Stefan T.M. Peters^a,b, Carsten Münker^a,b,1, Harry Becker^c,2, Toni Schulz^d,3

^aInstitut für Geologie und Mineralogie, Universität zu Köln, Zülpicherstr. 49b, 50674 Cologne, Germany
^bSteinmann-Institut, Poppelsdorfer Schloss, 53115 Bonn, Germany
^cInstitut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany
^dDepartment of Lithospheric Research, Universität Wien, Althanstrasse 14, A-1090, Vienna, Austria

The decay of the rare nuclide ¹⁸⁴Os by alpha emission to ¹⁸⁰W has been theoretically predicted, but was previously never observed in experiments. Variable excesses of ¹⁸⁰W were recently observed for iron meteorites, but the contribution to these excesses by ¹⁸⁴Os-decay was regarded as insignificant. Here, we present combined ¹⁸⁰W and Os–W concentration data for meteorites and terrestrial rocks, now indicating that the ¹⁸⁰W heterogeneities can be explained by α-decay of ¹⁸⁴Os. A combined ¹⁸⁴Os–¹⁸⁰W isochron for iron meteorites and chondrites yields a decay constant value of λ¹⁸⁴Os(α) of 6.49±1.34×10⁻¹⁴ a⁻¹ (half life 1.12±0.23×10¹³ yr), in good agreement with theoretical estimates. The ¹⁸⁴Os–¹⁸⁰W decay system may constitute a viable tracer and chronometer for important geological processes like core formation, silicate differentiation or late accretion processes. This is illustrated by a measured ¹⁸⁰W-deficit in terrestrial basalts relative to chondrites by 1.16±0.69 parts in 10 000, consistent with core formation ~4.5 Ga ago.

Reference
Peters STM, Münker C, Becker H and Schulz T (2014) Alpha-decay of 184Os revealed by radiogenic 180W in meteorites: Half life determination and viability as geochronometer. Earth and Planetary Science Letters 391:69–76.
[doi:10.1016/j.epsl.2014.01.030]
Copyright Elsevier

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