Research ArticlePLANETARY SCIENCE

Discovery of moganite in a lunar meteorite as a trace of H2O ice in the Moon’s regolith

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Science Advances  02 May 2018:
Vol. 4, no. 5, eaar4378
DOI: 10.1126/sciadv.aar4378
  • Fig. 1 Petrological photographs of NWA 2727.

    False-color elemental x-ray map of the NWA 2727 thin section, with red (R) = Mg Kα, green (G) = Fe Kα, and blue (B) = Al Kα x-rays obtained by the electron probe microanalyzer (EPMA). Areas enclosed by white lines indicate Mg/Fe-rich OC gabbroic clasts, including abundant coarse grains of euhedral olivine (orange and yellow) and clinopyroxene (brown and green) accompanied by minor amounts of anhedral plagioclase (blue) between these constituent minerals. The pyroxene phyric basaltic clasts (highlighted by dashed white lines) are characterized by clinopyroxene phenocrysts with a fine-grained groundmass of clinopyroxene, plagioclase, and fine quartz grains. The other areas filling the interstices between these clasts represent the breccia matrix composed of numerous fine to coarse grains of the OC gabbro and basalt lithic minerals with small amygdaloidal silica micrograins.

  • Fig. 2 Micro-Raman spectroscopy and mapping of an amygdaloidal silica micrograin in the breccia matrix.

    (A) High-magnification BSE image of an amygdaloidal silica micrograin (no. 1) in the breccia matrix of NWA 2727. White crosses indicate the analytical points for Raman spectroscopy (areas 1 to 3). (B) Raman spectra of areas 1 to 3 of the silica micrograin in (A) showing a negative correlation between the Raman intensities of the moganite and the quartz bands. Obvious coesite bands are present together with the moganite signature in the rim of the silica micrograin (area 2). a.u., arbitrary units. (C to E) Raman intensity maps of the moganite (502 cm−1) (C), coesite 21 cm−1) (D), and quartz (464 cm−1) (E) bands for the silica micrograin. Red and blue denote the high- and low-relative Raman intensities of each silica band, respectively. Moganite becomes dominant at the left part of the silica micrograin, whereas quartz is abundant in the right part. Coesite coexists with moganite and occurs in the outermost rim of the left, as indicated by the areas of overlap in the spectra.

  • Fig. 3 Micro-Raman analyses of another amygdaloidal silica micrograin in the breccia matrix.

    (A) High-magnification BSE image of another amygdaloidal silica micrograin (no. 2) in the breccia matrix of NWA 2727 far from the shock veins. White crosses and white boxes indicate the analytical points of Raman spectroscopy (areas 1 to 3) and the mapping areas, respectively. (B) Raman spectra of the different areas of the silica micrograin in (A). Raman intensities of the moganite bands increase, whereas those of the quartz bands decrease. The most intense coesite bands are detected together with moganite signatures in the rim of the silica micrograin (area 1). (C to E) Raman intensity maps of the moganite (502 cm−1) (C), coesite (521 cm−1) (D), and quartz (464 cm−1) (E) bands for the silica micrograin. Red and blue denote the high- and low-relative Raman intensities of each silica band, respectively. Moganite is abundant in the right and lower left of the silica micrograin, being present with coesite in the outermost rim of these parts. In contrast, quartz is located in the upper left, and its distribution is opposite to those of moganite and coesite.

  • Fig. 4 Micro-Raman analyses of high-pressure SiO2 phases.

    (A) High-magnification BSE image of an amygdaloidal silica micrograin (no. 3) within a shock vein. Tweed-like textures occur in the rim of this micrograin (lower right). White crosses and white boxes indicate the analytical points for Raman spectroscopy (areas 1 and 2) and the mapping areas, respectively. (B) Raman spectra of the silica micrograin in areas with and without the tweed-like textures. Only moganite and high-pressure silica glass signatures are detected in the area without the tweed-like textures (area 1). In contrast, stishovite and coesite signals become dominant in the tweed-textured rim (area 2). (C to F) Raman intensity maps of the stishovite (753 cm−1) (C), coesite (521 cm−1) (D), moganite (502 cm−1) (E), and shock-induced silica glass (602 cm−1) (F) bands. The relative Raman intensities are represented by color variations from red (high) to blue (low). Abundant stishovite coexists with coesite in the tweed-like textural rim (lower right). In contrast, moganite and shock-induced silica glass are quite depleted in the rim and enriched in the interior without the tweed-like texture.

  • Fig. 5 TEM observation of moganite.

    (A and B) Bright-field TEM images of nanocrystalline aggregates (A) and the largest grain (107.6 nm in radius) (B) of moganite (Mog) taken from the SiO2 region of the silica micrograin (no. 1) (red in fig. S8A). (C and D) SAED patterns of moganite along the [001] (C) and [0Embedded Image1] (D) zone axes. The diffraction patterns were acquired from the same largest grain within the SiO2 region, with an angular relationship of 25.2° between them. Both the aggregates of numerous fine nanoparticles (average radius of 4.5 nm) and the largest grain show spot diffraction patterns corresponding to a moganite structure rather than Debye-Scherrer rings, which indicates the occurrence of moganite nanoparticles with the same crystal orientation.

