Abstract

IN 1964 I showed1 that approximately 60 per cent of terrestrial 187Os should be regarded as having resulted from the decay of 187Re and that most of that decay occurred before the formation of the solar system. Measurements of the neutron capture cross-sections for the isotopes of several elements have now confirmed2 the correctness of the s-process theory on which that conclusion depended, and although the specific measurements of the cross-sections for the osmium isotopes have not yet been made there is little doubt that 187Os must be primarily radiogenic. This compelling conclusion is, however, awkward for current cosmological thought because the age of the 187Re seems to exceed the age of the universe. Recent chronological models (refs. 3, 4 and unpublished work of Hohenberg) for the natural radioactivities have therefore tended to ignore 187Re in favour of the short lived species 235U, 129I and 244Pu, and although the resulting models suggest significant enrichment shortly before the solar system formed, they do little to clarify when nucleosynthesis actually began. Here I wish to outline a potentially important factor for the 187Re age along with an isotopic measurement capable of clarifying the situation.

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