Abstract
BROWN et al.1 have treated the problem of the history of the argon degassing of the Earth, and claim to demonstrate that the 40Ar/36Ar ratio in the mantle has increased from ∼133, 770 Myr ago, to a value that “is at present not greater than the atmospheric ratio (295.5) and is probably a little lower”. Their Table 1 and Fig. 1 show the results of K–Ar isochron analyses of several suites of “mantle-derived igneous” samples. We have been unable to reproduce in detail their results from the references they cite, and conclude that the apparent trend in their Fig. 1 is an artefact of data selection and that the source of the observed argon has been misidentified. We think the 40Ar/36Ar ratios in the solid earth and the atmosphere have probably increased with time. Theoretical considerations and the available literature both indicate, however, that the 40Ar/36Ar ratio in the solid earth is much higher than the 40Ar/36Ar ratio in the atmosphere.
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ALEXANDER, E., SCHWARTZMAN, D. Argon isotopic evolution of upper mantle. Nature 259, 104–106 (1976). https://doi.org/10.1038/259104a0
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DOI: https://doi.org/10.1038/259104a0
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