Research ArticleASTRONOMY

Oxygen isotopic heterogeneity in the early Solar System inherited from the protosolar molecular cloud

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Science Advances  16 Oct 2020:
Vol. 6, no. 42, eaay2724
DOI: 10.1126/sciadv.aay2724
  • Fig. 1 Mineralogy and petrography of grossite-bearing CAIs in the metal-rich CH and CB carbonaceous chondrites.

    (A to D and F) Backscattered electron images and (E) combined x-ray elemental map in Mg (red), Ca (green), and Al (blue) of the grossite-bearing CAIs (A) 1579-2-1, (B) MB2-15, (C) 1573-4-11, (D) MB111-33, and (E and F) 4C from Acfer 214 (CH) and QUE 94627 (CB). The region outlined in (E) is shown in (F). The Acfer 214 CAIs are surrounded by single- or multilayered Wark-Lovering rims, which are considered to have resulted from melting, evaporation, and condensation processes in the CAI-forming region. CAI 4C is a fine-grained inclusion that probably represents an aggregate of nebular condensates. Cpx, Al-diopside; fo, forsterite; grs, grossite; hib, hibonite; krt, krotite; mel, melilite; pv, perovskite; sp., spinel.

  • Fig. 2 Oxygen isotopic compositions of grossite-bearing CAIs in the metal-rich CH and CB carbonaceous chondrites.

    (A) δ17O versus δ18O and (B) Δ17O of grossite-bearing CAIs, their Wark-Lovering rims, and CAI-bearing chondrules in CH, CH/CB, and CB metal-rich carbonaceous chondrites. In (B), individual CAIs are separated by dashed lines; numbered CAIs are shown in Fig. 1 and fig. S1. The vast majority of the CAIs studied have uniform Δ17O. There is, however, a large range of Δ17O among individual inclusions. CAI minerals are shown by color symbols: an, anorthite; cpx, Al-diopside; fo, forsterite; grs, grossite; hib, hibonite; krt, krotite; mel, melilite; pv, perovskite; sp., spinel. Chondrule minerals are shown by black-and-white symbols: cpx, high-Ca pyroxene; ol, olivine; pl. plagioclase; px, low-Ca pyroxene. CCAM, carbonaceous chondrite anhydrous mineral line (53); PCM, primitive chondrule mineral line (54); SW, Genesis solar wind (1); TF, terrestrial fractionation line.

  • Fig. 3 The inferred initial 26Al/27Al ratios and Δ17O values for CAIs from the CV3, CR2, CM2, CH3.0, and CO3.0-3.1 + Acfer 094 (C3.0 ungrouped) carbonaceous chondrites.

    (A) The initial 26Al/27Al [(26Al/27Al)0] ratios inferred from the internal and model 26Al-26Mg isochrons in CAIs from the CV3, CR2, CM2, CH3.0, and CO3.0-3.1 + Acfer 094 (C3.0 ungrouped) carbonaceous chondrites. The CM SHIBs and CAIs in CR, CO, and CO-like chondrites have approximately the canonical (26Al/27Al)0 of (5.25 ± 0.02) × 10−5. The CM PLACs, most FUN/F (fractionation ± unidentified nuclear isotope effects) CAIs from CV, CM, CR, and CO chondrites, and ~95% of the grossite-bearing CH CAIs have much lower (26Al/27Al)0 than the canonical value, <5 × 10−6. (B) Αverage Δ17O of the isotopically uniform CAIs in CM2, CR2, CO3.0, CH3.0, and Acfer 094, and FUN/F CAIs from CV3, CO3, CM2, and CR2 chondrites. FUN CAIs from metamorphosed CV chondrites experienced postcrystallization oxygen-isotope exchange and are isotopically heterogeneous (49). To calculate the average Δ17O of the CV FUN CAIs, only minerals that avoided postcrystallization oxygen-isotope exchange were used; these include spinel, hibonite, forsterite, and most Al,Ti-diopside grains. The predominantly 26Al-poor CAIs show a much larger range of Δ17O than the 26Al-rich CAIs. Note that not all CAIs measured for oxygen isotopes were analyzed for Al-Mg isotope systematics. Data from (810, 24, 26, 28, 32, 34, 37, 38, 49, 4043) and this study. grs, grossite; ; SW, Genesis solar wind (1); TF, terrestrial fractionation line.

  • Fig. 4 Volume fractions of key O-bearing species in the protostellar collapse model of Lee et al. at the inner boundary of the model (125 AU).

    Volume fractions of key O-bearing species in the Lee et al. (50) protostellar collapse model at the inner boundary of the model (125 AU). The radiation field is 100× the local interstellar medium FUV radiation field (G0 = 100). Collapse is defined to start at t = 0. The first CAIs are believed to have formed ~104 years after the start of the collapse. The photochemical loss of CO releases oxygen atoms that are converted to H2O.

  • Fig. 5 Time evolution of delta values (δ17O and δ18O) for CO and H2O at the inner boundary of the collapse model of Lee et al.

    Time evolution of delta values (δ17O and δ18O) for CO and H2O at the inner boundary of the collapse model of Lee et al. (50). Delta values are computed relative to initial (assumed) molecular cloud ratios; delta values relative to SMOW (Standard Mean Ocean Water) would be approximately 60‰ lighter. COtot = CO + COice and Otot = COtot + H2Oice + atomic O. At time t = 0, the protosun begins to accumulate mass. At the end of the model run, the protosun is 1 solar mass. Photodissociation of CO is accompanied by a massive isotope enrichment in 17O and 18O in the newly formed H2O due to self-shielding by 12C16O. CO isotope fractionation during photodissociation is computed using shielding functions.

  • Fig. 6 Time evolution of Δ17O of fractionated nebular material (CO + H2O + O) at the inner edge of the collapse model.

    Time evolution of Δ17O of fractionated nebular material (CO + H2O + O) at the inner edge of the collapse model. Dust/gas fractionation between CO + O and H2O is accounted for by the factor β, which is the fraction of H2O remaining in nebular material transported to the inner disk. This would be the Δ17O value of the CAI-forming region. The Δ17O values are relative to the bulk initial cloud, which we assume to be identical to the modern bulk Sun. Δ17O values of ~−10‰ are required to explain the lowest Δ17O value of 26Al-poor CH CAI data presented here, indicated by the red ellipse at a time ~20,000 to 30,000 years after the start of collapse. In the collapse model, the protosun is fully formed by 4.6 × 105 years. Results from the model described in Lee et al. (50).

Supplementary Materials

  • Supplementary Materials

    Oxygen isotopic heterogeneity in the early Solar System inherited from the protosolar molecular cloud

    Alexander N. Krot, Kazuhide Nagashima, James R. Lyons, Jeong-Eun Lee, Martin Bizzarro

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    This PDF file includes:

    • Mineralogical observations
    • Gas/dust fractionation during disk accretion
    • Solar nebula model
    • Radial transport time scales assuming an active MRI
    • FUV and x-ray surfaces of solar nebula
    • Inward flux of surface disk material
    • Time scales for CO photodissociation by FUV and H ionization by x-rays
    • Time scales for loss of O onto grains in disk surface and for grains settling
    • Δ17O of mixed material in CAI-forming region
    • Table S1
    • Figs. S1 to S5
    • References

    Files in this Data Supplement:

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