Research ArticlePLANETARY SCIENCE

Discovery of fossil asteroidal ice in primitive meteorite Acfer 094

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Science Advances  20 Nov 2019:
Vol. 5, no. 11, eaax5078
DOI: 10.1126/sciadv.aax5078
  • Fig. 1 XCT slice images of equant samples of the Acfer 094 matrix and their 2D histograms of LAC and RID values at 7 and 8 keV.

    Absorption XCT images at 7 keV (A) and 8 keV (B), as well as a phase XCT image at 8 keV (C), indicate an UPL embedded in the matrix. 2D histograms of LAC values at 7 and 8 keV (D) and LAC and RID values at 8 keV (E) of the matrix show peaks around the air, resin [polyacetal (POM)], forsterite (Fo), enstatite (En), and serpentine/saponite (Serp/Sap)–cronstedtite (Cro). Those plots of UPL have peaks in the areas surrounded by white dashed lines in (D) and (E). The density scale corresponding to the RID values is shown in (E). Fa, fayalite; Di, diopside; Hd, hedenbergite; Fs, ferrosilite; Po, pyrrhotite; PE, polyethylene.

  • Fig. 2 SEM–back-scattered electron (BSE) and XCT slice images of a porous object on an Acfer 094 polished section surface.

    SEM-BSE image of the porous object (A) and its absorption XCT cross-sectional image along the black dashed line in (A) at 8 keV (B) show that the object corresponds to UPL. Pt, Pt-deposition applied during FIB sample preparation.

  • Fig. 3 STEM-EDS maps and chemical and oxygen isotopic compositions of UPL and the matrix.

    An annular dark field (ADF)–STEM image (A) and a combined STEM-EDS map (B) for Fe (red), Mg (green), Si (blue), and S (yellow) show that both UPL and the matrix consist mainly of amorphous silicates (Amo; magenta), forsterite (Fo; light green), enstatite (En; cyan), and Fe─Ni sulfides (Sulf; yellow). Spongy organics (OM; red) (C) are heterogeneously distributed in the UPL. Compositions of GEMS-like materials (Amo + Sulf) in the UPL and the matrix, plotted in the Si─Mg─Fe diagram (D), are relatively homogeneous and enriched in Fe compared with those of GEMS in CP-IDPs (1, 3, 4). Olivine (Ol), pyroxene (Px), and serpentine (Serp) solid solution lines are also shown in (D). at %, atomic %. The oxygen isotopic compositions of UPL and the matrix are plotted in the oxygen three-isotope diagram (E). GEMS-like materials in UPLs and the matrix plot around the GEMS (2) mass fractionation (GF) line, suggesting their similar origins. The constituents in UPLs and the matrix show relatively heavy-oxygen isotopic compositions compared with the bulk meteorite (15). SMOW, standard mean of ocean water; TF, terrestrial mass fractionation line; CCAM, carbonaceous chondrite anhydrous mineral line.

  • Fig. 4 Bright-field (BF)–TEM images of UPLs and the matrix.

    (A to C) BF-TEM images of UPLs show that amorphous silicates (Amo) contain various quantities of small Fe─Ni sulfide (Sulf) inclusions and that pores are partially filled with spongy organics (OM). The 2D SAED pattern of the amorphous silicate in (C) contains weak rings of ~0.15 and ~0.25 nm. A BF-TEM image of an enstatite (En) whisker in UPL (D) shows that it is elongated along the crystallographic a axis. The enstatite and forsterite (Fo) in (C) are surrounded by thin, Fe-rich layers, indicated by black arrows. A BF-TEM image of the matrix (E) shows that it mainly consists of densely packed amorphous silicates with Fe─Ni sulfide inclusions. The interspaces between the amorphous silicate grains are filled with poorly crystallized phyllosilicates with a d spacing of ~0.7 nm (F).

  • Fig. 5 Schematic illustration of the Acfer 094 parent body formation model.

    The parent body grew by agglomeration of fluffy source dust with and without ice through its radial migration from the outer to the inner regions of the solar nebula across the H2O snow line. The process produced a layered structure inside the parent body, with an ice-rich core and an ice-poor mantle. Around the H2O snow line, ice-bearing UPLs were incorporated into the mantle. Subsequently, the melting of ice, mainly in the core, induced an aqueous alteration in the parent body. The Acfer 094 meteorite was subsequently ejected from the mantle of the parent body by some destructive processes. Note that we did not describe organics, which might have existed in ice in FISA and ice-bearing UPLs to make it easier to understand. FSA might also have contained some refractory organics.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/11/eaax5078/DC1

    Fig. S1. Schematic illustration of the analytical protocol in this study.

    Fig. S2. SEM images of the two Acfer 094 polished sections, #1 and #2.

    Fig. S3. Histogram showing the size distribution of UPLs.

    Fig. S4. BF-TEM image of a UPL.

    Fig. S5. SAED patterns of amorphous silicates in UPLs and in the matrix.

    Fig. S6. STEM-EDS maps of equilibrated aggregate–like objects in a UPL.

    Fig. S7. BF-TEM image and SAED pattern of an enstatite whisker in the matrix.

    Table S1. Compositions of GEMS-like materials in UPLs and in the matrix.

    Table S2. Brief summary of textural and mineralogical characteristics of UPL, CP-IDP, and UPL-like lithology in the Paris meteorite.

    Table S3. Oxygen isotopic compositions of UPL and matrix.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Schematic illustration of the analytical protocol in this study.
    • Fig. S2. SEM images of the two Acfer 094 polished sections, #1 and #2.
    • Fig. S3. Histogram showing the size distribution of UPLs.
    • Fig. S4. BF-TEM image of a UPL.
    • Fig. S5. SAED patterns of amorphous silicates in UPLs and in the matrix.
    • Fig. S6. STEM-EDS maps of equilibrated aggregate–like objects in a UPL.
    • Fig. S7. BF-TEM image and SAED pattern of an enstatite whisker in the matrix.
    • Table S1. Compositions of GEMS-like materials in UPLs and in the matrix.
    • Table S2. Brief summary of textural and mineralogical characteristics of UPL, CP-IDP, and UPL-like lithology in the Paris meteorite.
    • Table S3. Oxygen isotopic compositions of UPL and matrix.

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