Research ArticlePLANETARY SCIENCE

Chondrules as direct thermochemical sensors of solar protoplanetary disk gas

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Science Advances  11 Jul 2018:
Vol. 4, no. 7, eaar3321
DOI: 10.1126/sciadv.aar3321
  • Fig. 1 High-resolution CL images of representative type I chondrules.

    See sketch in section S1 of the important features to be observed. (A and C) Yamato 81020, CO3.05, Ch#30b. This porphyritic olivine (ol) pyroxene chondrule shows the complex CL figures in each olivine with a core and euhedral outer edges. Olivine cores always contain metal (met) and glassy (gl) inclusions. This porphyritic olivine pyroxene chondrule shows an example of the Mg-rich olivine core with asymmetric overgrowth toward the periphery of the chondrule, forming a more or less continuous shell of the Mg-rich olivine at the outer edge, formed by successive olivine overgrowth layers. Low-Ca pyroxene (px) postdates the olivine growth. (B and D) Yamato 81020, CO3.05, Ch#9. Barred olivine chondrule. CL images allow us to resolve the first dendritic growth of each olivine bar in this chondrule. Notice the notable absence of FeNi metal blebs in olivine bars. The olivine rim that partly covers the chondrule surface corresponds to a multilayer Mg-rich olivine shell, similar to those observed at the outer edge of porphyritic chondrule. The unconformity between the shell and the olivine bars and their common crystallographic orientation suggest that the Mg-rich olivine shell formed after the olivine bars by an epitaxial growth mechanism.

  • Fig. 2 High-resolution CL images of representative type I chondrules.

    (A) Vigarano, CV3, Ch#25. Barred olivine chondrule with high CL dendritic growth in olivine bars and the thick and complex multilayer Mg-rich olivine shell at chondrule edge. (B) Yamato 81020, CO3.05, Ch#8. Porphyritic olivine pyroxene chondrule showing heterogeneous grain size with a large porphyritic grain. CL zoning textures in the small olivine grains are similar to those preserved at the edge of the large porphyritic grain. Low-Ca pyroxenes are characterized by CL well-resolved striated microstructures of ortho-clino inversion. Inset: Elemental map Al Ks x-ray showing the remarkable correspondence between Al concentration and CL intensity map. Notice, however, the small rotating shift between the two images. (C) Vigarano, CVRed 3.3, Ch#4. Details of a euhedral Mg-rich olivine bathed in the glassy mesostasis of a porphyritic olivine chondrule. Notice the several crystallization and dissolution fronts despite the euhedral shape of the whole crystal. These CL images also reveal that olivine overgrowth episodes proceed by epitaxial growth on preexisting remnant grains. (D) DOM 08004, CO3.1, Ch#8. Mg-rich olivine grain showing multiple fronts of dissolution and growth. Notice that the high CL intensity recorded by the olivine core is similar to Ch#8 in (B). (E) Yamato 81020, CO3.05, Ch#25b. Porphyritic olivine pyroxene chondrule. Notice the significant difference between CL features of the Mg-rich olivines and the low-Ca pyroxenes, and that low-Ca pyroxene growth postdates the Mg-rich olivine. Mg-rich olivine grains show several dissolution fronts. Unzoned low-Ca pyroxene CL features present exceptionally well-resolved striated microstructures of thin alternations of orthoenstatite and clinoenstatite in response to the ortho-clino inversion upon cooling.

  • Fig. 3 High-resolution CL images of representative type I chondrules revealing asymmetric growth of Mg-rich olivines with columnar-like textures at chondrule edge and multilayer Mg-rich olivine shell surrounding both porphyritic and barred chondrules.

