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Early episodes of high-pressure core formation preserved in plume mantle

Abstract

The decay of short-lived iodine (I) and plutonium (Pu) results in xenon (Xe) isotopic anomalies in the mantle that record Earth’s earliest stages of formation1,2,3,4,5,6,7,8. Xe isotopic anomalies have been linked to degassing during accretion2,3,4, but degassing alone cannot account for the co-occurrence of Xe and tungsten (W) isotopic heterogeneity in plume-derived basalts9,10 and their long-term preservation in the mantle. Here we describe measurements of I partitioning between liquid Fe alloys and liquid silicates at high pressure and temperature and propose that Xe isotopic anomalies found in modern plume rocks (that is, rocks with elevated 3He/4He ratios) result from I/Pu fractionations during early, high-pressure episodes of core formation. Our measurements demonstrate that I becomes progressively more siderophile as pressure increases, so that portions of mantle that experienced high-pressure core formation will have large I/Pu depletions not related to volatility. These portions of mantle could be the source of Xe and W anomalies observed in modern plume-derived basalts2,3,4,9,10. Portions of mantle involved in early high-pressure core formation would also be rich in FeO11,12, and hence denser than ambient mantle. This would aid the long-term preservation of these mantle portions, and potentially points to their modern manifestation within seismically slow, deep mantle reservoirs13 with high 3He/4He ratios.

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Figure 1: Compositional maps of Fe, Mg and I from the sample recovered from DAC_I_EXP5, spot 3 at 4,500 K and 40 GPa.
Figure 2: I partitioning (γFe-corrected) during core formation with corresponding Xe isotopic evolution in different mantle domains.
Figure 3: P–timing conditions of core extraction scenarios that lead to co-evolution of observed Xe and W isotopic anomalies.
Figure 4: Cartoon illustrating our model for the co-generation and preservation of Xe and W anomalies.

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Acknowledgements

Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), the Advanced Photon Source, Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR-1128799) and the Department of Energy—GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract number DE-AC02-06CH11357. We acknowledge the support provided by C. Prescher, E. Greenberg, and V. Prakapenka during the experiments conducted at GSECARS and in data reduction. We acknowledge E. Bullock and S. Vitale for their assistance with the electron microscopy, electron microprobe analysis, and focused ion beam recovery. We thank T. Gooding for his assistance in preparing the piston cylinder experiments for analysis. We acknowledge discussions with R. Fischer, R. Parai, S. Parman, J. Tucker and M. Nakajima during the development of the manuscript. C.R.M.J., N.R.B. and Z.D. acknowledge fellowship support from the Carnegie Institution for Science, and C.R.M.J. additionally acknowledges fellowship support from the Smithsonian Institution. The research was also supported by NSF grants to C.R.M.J. (EAR-1725315), to E.C. (EAR-0738654) and to Y.F. (EAR-1447311).

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Contributions

C.R.M.J. conceived the project. C.R.M.J. conducted the experiments and analyses in collaboration with N.R.B. and Z.D. C.R.M.J., N.R.B. and Z.D. all contributed to the model framework. N.R.B. conducted the W isotope and major-element chemistry calculations. C.R.M.J., N.R.B., Z.D., E.C. and Y.F. contributed to data interpretation, presentation and manuscript preparation. E.C. facilitated the project and piston cylinder experiments. Y.F. facilitated the project and DAC experiments.

Corresponding author

Correspondence to Colin R. M. Jackson.

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Reviewer Information Nature thanks G. Avice and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Parameterization of I partitioning between liquid Fe alloy and silicate liquid.

a, Partitioning of I plotted against the O content (atomic) of metal phases. Oxygen content of the metal is the first parameter identified in the stepwise fitting approach. Higher O contents of metal are associated with greater partitioning of I into metal over silicate. b, Partitioning of I corrected to O-free metal plotted against S content of metal. The S content of the metal is the second parameter identified in the stepwise fitting approach. c, Partitioning of I corrected to O- and S-free metal plotted against P/T. The P/T term is the third parameter identified in the stepwise fitting approach. d, A comparison of observed and predicted I partitioning. I partitioning is predicted using equation (1) (R2 = 0.86). eh, Covariance plots of parameterization. ‘Intercept’ refers to the constant term in equation (1).

Extended Data Figure 2 Piston cylinder data comparison with DAC regression.

a, A comparison of I partition coefficients determined in piston cylinder (PC) experiments and the I partition coefficient predicted from equation (1). Piston cylinder data are corrected to remove the effect of S and to the O contents of metal predicted along the mantle liquidus of ref. 22, allowing a direct comparison to the DAC data model. Upper-limit partition coefficients are consistent with the lower end of the uncertainty envelope. The difference in measured I partitioning coefficients in the piston cylinder series suggests that I partitioning is redox sensitive. b, Measured partition coefficients in piston cylinder series are corrected to ΔIW-1.3 and offset in pressure for clarity. Error bars and the uncertainty envelope are 2σ.

