Nature 259, 104 - 106 (15 January 1976); doi:10.1038/259104a0

Argon isotopic evolution of upper mantle

E. CALVIN ALEXANDER JR^* & DAVID W. SCHWARTZMAN^†

^*Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455
^†Department of Geology and Geography, Howard University, Washington DC 20001

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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