    (A) GRA 95229, CR2, Ch#5E. Porphyritic olivine chondrule showing spectacular columnar-like textures at chondrule edge. (B) Yamato 81020, CR3.05, Ch#5. Porphyritic olivine chondrule showing asymmetric growth of Mg-rich olivines at chondrule edge forming granoblastic texture with triple junctions (arrow). At the outer edge, subparallel layers of Mg-rich olivine are uninterrupted by grain boundary between two neighboring olivines delineating a more or less continuous olivine shell surrounding the whole chondrule. (C) Yamato 81020, CR3.05, Ch#20. Part of a porphyritic olivine chondrule surrounded by a multilayer Mg-rich olivine shell. Notice the frequent occurrence of FeNi metal blebs in the Mg-rich olivine core. (D) Yamato 81020, CR3.05, Ch#12. Part of barred olivine chondrule with two sets of olivine bars crosscut by a multilayer Mg-rich olivine shell that surrounds the whole chondrule. Dendritic crystals are particularly well preserved (high CL intensity) in each Mg-rich olivine bar of this chondrule (see sections S1 and S2).

  • Fig. 4 High-resolution CL images of representative type I chondrules showing FeNi metal and glass inclusions in Mg-rich olivines.

    (A and B) Yamato 81020, CR3.05, Ch#4. Metal-rich porphyritic olivine chondrule. Notice that almost all Mg-rich olivine cores (as revealed by CL) host FeNi metal blebs, acting very likely as seeds favoring the olivine growth. See text for explanation. Note also how FeNi metal blebs are incorporated at different steps of the Mg-rich olivine growth. Little arrows outline how FeNi metal is pinched by the Mg-rich olivine growth, suggesting unambiguously that FeNi metal blebs were liquid at the time of Mg-rich olivine crystallization. The absence of CL quenching at the olivine-metal interface is remarkable. (C) Yamato 81020, CO3.05, Ch#9. Euhedral Mg-rich olivine with a core (low CL) sprinkled by both FeNi metal inclusions and Ca-Al–rich melt inclusions in a porphyritic olivine pyroxene chondrule. Some glass inclusions host spherical metal beads. (D) Acfer 094, CO3.1, Ch33b. Porphyritic olivine chondrule with large euhedral grain of Mg-rich olivine hosts a glass inclusion, which, in turn, hosts spherical FeNi metal blebs, suggesting the minimization of surface energy of two immiscible liquids.

  • Fig. 5 Gas-assisted near-equilibrium (liquid epitaxial) growth of Mg-rich olivines.

    (A), porphyritic olivine chondrules; (B), barred olivine chondrules. We suggest that the main difference in the textures of type I chondrules, that is, porphyritic or barred, is linked to the presence or absence of FeNi metal blebs. In porphyritic chondrules, the FeNi metal blebs act as a seeding agent during growth mechanism of the Mg-rich olivine. In the case of barred chondrules, Mg-rich olivine growth starts at the edge of the chondrule (thin layer) and is followed by a dendritic growth in a given crystallographic direction toward the inside of the chondrule in response to the inward Mg and Si chemical gradient (34). The dendritic step of growth is very similar to the one depicted in Al-rich chondrules [figures 1 and 2 in (10)]. The current growth mechanism can be divided in three main steps: incorporation, diffusion, and crystallization (see text). In this cartoon, the pristine Ca-Al–rich melt (dark blue) is progressively diluted by MgO and SiO2 components entering the chondrule melt from the gas phase (pale blue and then yellow) (see Eq. 1).

  • Fig. 6 Chondrule thermal history.

