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Capture of nebular gases during Earth’s accretion is preserved in deep-mantle neon

Nature (2018) | Download Citation


Evidence for the capture of nebular gases by planetary interiors would place important constraints on models of planet formation. These constraints include accretion timescales, thermal evolution, volatile compositions and planetary redox states1,2,3,4,5,6,7. Retention of nebular gases by planetary interiors also constrains the dynamics of outgassing and volatile loss associated with the assembly and ensuing evolution of terrestrial planets. But evidence for such gases in Earth’s interior remains controversial8,9,10,11,12,13,14. The ratio of the two primordial neon isotopes, 20Ne/22Ne, is significantly different for the three potential sources of Earth’s volatiles: nebular gas15, solar-wind-irradiated material16 and CI chondrites17. Therefore, the 20Ne/22Ne ratio is a powerful tool for assessing the source of volatiles in Earth’s interior. Here we present neon isotope measurements from deep mantle plumes that reveal 20Ne/22Ne ratios of up to 13.03 ± 0.04 (2 standard deviations). These ratios are demonstrably higher than those for solar-wind-irradiated material and CI chondrites, requiring the presence of nebular neon in the deep mantle. Furthermore, we determine a 20Ne/22Ne ratio for the primordial plume mantle of 13.23 ± 0.22 (2 standard deviations), which is indistinguishable from the nebular ratio, providing robust evidence for a reservoir of nebular gas preserved in the deep mantle today. The acquisition of nebular gases requires planetary embryos to grow to sufficiently large mass before the dissipation of the protoplanetary disk. Our observations also indicate distinct 20Ne/22Ne ratios between deep mantle plumes and mid-ocean-ridge basalts, which is best explained by addition of a chondritic component to the shallower mantle during the main phase of Earth’s accretion and by subsequent recycling of seawater-derived neon in plate tectonic processes.

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The main data supporting the findings of this study are available within the article, its Extended Data and Supplementary Table 1, as well as in the EarthChem database (https://doi.org/10.1594/IEDA/111217).

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This work is supported by an NSF EAR Postdoctoral Fellowship and NSF grant EAR-1250419. Discovery samples were obtained during the cruise EW9309 of RV Maurice Ewing (in November–December 1993) and provided by the Marine Geological Samples Laboratory, of the Graduate School of Oceanography, University of Rhode Island.

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Nature thanks D. Graham and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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  1. Department of Earth and Planetary Sciences, University of California-Davis, Davis, CA, USA

    • Curtis D. Williams
    •  & Sujoy Mukhopadhyay


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Both authors contributed to the design of the study, analyses and data processing. C.D.W. wrote the manuscript with input from S.M.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Curtis D. Williams.

Extended data figures and tables

  1. Extended Data Fig. 1 Lack of mass-dependent isotope fractionation in the step-crushing neon data.

    Péron et al.8 suggested that using the highest measured 20Ne/22Ne ratios for characterizing the plume mantle is inappropriate because mass-dependent isotope fractionation may occur during bubble formation8. In this scenario, mass-dependent fractionation during bubble formation would lead to 20Ne/22Ne ratios scattering about the ‘true’ mean value, with some bubbles characterized by relatively high 20Ne/22Ne ratios while other bubbles displayed relatively low 20Ne/22Ne ratios. This hypothesis can be tested by measuring 4He/3He and 38Ar/36Ar ratios during the same step-crushes as the neon isotopes as illustrated here. a, b, The measured neon isotopic compositions of individual step-crushes plotted against those of helium (a) and argon (b) along with predicted trajectories of mass-dependent isotope fractionation during bubble formation obtained by applying a Rayleigh fractionation model78. Here, the parental melt is assumed to have an initial 20Ne/22Ne ratio of 12.65 ± 0.08, similar to the value of ref. 8. Initial helium and argon isotopic compositions are from the mean values determined for sample EW9309_5D in this study (Supplementary Table 1). The curves show the trajectory of the melts during degassing, the evolution of an instantaneously lost vapour phase (bubbles; short-dashed line) and the cumulative evolution of the vapour phase (bubbles; solid line). Plotted along with these curves are the individual step-crushes (circles) from this study (sample EW9309_5D) with their associated 2σ uncertainties (error bars). Note that the 4He/3He ratios were measured only on one aliquot of EW9309_5D. The helium–neon–argon isotopic compositions measured in the individual step-crushes do not follow the predicted Rayleigh fractionation trends. Rather, the data cloud is at a high angle to the predicted isotope fractionation trend. For example, for our highest measured 20Ne/22Ne of 13.03 ± 0.04 (2σ) to be a result of mass fractionation, the measured 38Ar/36Ar should be 0.1847 (b). However, the measured 38Ar/36Ar of 0.1885 ± 0.0014 (2σ) is identical to the atmospheric value and similar to other determinations of 38Ar/36Ar ratios in plumes and MORBs. Given this, we conclude that mass-dependent isotope fractionation during bubble formation is not responsible for generating the highest 20Ne/22Ne ratios determined in these studies.

