Research ArticleASTRONOMY

Primordial formation of major silicates in a protoplanetary disc with homogeneous 26Al/27Al

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Science Advances  11 Mar 2020:
Vol. 6, no. 11, eaay9626
DOI: 10.1126/sciadv.aay9626
  • Fig. 1 Illustration of two Δ′26Mg evolution models for chondrite parent bodies.

    The canonical model (purple curve), consistent with widespread (26Al/27Al)0 homogeneity, uses the modern composition of CI chondrites (9, 37, 47) and (26Al/27Al)0 of (5.32 ± 0.11) × 10−5 (8, 9) to yield Δ′26Mg0 = −34.7 ± 1.4 ppm. Ordinary chondrites (OC) and enstatite chondrites (EC), two major classes of chondrites, yield statistically identical Δ′26Mg0 based on their modern compositions (9, 37, 47). (ii) The alternative “AOA-CAI” model (orange curve) assumes Δ′26Mg0 of −15.8 ppm (9), consequently requiring (26Al/27Al)0 a factor of ~2 lower than the canonical model to evolve to modern CI composition, reflecting (26Al/27Al)0 heterogeneity between the portion of the protoplanetary disc that condensed CAIs and that which contributed to bulk chondrites. Uncertainty bars/areas are ±2 SE.

  • Fig. 2 Examples of RFs as isolated matrix grains (left and right) and in situ phenocrysts in a magnesium-rich (type I) chondrule (middle, dashed outlines).

    Careful high-resolution microexcavation of material adjacent to RFs before microsampling (bottom panels; see also the Supplementary Materials) reduces the risk of inadvertently sampling unwanted neighboring material. Top panels are backscattered electron maps, middle panels are Kα x-ray maps (green, magnesium; blue, silicon; red, aluminum; green, olivine; light-blue, pyroxene; pink/red, Al-rich phases), and bottom panels are optical images.

  • Fig. 3 The chemical and isotopic compositions of RFs compared to CAIs, AOAs, chondrules, and both Δ′26Mg evolution models.

    (A) RFs (Fo>98.5) are Ca-rich relative to AOAs and CAIs. (B) Oxygen isotope compositions similar to bulk carbonaceous chondrite (CC) chondrules distinguish RFs from AOAs and CAIs, linking them to the major silicates in chondrites. We show the primitive chondrule mineral (PCM) line (48), the terrestrial fractionation line, and a fractionation line at Δ′17O = −5.6‰ around which our RF data cluster. Measured Δ′26Mg0 of RFs relative to the end-member Δ′26Mg0 models (vertical bars) plotted against (C) calcium and (D) aluminum concentrations. Four RFs are well resolved from the AOA-CAI model. All uncertainties are ±2 SE (omitted on literature data and smaller than symbols for our oxygen data). Literature references are given in the Supplementary Materials.

  • Fig. 4 The onset of the solar system’s rock record as recorded by Al-Mg and Pb-Pb systematics in chondrites.

    (A) Magnesium Δ′26Mg0 model ages of RFs (this study), which span from CAI formation (time zero) to ~3 to 4 Ma. (B) Kernel density estimate curves of Al-Mg bulk CAIs and internal chondrule ages (literature sources; see the Supplementary Materials), showing a well-defined CAI peak and a broad chondrule peak ~2 to 3 Ma later. (C) Pb-Pb ages of individual chondrules (literature sources; see the Supplementary Materials) range from CAI formation to ~4 Ma, similar to the distribution of our RF model ages. All uncertainties are ±2 SE.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/11/eaay9626/DC1

    Supplementary Text

    Fig. S1. Δ′26Mg evolution models for three major classes of chondritic meteorites.

    Fig. S2. Aluminum blank correction models.

    Fig. S3. The empirically derived terrestrial oxygen isotope fractionation line.

    Fig. S4. Microexcavation of an RF.

    Fig. S5. Magnesium isotope measurements of reference solutions.