  • Fig. 6 Chronological schematic of the history of subsurface H2O in the Moon and the formation of moganite.

    This history can be drawn in chronological order, based on the present study and previous works. At 2.993 ± 0.032 Ga, mare basalt (gray) solidified on the lunar surface, and a gabbroic intrusive chamber (green) crystallized in the anorthositic crust (brown) of the PKT. (i) Carbonaceous chondrite (CC) collisions are considered to have occurred at <2.67 ± 0.04 Ga, which led to the delivery of alkaline water to the PKT. (ii) After these collisions, constituent rocks of the PKT and carbonaceous chondrite fragments (that is, a serpentine relict) would have been ejected and brecciated in the impact basin. During breccia consolidation, water delivered by the carbonaceous chondrites was captured as fluid inside the breccia. (iii) On the sunlit surface at 363 to 399 K, the captured H2O (pH 9.5 to 10.5) is likely to have become a silicic acid fluid and have migrated to space and the colder regions. Then, it would have precipitated moganite nanoparticles under high consolidation pressure after (ii) the first collisions at <2.67 ± 0.04 Ga and before (iv) the most recent impact at 1 to 30 Ma as expressed below, or perhaps at <130 Ma. Below its freezing point, it would have been simultaneously cold-trapped in the subsurface down to the depth of the impact basin and PSR. (iv) A subsequent heavy impact event at 1 to 30 Ma may have been launched NWA 2727 from the Moon, which resulted in partial phase transitions from moganite to coesite, stishovite, and cristobalite. The estimated size of impactor that collided on the Moon in this period is at least ~0.1 km in diameter, which formed an impact crater with a diameter of at least ~1 km, based on the shock pressure, temperature, and grain size of stishovite obtained by the present study and a calculation procedure reported in a previous study (23). NWA 2727 eventually may have fallen to Earth at 17 ± 1 thousand years (ka) ago. A subsurface H2O concentration higher than the estimated bulk content of 0.6 wt % is expected to still remain as ice.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/5/eaar4378/DC1

    fig. S1. BSE images of silica in the breccia matrix and the shock veins of NWA 2727.

    fig. S2. BSE image of the shock vein of NWA 2727.

    fig. S3. Micro-Raman analyses of high-pressure and high-temperature SiO2 phases.

    fig. S4. BSE images of silica varieties in various lithologies of NWA 2727.

    fig. S5. Single crystals of anhedral quartz in a basaltic clast.

    fig. S6. Spectroscopic identification of maskelynite and plagioclase.

    fig. S7. SR-XRD analyses of various SiO2 phases.

    fig. S8. TEM chemical composition analyses.

    fig. S9. TEM observations of coesite, stishovite, and cristobalite.

    fig. S10. Micro-Raman spectroscopy of unshocked and experimentally shock-recovered moganite.

  • Supplementary Materials

    This PDF file includes:

    • Modeling and simulation details
    • table S1. Elemental analysis of C, N, O, and Si in original CNS, O-etched CNS, and glassy carbon by energy dispersive x-ray spectrometry elemental mapping.
    • table S2. Partial current densities (mA cm−2) for CNS, oxygen-etched CNS, glassy carbon, and CNS with argon gas.
    • table S3. Comparison of open-circuit potentials and polarizations of original CNS, O-etched CNS, and glassy carbon in 0.25 M KClO4.
    • fig. S1. Representative TEM images of the CNS electrode.
    • fig. S2. The variation of surface electric field Es calculated along the normal direction at the tip of a CNS for different tip radii in the case of desolvated Li+ counterion and of solvated Li+ counterion.
    • fig. S3. Molecular dynamics simulation of electric double layers near a carbon nanosphere immersed in LiCl solution.
    • fig. S4. Regression curves for ammonia quantification.
    • fig. S5. SEM micrographs of CNS surface.
    • fig. S6. SEM micrographs for the side view of CNS.
    • fig. S7. XPS spectra of CNS.
    • fig. S8. The overall current density (red curve) and formation rate (blue dots) with time at −1.19 V versus RHE.
    • fig. S9. CP experiment to investigate stability of the electrode during the initial 5 hours of the reaction, using a larger (4.8 cm2) electrode to observe changes with respect to electrolyte composition.
    • fig. S10. Oxygen-etched CNS showing smoother texture compared to unetched CNS.
    • fig. S11. Correlated orbital levels calculated for three outer valence orbitals and three virtual orbitals at the level of EPT/aug-cc-pVTZ as a function of electric field strength.
    • fig. S12. Ultravoilet photoelectron spectroscopy and work functions of emersed CNS.
    • fig. S13. Ultraviolet photoelectron spectroscopy of pristine, unemersed CNS.
    • fig. S14. Ultraviolet photoelectron spectroscopy of O-etched CNS and glassy carbon.
    • fig. S15. Mass spectra of double-silylated product for ammonia from electrochemical N2 reduction.
    • fig. S16. Mass spectra of 14N and 15N products in the mass region of the molecular ion.
    • References (44–54)

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