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A nucleosynthetic origin for the Earth’s anomalous 142Nd composition

Nature volume 537, pages 394–398 (2016)Cite this article

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Abstract

A long-standing paradigm assumes that the chemical and isotopic compositions of many elements in the bulk silicate Earth are the same as in chondrites1,2,3,4. However, the accessible Earth has a greater 142Nd/144Nd ratio than do chondrites. Because 142Nd is the decay product of the now-extinct 146Sm (which has a half-life of 103 million years5), this 142Nd difference seems to require a higher-than-chondritic Sm/Nd ratio for the accessible Earth. This must have been acquired during global silicate differentiation within the first 30 million years of Solar System formation6 and implies the formation of a complementary 142Nd-depleted reservoir that either is hidden in the deep Earth6, or lost to space by impact erosion3,7. Whether this complementary reservoir existed, and whether or not it has been lost from Earth, is a matter of debate3,8,9, and has implications for determining the bulk composition of Earth, its heat content and structure, as well as for constraining the modes and timescales of its geodynamical evolution3,7,9,10. Here we show that, compared with chondrites, Earth’s precursor bodies were enriched in neodymium that was produced by the slow neutron capture process (s-process) of nucleosynthesis. This s-process excess leads to higher 142Nd/144Nd ratios; after correction for this effect, the 142Nd/144Nd ratios of chondrites and the accessible Earth are indistinguishable within five parts per million. The 142Nd offset between the accessible silicate Earth and chondrites therefore reflects a higher proportion of s-process neodymium in the Earth, and not early differentiation processes. As such, our results obviate the need for hidden-reservoir or super-chondritic Earth models and imply a chondritic Sm/Nd ratio for the bulk Earth. Although chondrites formed at greater heliocentric distances and contain a different mix of presolar components than Earth, they nevertheless are suitable proxies for Earth’s bulk chemical composition.

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Figure 1: Nd isotope compositions of enstatite and ordinary chondrites.
Figure 2: Nd and Sm isotope variations among meteoritic and terrestrial samples.
Figure 3: Nd and Sm isotope variations among meteoritic and terrestrial samples.

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Acknowledgements

We thank the Field Museum for providing samples, S.-G. Lee for help setting up the chemistry in Chicago, R. Carlson for discussions. This work was funded through SNF PBE2PZ-145946 (CB); NASA (NNX14AK09G, OJ-30381-0036A, NNX15AJ25G), NSF (EAR144495, EAR150259) (ND); NASA NNH12AT84I (LB) and the ERC (Grant Agreement 616564 ‘ISOCORE’) (TK). The work performed by L.E.B., G.A.B., and Q.R.S. was done under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

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Authors and Affiliations

  1. Department of the Geophysical Sciences and Enrico Fermi Institute, Origins Laboratory, The University of Chicago, 5734 South Ellis Avenue, Chicago, 60637, Illinois, USA

    C. Burkhardt & N. Dauphas

  2. Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm Klemm-Strasse 10, Münster, 48149, Germany

    C. Burkhardt, G. A. Brennecka, Q. R. Shollenberger & T. Kleine

  3. Lawrence Livermore National Laboratory, L231, Livermore, 94550, California, USA

    L. E. Borg, G. A. Brennecka & Q. R. Shollenberger

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Contributions

C.B. initiated the project in collaboration with L.E.B., N.D. and T.K., acquired and processed the samples in Chicago and wrote a first draft of the manuscript. L.E.B., G.A.B. and Q.R.S. performed additional chemistry and measured all samples in Livermore. All authors contributed to the data interpretation and editing of the manuscript.

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Correspondence to C. Burkhardt.

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Additional information

Data are available at the EarthChem library (http://dx.doi.org/10.1594/IEDA/100597).

Extended data figures and tables

Extended Data Figure 1 Nucleosynthetic pathways and calculated anomaly patterns for Nd and Sm.