    Fig. S6. Measurements of samples using long measurement times.

    Fig. S7. 27Al/24Mg measurements of the JP-1 reference material.

    Fig. S8. A summary of false-color Kα x-ray maps of the 13 RFs analyzed in this study.

    Fig. S9. Detailed scanning electron microscope image of RF C9 (Felix).

    Fig. S10. Detailed scanning electron microscope image of RF C9a (NWA 4502).

    Fig. S11. Detailed scanning electron microscope image of RFs C19a and C19b (NWA 4502).

    Fig. S12. Detailed scanning electron microscope image of RF C39 (NWA 4502).

    Fig. S13. Detailed scanning electron microscope image of RF C4 (NWA 4502).

    Fig. S14. Detailed scanning electron microscope image of RF C18 (NWA 4502).

    Fig. S15. Detailed scanning electron microscope image of RFs C1a, C1b, and C1c (NWA 4502).

    Fig. S16. Detailed scanning electron microscope image of RF C21 (NWA 4502).

    Fig. S17. Detailed scanning electron microscope image of RF C34 (NWA 4502).

    Fig. S18. Detailed scanning electron microscope image of RF C6 (NWA 4502).

    Table S1. A summary of the chemical composition and 27Al/24Mg of RFs measured in situ by EPMA and ex situ by inductively coupled plasma source mass spectrometry.

    Table S2. A summary of the oxygen isotope composition of RFs measured in situ by secondary ionization mass spectrometry.

    Table S3. A summary of the magnesium isotope composition of RFs measured ex situ by MC-ICP-MS and their associated Δ′26Mg0 model ages.

    References (49106)

  • Supplementary Materials

    The PDF file includes:

    • Supplementary Text
    • Fig. S1. Δ′26Mg evolution models for three major classes of chondritic meteorites.
    • Fig. S2. Aluminum blank correction models.
    • Fig. S3. The empirically derived terrestrial oxygen isotope fractionation line.
    • Fig. S4. Microexcavation of an RF.
    • Fig. S5. Magnesium isotope measurements of reference solutions.
    • Fig. S6. Measurements of samples using long measurement times.
    • Fig. S7. 27Al/24Mg measurements of the JP-1 reference material.
    • Fig. S8. A summary of false-color Kα x-ray maps of the 13 RFs analyzed in this study.
    • Fig. S9. Detailed scanning electron microscope image of RF C9 (Felix).
    • Fig. S10. Detailed scanning electron microscope image of RF C9a (NWA 4502).
    • Fig. S11. Detailed scanning electron microscope image of RFs C19a and C19b (NWA 4502).
    • Fig. S12. Detailed scanning electron microscope image of RF C39 (NWA 4502).
    • Fig. S13. Detailed scanning electron microscope image of RF C4 (NWA 4502).
    • Fig. S14. Detailed scanning electron microscope image of RF C18 (NWA 4502).
    • Fig. S15. Detailed scanning electron microscope image of RFs C1a, C1b, and C1c (NWA 4502).
    • Fig. S16. Detailed scanning electron microscope image of RF C21 (NWA 4502).
    • Fig. S17. Detailed scanning electron microscope image of RF C34 (NWA 4502).
    • Fig. S18. Detailed scanning electron microscope image of RF C6 (NWA 4502).
    • Legends for tables S1 to S3
    • References (49106)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Table S1 (Microsoft Excel format). A summary of the chemical composition and 27Al/24Mg of RFs measured in situ by EPMA and ex situ by inductively coupled plasma source mass spectrometry.
    • Table S2 (Microsoft Excel format). A summary of the oxygen isotope composition of RFs measured in situ by secondary ionization mass spectrometry.
    • Table S3 (Microsoft Excel format). A summary of the magnesium isotope composition of RFs measured ex situ by MC-ICP-MS and their associated Δ′26Mg0 model ages.

    Files in this Data Supplement:

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