Early differentiation and volatile accretion recorded in deep-mantle neon and xenon

Journal name:
Nature
Volume:
486,
Pages:
101–104
Date published:
DOI:
doi:10.1038/nature11141
Received
Accepted
Published online

The isotopes 129Xe, produced from the radioactive decay of extinct 129I, and 136Xe, produced from extinct 244Pu and extant 238U, have provided important constraints on early mantle outgassing and volatile loss from Earth1, 2. The low ratios of radiogenic to non-radiogenic xenon (129Xe/130Xe) in ocean island basalts (OIBs) compared with mid-ocean-ridge basalts (MORBs) have been used as evidence for the existence of a relatively undegassed primitive deep-mantle reservoir1. However, the low 129Xe/130Xe ratios in OIBs have also been attributed to mixing between subducted atmospheric Xe and MORB Xe, which obviates the need for a less degassed deep-mantle reservoir3, 4. Here I present new noble gas (He, Ne, Ar, Xe) measurements from an Icelandic OIB that reveal differences in elemental abundances and 20Ne/22Ne ratios between the Iceland mantle plume and the MORB source. These observations show that the lower 129Xe/130Xe ratios in OIBs are due to a lower I/Xe ratio in the OIB mantle source and cannot be explained solely by mixing atmospheric Xe with MORB-type Xe. Because 129I became extinct about 100 million years after the formation of the Solar System, OIB and MORB mantle sources must have differentiated by 4.45 billion years ago and subsequent mixing must have been limited. The Iceland plume source also has a higher proportion of Pu- to U-derived fission Xe, requiring the plume source to be less degassed than MORBs, a conclusion that is independent of noble gas concentrations and the partitioning behaviour of the noble gases with respect to their radiogenic parents. Overall, these results show that Earth’s mantle accreted volatiles from at least two separate sources and that neither the Moon-forming impact nor 4.45 billion years of mantle convection has erased the signature of Earth’s heterogeneous accretion and early differentiation.

At a glance

Figures

  1. Differences in neon and argon isotopic composition between MORB and the Iceland plume.
    Figure 1: Differences in neon and argon isotopic composition between MORB and the Iceland plume.

    a, Neon three-isotope plot showing the new analyses of the DICE 10 sample (filled circles) from Iceland in comparison to previously published data for this sample (open circles; ref. 18) and the gas-rich ‘popping rock’ ( ) from the north Mid-Atlantic Ridge (open triangles; ref. 17). Error bars are 1σ, and for clarity, two previous analyses18 with large error bars have not been shown. Step-crushing of a mantle-derived basalt produces a linear trend that reflects variable amounts of post-eruptive air contamination in vesicles containing mantle Ne. The slope of the line is a function of the ratio of nucleogenic 21Ne to primordial 22Ne, with steeper slopes indicating a higher proportion of primordial 22Ne and, thus, a less degassed mantle source. The slope of the Iceland line based on the new analyses is consistent with that obtained previously18. Importantly, 20Ne/22Ne ratios of 12.88±0.06 are distinctly higher than the MORB source 20Ne/22Ne of ≤12.5 as constrained from continental well gases20. b, Ne–Ar compositions of individual step crushes of the DICE 10 sample. 40Ar is generated by radioactive decay of 40K, and low 40Ar/36Ar ratios are indicative of a less degassed mantle. The data reflect mixing between a mantle component and post-eruptive atmospheric contamination. A least-squares hyperbolic fit through the data yields a 40Ar/36Ar ratio of 10,745±3,080, corresponding to a mantle solar 20Ne/22Ne ratio of 13.8. This Ar isotopic ratio is used as the mantle source value for Iceland in Figs 2 and 3. Symbols as in a; error bars are 1σ.

  2. Differences in elemental abundances and isotope ratios between MORB and the Iceland plume.
    Figure 2: Differences in elemental abundances and isotope ratios between MORB and the Iceland plume.

