Research ArticlePLANETARY SCIENCE

Potassium isotope anomalies in meteorites inherited from the protosolar molecular cloud

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Science Advances  09 Oct 2020:
Vol. 6, no. 41, eabd0511
DOI: 10.1126/sciadv.abd0511
  • Fig. 1 Chemical and isotopic variations of K among chondrites and planetary bodies.

    Each data point is the group mean calculated using available samples within each group. The corresponding uncertainties on the group means are larger than the analytical uncertainty because of the heterogeneity among different meteorites within groups (details in Table 1). Note that evaporation/condensation cannot explain our K isotopic data. The K and Ca concentrations in each group are taken from the literature (32, 4547). The group mean and uncertainties (2SE) for the CI group in this study are based on Orgueil.

  • Fig. 2 Variations of δ41K, ε54Cr, and ε64Ni in bulk samples.

    The concentrations of each element are from (32, 45), and mean ε54Cr and ε64Ni data for each group are from (17) (see fig. S8 for complete references). The gray regions show the three-component mixing model [modified from (32)] of chondrules, matrix, and CAIs. The chondrule and matrix components are estimated by the average ordinary and CI chondrites, respectively. The correlations between δ41K, ε54Cr, and ε64Ni suggest that these anomalies are primarily caused by nucleosynthetic processes and cannot be generated by a multicomponent mixing model between major chondrite components. The correlations also cannot be explained by the mixing of two isotopically distinct reservoirs, such as CM (or CI) and ordinary chondrites. This is because mixing between two end members would yield straight lines in δ41K-Ca/K plots (Fig. 1, and similar plots in fig. S7B), which is not observed. In addition, mixing the two end members’ compositions is unlikely to reproduce both volatile-depleted Earth and volatile-enriched enstatite chondrites that both have intermediate values of δ41K, ε64Ni, and ε54Cr. pptt, parts per ten thousand.

  • Fig. 3 Models explaining the observed isotopic heterogeneity and MVE depletions in the solar protoplanetary disk.

    (A) A nebular thermal processing of infalling, isotopically homogeneous material from the molecular cloud results in both isotopic heterogeneity (for ε46Ti and ε50Ti) and variable depletions in MVEs in the disk (39). An expected correlation between isotope compositions and depletion of MVEs is not observed for K, ruling out this model. (B) Isotopic heterogeneity (for μ48Ca and others) in the inner disk (depleted in n-rich isotopes) results from thermal processing of a homogeneous molecular cloud material. The inner disk gradually mixes with materials from the outer disk, making the isotopic composition of solid bodies in the inner solar system a function of time (increasing n-rich isotopes with time) (16). This would produce correlations among isotope compositions, MVE depletions, and sizes of planetary bodies in the inner solar system, which is not observed for K, ruling out this model too. (C) Isotopic heterogeneity inherited from an isotopically heterogeneous molecular cloud. Thermal processing in the disk produces the MVE depletions in the solid objects of the inner disk. No correlation between isotope compositions and MVE depletions is expected as long as mixing in the disk occurs on a local scale only, but the correlation between MVEs (δ41K) and other n-rich refractory nuclides (ε64Ni, ε54Cr) can be preserved. This is the model favored by our K isotope data.

  • Table 1 K isotopic composition of each sample in this study.

    The interior portion and fusion crust of Holbrook-1 were powdered independently; the fusion crust, Holbrook-1 (c), is not included in calculating the group mean. NIST SRM-3141-a was measured with respect to Suprapur = +0.047 ± 0.003‰. The group mean and the corresponding 2SE are calculated on the basis of different meteorites. Details are in Materials and Methods. N, numbers of independent analytical runs; n, total number of the bracketed δ41K values from N days; ANSMET, Antarctica Meteorite Collection (NASA); ASU, Center for Meteorite Studies, Arizona State University; Caltech (JW), Caltech Jerry Wasserburg; HMNH, Harvard Museum of Natural History; Harvard (CL), Harvard University (Charlie Langmuir); MNHN, Museum national d’Histoire naturelle; NEMS, New England Meteoritical Services; SAO (JW), Smithsonian Astrophysical Observatory (John Wood); SAO (UM), Smithsonian Astrophysical Observatory (Ursula Marvin); USNM3529, Smithsonian USNM3529, 4-kg Allende powder.

