intergrown with acicular plagioclase and ilmenite, accompanied by Fe-rich olivine, K-feldspar, silica, troilite, apatite, merrillite, and Zr-bearing phases (Fig. 1). Fayalite-silica symplectites occur along with apatite. Minor impact melt pockets are also observed. Pyroxene exhibit Mg-rich augite core and Fe-rich pigeonite rim, with Fe/Mn ratios of ~70–80 (Fig. 2). Their Ti# and Fe# show a broad agreement with lunar low-Ti basalts (Fig. 2). Plagioclase is Ca-rich and associated with K-feldspar. Apatite is rich in F, Fe, and Si. EBSD analyses reveal minor shock-related deformation in phosphates, while plagioclase is largely transformed into maskelynite.
A 207Pb–206Pb isochron obtained from the analyses on phosphates, Zr-bearing phases, impact melt glass, silica, fayalite, and plagioclase yields an age of 2261 ± 110 Ma (MSWD = 1.1; Fig. 3). The isochron slope indicates a highly radiogenic initial Pb component, corresponding to a source µ-value of ~1500. Bulk-rock compositions are enriched in Fe, alkali elements (K, Na), Th and U, depleted in Al, and moderately low in Ti. REE abundances are ~100 × CI chondrites, with a flat pattern and a negative Eu anomaly. The oxygen isotopic composition (Δ17O) is close to 0 ‰, confirming a lunar origin.
The textural and chemical characteristic identify NWA 16895 as a moderately low-Ti, low-Al, high-alkali, and KREEP-rich lunar basalt. The largely preserved crystal orientations of phosphates support an igneous crystallization age of 2261 ± 110 Ma. This age closely matches that of a recently identified lunar meteorite, NWA 16286 (2201 ± 13 Ma; [9]), and older than the Chang’e-5 basalts collected from lunar Oceanus Procellarum (2030 ± 4 Ma; [8]), but distinctly younger than Chang’e-6 basalts from lunar far-side (2807 ± 3 Ma; [10]). The geochemical and chronological similarities suggest that NWA 16895 and NWA 16286 may be paired, although the lithologies exhibit some differences.
References:
[1] Driscoll, E. and Herd, C. D. K. (2025) 56th LPSC Abstr. 2356. [2] Koike, M. et al. (2025) 87th MetSoc Abstr. 5193. [3] Koike, M. et al. (2025) 16th Symp. Polar Sci. Abstr. OAo13. [4] Koike, M. et al. (2020) EPSL 549, 116497. [5] Rider-Stokes, B. G. et al. (2023) Nature Astron. 7, 836–842. [6] Rider-Stokes, B. G. et al. (2024) MaPS 59, 23–29. [7] Nakada, R. et al. (2019) Geochem J. 53, 293–304. [8] Li, Q-L. et al. (2021) Nature 600, 54–60. [9] Yang, M-H. et al. (2025) Science Bulletin 70, 3265–3271. [10] Zhang, W.W.L. et al. (2024) Nature 643, 356–360.