    Error bars are 1σ. a, 3He/22Ne versus 20Ne/22Ne; b, 3He/36Ar versus 40Ar/36Ar; and c, 22Ne/36Ar versus 40Ar/36Ar. The mantle source composition for (filled grey square in all panels) is based on the 40Ar/36Ar and 20Ne/22Ne ratios as defined in ref. 30, and the mantle source composition for Iceland (filled black square in all panels) is based on Fig. 1. The grey and black arrows at the top of the figure indicate how elemental ratios evolve as a result of kinetic fractionation and solubility controlled degassing, respectively. Good linear relationships are observed between isotope ratios and elemental ratios, which reflect mixing between mantle-derived noble gases and post-eruptive atmospheric contamination. Lines are robust linear regressions through the data with the atmospheric contaminant near the origin and the mantle source at the other end. Arrow in c indicates the minimum 22Ne/36Ar ratio of the Iceland mantle source given the measured 40Ar/36Ar ratio of 7,047 (Supplementary Table 6). Because of systematic differences in noble gas solubilities and diffusivities, the differences in elemental abundances are not likely to be generated through ancient fractionation associated with diffusion or magmatic outgassing. For example, kinetic fractionation should lead to higher 3He/22Ne and higher 3He/36Ar–22Ne/36Ar ratios. However, the Iceland source has a lower 3He/22Ne and higher 3He/36Ar–22Ne/36Ar. Likewise, adding recycled atmospheric gases to the MORB source cannot produce the plume noble gas compositions. Finally, c shows that preferential recirculation of atmospheric Ar into the plume source does not explain the higher 22Ne/36Ar of the plume source and because of the difference in MORB and OIB 22Ne/36Ar ratios, adding radiogenic 40Ar to the plume composition is not likely to generate the 40Ar/36Ar ratio in MORBs.

  3. Differences in Xe isotopic composition between MORB and the Iceland plume.
    Figure 3: Differences in Xe isotopic composition between MORB and the Iceland plume.

    a, Correlation between 129Xe and 3He in the ‘popping rock’ MORB ( )17 and Iceland (DICE 10). Error bars are 1σ. Data points are individual step crushes that reflect different degrees of post-eruptive atmospheric contamination in the vesicles. Air lies near the origin and the mantle compositions at the other end of the linear arrays. The straight lines are robust regressions through the data. Because mixing in this space is linear, the lines also represent the trajectories along which the mantle sources will evolve when mixed with subducted air. The new observations from Iceland demonstrate that the Iceland plume 129Xe/130Xe ratio cannot be generated solely through adding recycled atmospheric Xe to the MORB source, and vice versa. Thus, two mantle reservoirs with distinct I/Xe ratios are required. The mantle 129Xe/130Xe ratio of 6.98±0.07 for Iceland was derived from a hyperbolic least-squares fit through the Ar-Xe data (b) corresponding to a mantle 40Ar/36Ar ratio of 10,745. Note that given the curvature in Ar–Xe space, the 129Xe/130Xe in the Iceland mantle source is not particularly sensitive to the exact choice of the mantle 40Ar/36Ar ratio.

  4. Difference in the measured 129Xe/136Xe ratio between MORB and the Iceland plume.
    Figure 4: Difference in the measured 129Xe/136Xe ratio between MORB and the Iceland plume.

    Unlike the traditional 129Xe/130Xe–136Xe/130Xe plot, the x and y errors are de-correlated. The arrows illustrate how the MORB and OIB source compositions evolve as subducted air is added. Error bars are 1σ. The figure demonstrates a small Xe isotopic difference between the Iceland plume and MORBs that cannot be related solely through recycling atmospheric Xe or by adding fissiogenic 136Xe to MORB Xe. The data points represent the weighted means of the different step crushes for MORBs (n = 38) and Iceland (n = 51; this study). The MORB 129Xe/136Xe ratio was calculated from the weighted means of the 129Xe/130Xe and 136Xe/130Xe ratios25.

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Affiliations

  1. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA

    • Sujoy Mukhopadhyay

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  1. Supplementary Information (793K)

    This file contains Supplementary Figures 1-4, Supplementary Tables 1-7 and Supplementary References.

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