Abstract
The extent to which primordial mantle domains have survived billions of years of convective mixing is a fundamental question in mantle dynamics and geochemistry. The observation that around half of Earth’s 40Ar is missing from the atmosphere has been used to argue for a largely primordial, convectively isolated lower mantle. This hypothesis is apparently supported by lower 40Ar/36Ar ratios in the mantle source of ocean-island basalts compared with mid-ocean-ridge basalts. However, strongly layered convection is contradicted by seismic tomographic observations and geodynamic constraints. Using joint geodynamic–geochemical modelling of mantle convection, we show that high 40Ar concentrations associated with K-rich subducted oceanic crust plus unmelted material dispersed throughout the mantle can fully account for Earth’s 40Ar budget. This solution to the missing Ar paradox requires neither a substantial reduction in Earth’s assumed K concentration nor large isolated domains in the mantle. We additionally show that subducted atmosphere-derived Ar has little effect on the mantle 40Ar budget but can substantially reduce mantle 40Ar/36Ar ratios. Unlike He and Ne isotope systems, whose variations reflect primarily incorporation of primordial material, mantle 40Ar/36Ar ratios may instead result from subduction of atmosphere-derived Ar into the deep mantle.
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Data availability
No new data were collected as part of this study. All data used in this study are available from the cited references.
Code availability
The geodynamic and geochemical model codes are available from the authors upon request. The code used for MCMC sampling is available from https://github.com/dfm/emcee.
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Acknowledgements
J.M.T. and P.E.v.K. acknowledge funding from NSF CSEDI grant 1664642. J.M.T. acknowledges support from a Peter Buck Postdoctoral Fellowship. C.J.B. acknowledges funding from the NERC Deep Mantle Volatiles consortium, NE/M000427/1.
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P.E.v.K. developed the geodynamic models; J.M.T. and P.E.v.K. developed the geochemical models with input from C.J.B.; all authors analysed the results; J.M.T. wrote the manuscript with input from P.E.v.K. and C.J.B.
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Extended data
Extended Data Fig. 1 Fraction of terrestrial 40Ar in the atmosphere for different BSE K concentrations (red line).
The other colored lines assume that all 40Ar produced before the specified time is lost to space, for example by impacts. Due to the long half-life of 40K, the amount of 40Ar produced and lost even after 200 Ma hardly affects the terrestrial 40Ar budget and the missing Ar paradox. Furthermore, these calculations assume efficient degassing such that all 40Ar produced within the specified time is subject to atmospheric loss. Any 40Ar retained within the Earth would reduce the effect. A95 = Allègre et al.78; HZ86 = Hart and Zindler79; J79 = Jagoutz et al.80 LK07 = Lyubetskaya and Korenaga81; MS95 = McDonough and Sun82; PO03 = Palme and O’Neill83.
Extended Data Fig. 2 K/U ratios in the upper mantle and continental crust.
Bulk continental crust is \({1160{0}_{-2500}}^{+3500}\) ref. 84); sediments are 10600 ± 900 (2.21 ± 0.14$% K2O, 1.73 ± 0.09 ppm U; ref. 85); arc lavas are \({1100{0}_{-2900}}^{+9700}\) based on the median and 1 − σ range of arc segment averages86; MORB is 12300 ± 400 (ref. 32). The similarity between K/U ratios of the upper mantle and continental crust suggest that K/U fractionation during subduction does not substantially alter the global K and U budgets, and that MORBs provide a reasonable estimate for the BSE.
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Supplementary Figs. 1–16 and Tables 1–7.
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Tucker, J.M., van Keken, P.E. & Ballentine, C.J. Earth’s missing argon paradox resolved by recycling of oceanic crust. Nat. Geosci. 15, 85–90 (2022). https://doi.org/10.1038/s41561-021-00870-6
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DOI: https://doi.org/10.1038/s41561-021-00870-6
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