  2. Extended Data Fig. 2 Frequency functions for non-plume-influenced MORBs MORBs and plume-influenced basalts.

    Histograms and kernel density estimates were constructed from the maximum measured neon isotopic composition (20Ne/22Nemax) of globally distributed, mantle-derived samples from non-plume-influenced MORBs (solid curve) and plume-influenced basalts (dashed curve), similar to Fig. 2. Kernel density estimates were calculated using the Matlab Curve Fitting Toolbox with bandwidths of 0.16 and 0.12 for non-plume-influenced MORBs and plume-influenced materials, respectively. The kernel density estimator results in slightly broader distributions than observed in Fig. 2 but does not change the main conclusions that there is a clear difference between the maximum measured 20Ne/22Ne ratios for non-plume-influenced MORBs and for plume-influenced basalts, with MORBs displaying a sharp cut-off at a 20Ne/22Ne ratio of 12.5.

  3. Extended Data Fig. 3 Collection depths of non-plume-influenced MORB samples and plume-influenced basalts.

    Depths are in metres. MORB samples (squares) highlighted in this study all erupted from depths between 2,000 and 5,000 m below sea level, and plume-influenced basalts (circles) on average erupted at comparable or shallower depths. For comparable eruption depths, plume-influenced basalts show higher 20Ne/22Ne ratios than non-plume-influenced MORBs. Moreover, if atmospheric contamination played a role in generating the difference between these two populations, plume-influenced basalts should have lower 20Ne/22Ne ratios, given their shallower eruption depth in the sample suite. However, such a relationship is not observed. Therefore, we conclude that different eruption depths are not responsible for the two distinct modes observed for non-plume-influenced MORBs and for plume-influenced basalts shown in Fig. 2. We note that the depth of eruption for Iceland sample DICE 10 is unknown, as it was erupted subglacially. Here, we have assigned a value of zero metres below sea-level to the DICE 10 samples, but deeper eruption depths will not change the results of this study. Data sources are reported in Extended Data Table 1.

  4. Extended Data Fig. 4 Long-term external reproducibility of bracketing standards.

    a, Mass discrimination of the 20Ne/22Ne ratio as a function of the 20Ne beam size. The 20Ne/22Ne ratios for the different size standards were normalized to the 20Ne/22Ne ratio of the largest standard (10−14 moles of 20Ne). The error bars (2σ uncertainties) reflect the relative errors in the 20Ne/22Ne isotope ratio based on the reproducibility of the standards. bd, Reproducibility of standard 20Ne/22Ne ratios that were interspersed with the sample measurements over the 3-month period that it took to conduct all step-crushes. Error bars on the individual air standards represent the internal measurement error (2SE), while the dashed lines represent the long-term (2σ) external reproducibility. The external reproducibilities on the 20Ne/22Ne ratios were 0.03, 0.03 and 0.01 (2σ) for 20Ne beam sizes of 1.6 × 10−15 moles (n = 26), 3.7 × 10−15 moles (n = 17) and 1 × 10−14 moles (n = 114), respectively.

  5. Extended Data Table 1 Maximum measured 20Ne/22Ne and 21Ne/22Ne ratios for non-plume-influenced MORBs and plume-influenced materials
  6. Extended Data Table 2 Noble gas end-member compositions

Supplementary information

  1. Supplementary Table 1

    Noble gas abundances and isotope ratios for the individual step-crushes of the two plume-influenced MORBs. Two to five grams of basaltic glass was chipped from pillow lavas, loaded into a stainless-steel piston crusher, step-crushed under ultra-high vacuum, and then let into the Nu Noblesse mass spectrometer in the UC Davis Noble Gas Laboratory.

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