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40Ar retention in the terrestrial planets


The solid Earth is widely believed to have lost its original gases through a combination of early catastrophic release and regulated output over geologic time. In principle, the abundance of 40Ar in the atmosphere represents the time-integrated loss of gases from the interior, thought to occur through partial melting in the mantle followed by melt ascent to the surface and gas exsolution. Here we present data that reveal two major difficulties with this simple magmatic degassing scenario—argon seems to be compatible in the major phases of the terrestrial planets, and argon diffusion in these phases is slow at upper-mantle conditions. These results challenge the common belief that the upper mantle is nearly degassed of 40Ar, and they call into question the suitability of 40Ar as a monitor of planetary degassing. An alternative to magmatism is needed to release argon to the atmosphere, with one possibility being hydration of oceanic lithosphere consisting of relatively argon-rich olivine and orthopyroxene.

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Figure 1: Ar diffusive-uptake profiles in olivine and enstatite.
Figure 2: Diffusivities of Ar in olivine at T  ≈ 750 °C from experiments of differing duration.
Figure 3: Summary of diffusion data for olivine and enstatite.
Figure 4: Summary of Ar solubility measurements for olivine and enstatite compared with basaltic melt.
Figure 5: Diffusive loss of Ar from spherical pyroxene grains.
Figure 6: Transfer of Ar from olivine to melt for various equilibration scenarios.


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We are grateful to F. M. Richter and K. K. Turekian for discussions about noble gas systematics and bulk-Earth model constraints, and to M. J. Drake for comments. This research was supported by the NSF.

Author Contributions E.B.W. ran exploratory experiments, developed the melting models and wrote the manuscript; J.B.T. performed most of the experiments and interpreted the data; D.J.C. conducted the Rutherford backscattering spectroscopy analyses and reduced the raw spectra.

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Correspondence to E. Bruce Watson.

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Watson, E., Thomas, J. & Cherniak, D. 40Ar retention in the terrestrial planets. Nature 449, 299–304 (2007).

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