Research ArticleSPACE SCIENCES

Origin of uranium isotope variations in early solar nebula condensates

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Science Advances  04 Mar 2016:
Vol. 2, no. 3, e1501400
DOI: 10.1126/sciadv.1501400
  • Fig. 1 δ235U plotted as a function of the 144Nd/238U atomic ratio in meteoritic samples.

    Open circles, previous studies (11, 1719, 33, 34); blue circles, Allende CAIs from this work; light-blue square, bulk Allende from this work]. The +59‰ δ235U value observed in the Curious Marie CAI is well outside the range of variations expected from fractionation during condensation (gray rectangle) and is thus interpreted as definitive evidence for live 247Cm in the ESS. The scatter in the data (for example, at very low Nd/U ratios) suggests that stable isotopic fractionation during evaporation/condensation also influenced the U isotopic composition of CAIs. The slope of the two-point isochron between Curious Marie and the rest of the samples translates into a 247Cm/235U of (5.6 ± 0.3) × 10−5 at the time of Nd/U fractionation in Curious Marie. Accounting for a possible delay between this fractionation event (possibly related to the extensive alteration of this CAI) and the formation of the SS of 5 ± 5 My, the inferred 247Cm/235U at SS formation is (7.0 ± 1.6) × 10−5 (red line).

  • Fig. 2 Correlation between U and Yb abundances relative to solar composition and the abundance of Nd (a refractory lithophile element).

    Circles denote data from different studies (3537) and this study, and triangles denote data from Brennecka et al. (11). This correlation indicates that U and Yb have similar behaviors during evaporation/condensation processes under solar nebula conditions. The Curious Marie CAI plots off the 1:1 line, with a CI-normalized U/Nd ratio 20 times lower than that of Yb/Nd. This CAI is extremely altered (see main text and table S1), and such alteration may have mobilized U, producing a 20-fold U depletion on top of the 50-fold depletion associated with condensation.

  • Fig. 3 Meteoritic abundance ratios of extinct radionuclides to stable nuclides produced by the same process (for example, 129I/127Ir), normalized to stellar production ratios versus mean lives (τ = t1/2 /ln2).

    The superscript “r” refers to the r-process component of the cosmic abundance [obtained after subtracting the s-process contribution from solar abundances (3)]. When the normalizing isotope is not stable (for example, X/238U or X/232Th), the R/P ratio [that is, (NSLR/NStable)/(PSLR/PStable)] is corrected for the decay of the long-lived isotope by multiplication by the N/P ratio of the normalizing isotope in the ESS [0.71 for 238U and 0.89 for 232Th, values from Nittler and Dauphas (10)]. Triangles denote p-process isotopes, diamonds denote r-process nuclides with possible large s-process contributions, and circles denote r-process isotopes. The data are compared to model steady-state abundances in the ISM, using the model of Dauphas et al. (28) with k = 1.7 and a presolar age of the galaxy of 8.7 Gy. Dotted curves show model abundances assuming free-decay intervals of 10, 35, and 100 My, respectively. The abundances of all r-process nuclides can be explained by a single r-process environment that was active throughout the history of the galaxy and from which the solar system parent material was isolated about 100 My before SS formation, provided that 182Hf and 107Pd in meteorites originate from the s-process (6, 31). See the Supplementary Materials for source data.

  • Table 1 Type, REE pattern, mass, U content, 144Nd/238U atomic ratio, and U isotopic composition of the samples analyzed in this study.

    n is the number of replicate analyses for each sample (starting from digested sample). Nd/U ratios and isotopic compositions are corrected for blank contributions (see table S2). δ235U = [(235U/238U)sample/(235U/238U)CRM-112a − 1] × 103. Error bars are 95% confidence intervals. Nd/U ratio was calculated using the U concentration from the double-spike technique, and Nd concentration from the standard addition technique (see the Supplementary Materials).

    Average 144Nd/238U ratios and U isotopic compositions of CAIs
    SampleTypeREE patternMass (mg)U (ng)n144Nd/238U±δ235U (‰) blk corr.±
    AllendeCV3101621.9220.22.50.490.16
    FG-1Fine-gr. CAIGroup II35.20.772156.711.62.081.54
    Curious MarieFine-gr. CAIGroup II717.90.37322,64078058.932.08
    CG-1Coarse-gr. CAIGroup I48.40.762120.88.81.351.52
    FG-2Fine-gr. CAIGroup II50.41.532173.96.50.120.69
    FG-3Fine-gr. CAIGroup II98.34.052104.84.51.010.50
    FG-4Fine-gr. CAIGroup II349.715.7266.84.7−0.830.25
    FG-5Fine-gr. CAIGroup II48.72.82254.23.90.300.59
    FG-6Fine-gr. CAIGroup II54.90.3521009642.981.14
    FG-7Fine-gr. CAIGroup II61.40.982555162.130.96
    FG-8Fine-gr. CAIGroup II377.38.502653103.320.21
    FG-9Fine-gr. CAIGroup II330.723.9244.23.11.950.14
    FG-10Fine-gr. CAIGroup II15.610.2428341563.141.75
    CG-2Coarse-gr. CAIGroup V200.423.0231.52.20.430.14
    FG-11Fine-gr. CAIGroup II68.60.832768166.031.02
    TS32Coarse-gr. CAIGroup V41.84.892322.4−0.160.30