Extended Data Figure 3 Calculated concentration of MSEs in plume mantle as a function of the pressure of core–mantle equilibrium.

W (a), Ni (b), and Co (c) concentrations in plume mantle all increase with increasing core–mantle equilibrium pressure. Results are normalized to the calculated concentration of MORB MSEs. MORB MSE concentrations are calculated using the average PTX conditions that satisfy BSE [W] and [FeO] (Supplementary Table 4). Temperatures are assumed to follow the mantle liquidus of ref. 22. FeO abundances are calculated following ref. 11. W partitioning is from ref. 18. Ni and Co partitioning are from ref. 37. Plume mantle data points are plotted only for PTX conditions that satisfy W and Xe isotopic constraints (see Methods subsection ‘Calculation of W and Xe isotopic anomalies resulting from discrete stages of core formation’).

Extended Data Figure 4 Imaging of silicate following analysis with low electron microprobe total.

a, Secondary electron (SE) image of the DAC_I_EXP5 spot 4 silicate shows evidence of vesiculation in the area that was analysed. b, C map showing no obvious concentration of C local to vesiculation. The dark material in the lower left corner of a is diamond. Scale bar is 1 μm.

Extended Data Figure 5 Demonstrations of DAC experiment equilibrium.

a, Si–Fe exchange coefficient plotted against inverse temperature. Data from this study (circles) are corrected for S–O–C interactions with silicon. Squares are from the compilation of ref. 37 corrected from O–C interactions with silicon. b, (Mg+Fe)/Si ratios of the silicate phase from this study (circles) and ref. 44 (squares) plotted against pressure. Symbols are grouped for Mg#, with darker symbols corresponding to lower Mg#. The offset of lower Mg# silicates within a pressure range to higher (Mg+Fe)/Si ratios and the similar pressure slopes within Mg# groupings support the accuracy of our pressure calculations.

Extended Data Figure 6 Secondary electron image and EDS maps of experiment DAC_I_EXP9 spot 4 showing I mobility.

The sequence of images shows the nature and distribution of I-rich materials that are mobilized to the surface of heating spots after storage in desiccators. a, Map of I distribution. I-rich materials are concentrated along the left and right edges of the heating spot, within isolated regions of the metal and silicate phases, and surrounding the Fe alloy phase. b, Map of Fe distribution. The metallic phase is mantled by Fe-rich and I-rich material. This material is also rich in C and O (not shown). The I-rich material that is present along the edges of the heating spot is also enriched in Fe, C and O. c, Secondary electron image. I-rich materials are positive relief features. Images are 20 μm wide.

Extended Data Figure 7 Parent–daughter ratio variations required to account for Xe isotopic variability between plume and MORB mantle.

a, Closure ages plotted against initial 129I/244Pu ratios. Closure ages are calculated using 129Xe*/136XePu ratios from ref. 8 and equation (12). The MORB mantle 129I/244Pu ratio is fixed and derived from estimates for the bulk silicate Earth from refs 49 and 50. The grey shading delineates I–Pu–Xe closure ages that are equal to or less than that determined for MORB mantle. Vertical lines in the grey shaded area are fractional depletions of the I/Pu in plume mantle relative to MORB mantle, ordered sequentially. The ratios denote the fractional depletion of the I/Pu ratio for plume mantle relative to initial MORB mantle. An approximately 3× lower I/Pu ratio for plume mantle results in equal closure ages for MORB and plume mantle. b, Closure ages plotted against the ratio of radiogenic components for 129Xe/130Xe within MORB and plume mantle. The upper edge of the grey shaded area delineates the upper limit of MORB mantle I–Pu–Xe closure. Left and right edges delineate the uncertainty on the ratio of radiogenic components observed in MORB and plume mantle. Curves are calculations for the ratio of radiogenic components of 129Xe/130Xe (denoted ‘–R’, see equation (13)), assuming different fractional depletions of the I/Xe ratio in plume mantle relative to initial MORB mantle (see fractions plotted along edge of graph). These calculations assume a MORB mantle closure age of 61 million years (Ma).

Supplementary information

Supplementary Table 1

This table contains metadata associated with diamond anvil cell experiments. (XLSX 28 kb)

Supplementary Table 2

This table contains metadata associated with piston cylinder experiments. (XLSX 19 kb)

Supplementary Table 3

This table contains the fitting parameters for equation 1. (XLSX 14 kb)

Supplementary Table 4

This table contains modelling parameters. (XLSX 19 kb)

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Jackson, C., Bennett, N., Du, Z. et al. Early episodes of high-pressure core formation preserved in plume mantle. Nature 553, 491–495 (2018). https://doi.org/10.1038/nature25446

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