    (A) Summary of chondrule heating and cooling conditions from experimental constraints (8). Cartoon showing maximum temperature relative to the liquidus, for (from right to left) porphyritic, barred, and radial textures, as expected from dynamic experiments. In general, porphyritic textures are produced when the peak temperature is below the liquidus, thus preserving multiple nucleation sites. Barred textures are produced when the peak temperature is slightly above the liquidus temperature such that most, but not all, nucleation embryos are destroyed. Radial textures are produced from temperatures that exceed the liquidus temperature, and in which few or no nucleation sites are available. (B) Current two-stage nonlinear cooling rate thermal history model for chondrule formation. The texture difference between porphyritic and barred chondrules is related to the presence or absence of FeNi metal droplets acting as seeds; the melt is maintained near Mg-rich olivine saturation due to high partial pressures: PMg(g), PSiO(g), and very likely PNa(g) of the surrounding gaseous environment. In contrast with the previous model (A), chondrules cool fast after a variable, but short (few tens of minutes maximum), time of near-equilibrium crystallization governed by gas-melt interaction at elevated temperatures of ≥1800 K. This model establishes the preponderance of the high-temperature gas-melt interaction to define the composition and texture of chondrules and that chondrules did not stay hot very long. Blue, melt; green, Mg-rich olivine; gray, low-Ca pyroxene.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/7/eaar3321/DC1

    Section S1. Further high-resolution CL images of chondrules

    Section S2. Preliminary CL spectral analyses

    Section S3. CL and EBSD in chondrules

    Section S4. Vapor-liquid-solid epitaxial growth mechanism

    Section S5. Phase diagram, nucleation, growth, Ostwald-Miers region, and seeding

    Section S6. Epitaxial growth mode in chondrules

    Section S7. Cartoons showing the inferred chondrule formation steps

    Fig. S1. CL image abbreviations used in the text and figures.

    Fig. S2. CL images of porphyritic chondrules from various chondrites.

    Fig. S3. CL images of barred chondrules from various chondrites.

    Fig. S4. CL spectral analysis of chondrule Mg-rich olivines.

    Fig. S5. CL activator distribution in Mg-rich olivines in porphyritic olivine chondrules.

    Fig. S6. Crystallographic fabrics of Mg-rich olivines in porphyritic olivine chondrule ch4 of Vigarano CV3.

    Fig. S7. Crystallographic fabrics of Mg-rich olivines in porphyritic olivine chondrule ch4 of Vigarano CV3.

    Fig. S8. Intracrystalline misorientations for Mg-rich olivine grains, as determined by EBSD.

    Fig. S9. Epitaxial growth crystallization in a simplified binary diagram.

    Fig. S10. Principal modes of epitaxial growth resulting mainly from competition between surface/interface energies.

    Fig. S11. Cartoons showing the inferred chondrule formation steps.

    Table S1. Examples of chemical composition of representative Mg-rich olivines and their associated phases from chondrule of Yamato 81020.

    References (5153)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Further high-resolution CL images of chondrules
    • Section S2. Preliminary CL spectral analyses
    • Section S3. CL and EBSD in chondrules
    • Section S4. Vapor-liquid-solid epitaxial growth mechanism
    • Section S5. Phase diagram, nucleation, growth, Ostwald-Miers region, and seeding
    • Section S6. Epitaxial growth mode in chondrules
    • Section S7. Cartoons showing the inferred chondrule formation steps
    • Fig. S1. CL image abbreviations used in the text and figures.
    • Fig. S2. CL images of porphyritic chondrules from various chondrites.
    • Fig. S3. CL images of barred chondrules from various chondrites.
    • Fig. S4. CL spectral analysis of chondrule Mg-rich olivines.
    • Fig. S5. CL activator distribution in Mg-rich olivines in porphyritic olivine chondrules.
    • Fig. S6. Crystallographic fabrics of Mg-rich olivines in porphyritic olivine chondrule ch4 of Vigarano CV3.
    • Fig. S7. Crystallographic fabrics of Mg-rich olivines in porphyritic olivine chondrule ch4 of Vigarano CV3.
    • Fig. S8. Intracrystalline misorientations for Mg-rich olivine grains, as determined by EBSD.
    • Fig. S9. Epitaxial growth crystallization in a simplified binary diagram.
    • Fig. S10. Principal modes of epitaxial growth resulting mainly from competition between surface/interface energies.
    • Fig. S11. Cartoons showing the inferred chondrule formation steps.
    • Table S1. Examples of chemical composition of representative Mg-rich olivines and their associated phases from chondrule of Yamato 81020.
    • References (5153)

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