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/3/e1501400/DC1

    Materials and Methods

    Curious Marie: not a FUN CAI.

    The 247Cm-235U chronometer and the initial abundance of 247Cm in the ESS.

    GCE model

    Fig. S1. Photos of typical fine-grained and coarse-grained CAIs.

    Figs. S2 to S15. Secondary electron, backscattered electron, and false-color RGB maps of all samples.

    Fig. S16. REE and U-Th abundance patterns of all 12 fine-grained CAIs analyzed in this study.

    Fig. S17. Results of the standard addition measurements conducted on the Curious Marie CAI.

    Fig. S18. U blank from new U/TEVA resin as a function of the volume of 0.05 M HCl passed through the column.

    Fig. S19. Results of precision tests of U isotopic measurements done using various instrumental setups.

    Fig. S20. Comparison of the δ235U determined on the CAIs using 80% (x axis) and 20% (y axis) of the sample.

    Fig. S21. Flowchart of the tests conducted on the Curious Marie CAI.

    Fig. S22. Evolution of the U isotopic composition of the gas, instantaneous solid, and cumulative solid, as a function of the fraction of U condensed.

    Table S1. Results of SEM analysis on small chips of CAIs mounted in epoxy (wt %).

    Table S2. Summary of U isotopic compositions and concentrations of CAIs and geostandards.

    Table S3. Specifics of U isotopic measurements on MC-ICPMS for low U samples.

    Table S4. Compilation of chemistry blanks and effect on U “stable” isotope ratio measurements.

    Table S5. Summary of the Ti data obtained on geostandards and the Curious Marie CAI.

    Table S6. Production ratios of selected SLRs produced by the s-, r-, and p-process and present in the ESS, normalized to a stable isotope produced in the same or similar nucleosynthetic process.

    Table S7. Compilation of Cm/U isochron data and free-decay interval data from experimental and theoretical studies.

    Table S8. Selected extinct radionuclides produced by the s-, r-, and p-process.

    References (3878)

  • Supplementary Materials

    This PDF file includes:

    • Materials and Methods
    • Curious Marie: not a FUN CAI.
    • The 247Cm-235U chronometer and the initial abundances of 247Cm in the ESS.
    • GCE model
    • Fig. S1. Photos of typical fine-grained and coarse-grained CAIs. Figs. S2 to S15. Secondary electron, backscattered electron, and false-color RGB maps of all samples.
    • Fig. S16. REE and U-Th abundance patterns of all 12 fine-grained CAIs analyzed in this study.
    • Fig. S17. Results of the standard addition measurements conducted on the Curious Marie CAI.
    • Fig. S18. U blank from new U/TEVA resin as a function of the volume of 0.05 M HCl passed through the column.
    • Fig. S19. Results of precision tests of U isotopic measurements done using various instrumental setups.
    • Fig. S20. Comparison of the δ235U determined on the CAIs using 80% (x axis) and 20% (y axis) of the sample.
    • Fig. S21. Flowchart of the tests conducted on the Curious Marie CAI.
    • Fig. S22. Evolution of the U isotopic composition of the gas, instantaneous solid, and cumulative solid, as a function of the fraction of U condensed.
    • Table S1. Results of SEM analysis on small chips of CAIs mounted in epoxy (wt %).
    • Table S2. Summary of U isotopic compositions and concentrations of CAIs and geostandards.
    • Table S3. Specifics of U isotopic measurements on MC-ICPMS for low U samples.
    • Table S4. Compilation of chemistry blanks and effect on U “stable” isotope ratio measurements.
    • Table S5. Summary of the Ti data obtained on geostandards and the Curious Marie CAI.
    • Table S6. Production ratios of selected SLRs produced by the s-, r-, and p-process and present in the ESS, normalized to a stable isotope produced in the same or similar nucleosynthetic process.
    • Table S7. Compilation of Cm/U isochron data and free-decay interval data from experimental and theoretical studies.
    • Table S8. Selected extinct radionuclides produced by the s-, r-, and p-process.
    • References (38–78)

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