    GroupSampleFind/FallTypeδ41K
    Suprapur
    (‰)
    ±2SE*NnGroup
    mean
    ±2SECrushed
    powder
    (g)
    Sourcesδ41K
    SRM-
    3141a
    (‰)
    CIOrgueil-2aFallCI1−0.1250.0305210.11SAO (JW)−0.172
    Orgueil-2b−0.1920.030526−0.239
    Orgueil-3FallCI1−0.1070.038314−0.133†0.0511024MNHN−0.154
    CMMurray-1aFallCM20.2050.0334190.29SAO (JW)0.158
    Murray-1b0.2410.046290.194
    Mighei-1aFallCM2−0.2550.0462101.94SAO (JW)−0.302
    Mighei-1b−0.2460.04629−0.293
    Murchison-1aFallCM2−0.1120.030105010.00SAO (UM)−0.159
    Murchison-1b−0.1550.06515−0.0540.285−0.202
    COOrnans-1FallCO3.4−0.0120.0295240.88SAO (JW)−0.059
    Ornans-2FallCO3.4−0.0570.0383146.45MNHN−0.104
    DaG 749FindCO3−0.2160.0383171.75ASU−0.263
    NWA7916FindCO30.1360.0334211.60ASU0.089
    KainsazFallCO3.2−0.1650.038314−0.0700.1571.70NEMS−0.212
    CVAllende-1aFallCV3−0.1870.030105034SAO (UM)−0.234
    Allende-1b−0.1970.030946−0.244
    Allende-2FallCV3−0.0350.0334194000USNM3529−0.082
    Vigarano-1aFallCV3−0.2370.0334190.67SAO (JW)−0.284
    Vigarano-1b−0.2700.04629−0.1840.140−0.317
    CKNWA6254FindCK4−0.2550.0462122.22ASU−0.302
    EHAbee-1aFallEH4−0.2930.0257361.00NEMS−0.340
    Abee-1b−0.2920.04629−0.339
    Indarch-1aFallEH4−0.1190.0383151.82NEMS−0.166
    Indarch-1b−0.1600.033422−0.207
    Indarch-2FallEH4−0.3330.038315−0.2640.0560.23HMNH−0.380
    ELMacHill 88481FindEL3−0.5560.0383150.44ANSMET−0.603
    EagleFallEL60.0820.046290.69NEMS0.035
    LLTuxuac-1aFallLL5−0.6090.0462112.46NEMS−0.656
    Tuxuac-1b−0.5430.038314−0.590
    ParnalleeFallLL3−0.9170.0383171.34ASU−0.964
    VicenciaFallLL3−0.7590.0383171.32ASU−0.806
    ChainpurFallLL3.4−0.7080.046210−0.7400.1410.99SAO (JW)−0.755
    LMarion IowaFallL6−0.6790.0305222.00HMNH−0.726
    HomesteadFallL5−0.7250.0306292.00HMNH−0.772
    BruderheimFallL6−0.6720.03831570SAO (UM)−0.719
    Peace River-1FallL6−0.6320.038315190SAO (UM)−0.679
    Peace River-2FallL6−0.6720.0334220.12Caltech (JW)−0.719
    Holbrook-1FallL6−0.6360.0383250.93SAO (JW)−0.683
    Holbrook-1 (c)Fall−0.6210.0383250.60−0.668
    Holbrook-2FallL6−0.6580.046210−0.6750.0283.07HMNH−0.705
    HKernouveFallH6−0.6280.0305232.00HMNH−0.675
    DresdenFallH6−0.6810.0305242.00HMNH−0.728
    Forest CityFallH5−0.7240.0306272.00HMNH−0.771
    GradyFindH3−0.7100.03831570Caltech (JW)−0.757
    GuareñaFallH6−0.6810.03831553.40Caltech (JW)−0.728
    EstacadoFindH6−0.6090.038315−0.6720.0372.80NEMS−0.656
    MarsNakhla-1FallNakhla−0.1930.0295270.25SAO (JW)−0.240
    Nakhla-2FallNakhla−0.1640.0295250.23SAO (JW)−0.211
    QUE94201.55FindShergottite−0.1520.0383150.10ANSMET−0.199
    Zagami-1FallShergottite−0.2790.0383150.11NEMS−0.326
    Zagami-2FallShergottite−0.1200.038314−0.1770.0270.94ASU−0.167
    VestaStannernFallEucrite0.5030.038390.26SAO (JW)0.456
    BouvanteFindEucrite0.2700.0383120.13SAO (JW)0.223
    Sioux CountyFallEucrite0.2020.0383160.20SAO (JW)0.155
    JuvinasFallEucrite0.7120.0462100.4220.2320.14SAO (JW)0.665
    EarthBHVO-2Basalt−0.3330.030946USGS−0.380
    BCR-2Basalt−0.3700.030946USGS−0.417
    CHEPRBasalt−0.5380.033418−0.4140.126Harvard (CL)−0.585
    Internal uncertainty = 30 ppm

    *The uncertainty (2SE) is 2SD (65 ppm)/N (fig. S2) and was replaced with 30 ppm if less than 30 ppm. Additional uncertainty is negligible when converting Suprapur to SRM-3141a.

    †The meteorite group means and 2SE are based on small numbers (numbers of different meteorites); CI group is based on two chunks of one meteorite, Orgueil.

    Supplementary Materials

    • Supplementary Materials

      Potassium isotope anomalies in meteorites inherited from the protosolar molecular cloud

      Y. Ku and S. B. Jacobsen

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