The top panel is a chart of the nuclides in the Ce–Nd–Sm–Gd mass region. Stable isotopes and their solar abundances are in black boxes on the chart, short-lived isotopes and their half-lives are in coloured boxes: blue indicates β- unstable, orange electron capture and yellow α-decay. Solid red arrows mark the main path of s-process nucleosynthesis, the dashed red arrows mark minor s-process branches and green arrows indicate the decay path of r-process nucleosynthesis. 148Sm and 150Sm are produced only by the s-process, 150Nd and 154Sm only by the r-process and 144Sm and 146Sm are p-process-only isotopes. The lower panels show expected μiNd (left) and μiSm (right) anomaly patterns for a p-process deficit (purple), an s-process deficit (red) and an r-process excess (green) for internal normalization to 146Nd/144Nd and 152Sm/147Sm, calculated using stellar model abundances27.

Extended Data Figure 2 Sm/Nd isochron diagrams of measured meteorite samples.

a, For 143Nd/144Nd, all but the disturbed Atlanta and Blithfield chondrites cluster in a narrow range around a 4.568 Ga chondrite isochron, consistent with literature data (grey). b, For 142Nd/144Nd, the meteorite data mostly fall below a 4.568 Ga isochron constructed through the accessible Earth value and only poorly correlate with Sm/Nd, indicating that, aside from Sm/Nd fractionation and 146Sm decay, other processes are responsible for setting the 142Nd/144Nd of meteorites. Error bars represent the external reproducibility (2 s.d. of the standards run in the same measurement campaign as the samples).

Extended Data Figure 3 Comparison of Nd and Sm isotope data.

The new data agree with literature data (in grey), but show less scatter, facilitating the calculation of more precise group averages. The error bars shown for our measurements represent external reproducibility (2 s.d. of the standards run in the same measurement campaign as the samples), whereas the uncertainties for the literature values are the 2 s.e. of the measurements. The solid lines denote mixing of the s-model prediction27 with the terrestrial composition. The dashed lines are the mixing line between CAIs and the CAI-free carbonaceous chondrite source reservoir as calculated by isotopic mass balance.

Extended Data Figure 4 Comparison of the slopes obtained from bulk meteorite anomaly data regressions and the slopes obtained from s-process modelling, SiC grain data and chondrite leachate data.

a, Slopes from the regression of enstatite chondrite, ordinary chondrite and NWA 5363 data. b, The same as a but including the processed standard data in the regression. c, Slopes from the regression of enstatite chondrite, ordinary chondrite and NWA 5363 values and calculated CAI-free Allende point (‘CV w/o CAI’). d, The same as c but including the processed standard data in the regression. Within uncertainties, the slopes from the bulk meteorite regressions are indistinguishable from the slopes from the literature data20,21,26,27, no matter which samples are used in the regressions. This implies that the Nd isotope variations in enstatite chondrites, ordinary chondrites, NWA 5363 and the CAI-free carbonaceous chondrite source are due to s-process heterogeneities. All regressions were performed using ISOPLOT. The slopes and μ142Nd intercepts of the regressions are also given in Extended Data Table 3. Error bars are the 95% CI.

Extended Data Figure 5 Effects of meteoroid exposure to galactic cosmic rays (GCRs) on the Sm and Nd isotope compositions.

a, Meteorites of this study show correlated μ149Sm and μ150Sm anomalies that are consistent with GCR exposure. Such reactions can also alter the Nd isotope signatures of planetary materials43. However, given the much smaller neutron capture cross-sections of the Nd isotopes relative to 149Sm, any effect of GCRs on μ142Nd is <1 p.p.m. b–e, Within a given meteorite group no obvious correlations are seen in μiNd versus μ149Sm, indicating the absence of significant GCR effects on the Nd isotope data. Error bars represent the external reproducibility (2 s.d. of the standards run in the same measurement campaign as the samples).

Extended Data Table 1 Measured and calculated 147Sm/144Nd and μ142Nd values
Full size table
Extended Data Table 2 Input parameters and the results of isotopic mass balance calculations for Nd and Sm
Full size table
Extended Data Table 3 μ142Nd values corrected for nucleosynthetic anomalies
Full size table
Extended Data Table 4 Collateral effects of the isotopic mass balance between Allende and CAIs for Ca, Ti, Cr, Ni, Sr, Zr, Mo and Ba
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Burkhardt, C., Borg, L., Brennecka, G. et al. A nucleosynthetic origin for the Earth’s anomalous 142Nd composition. Nature 537, 394–398 (2016). https://doi.org/10.1038/nature18956

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