Rhenium-osmium dating

Rhenium-Osmium dating is a form of radiometric dating based on the beta decay of the isotope ¹⁸⁷Re to ¹⁸⁷Os. This normally occurs with a half-life of 41.6 × 10⁹ y,^[1] but studies using fully ionised ¹⁸⁷Re atoms have found that this can decrease to only 33 y.^[2] Both rhenium and osmium are strongly siderophilic (iron loving), while Re is also chalcophilic (sulfur loving) making it useful in dating sulfide ores such as gold and Cu-Ni deposits.

This dating method is based on an isochron calculated based on isotopic ratios measured using N-TIMS (Negative – Thermal Ionization Mass Spectrometry).

Rhenium-Osmium isochron

Rhenium-Osmium dating is carried out by the isochron dating method. Isochrons are created by analysing several samples believed to have formed at the same time from a common source. The Re-Os isochron plots the ratio of radiogenic ¹⁸⁷Os to non-radiogenic ¹⁸⁸Os against the ratio of the parent isotope ¹⁸⁷Re to the non-radiogenic isotope ¹⁸⁸Os. The stable and relatively abundant osmium isotope ¹⁸⁸Os is used to normalize the radiogenic isotope in the isochron.

The Re-Os isochron is defined by the following equation:

$\left(\frac{{}^{187}\mathrm{Os}}{{}^{188}\mathrm{Os}}\right)_{\mathrm{present}} = \left(\frac{{}^{187}\mathrm{Os}}{{}^{188}\mathrm{Os}}\right)_{\mathrm{initial}} + \left(\frac{{}^{187}\mathrm{Re}}{{}^{188}\mathrm{Os}}\right) \cdot (e^{\lambda t}-1),$

where:

t is the age of the sample,

λ is the decay constant of ¹⁸⁷Re,

(e^λt-1) is the slope of the isochron which defines the age of the system.

A good example of an application of the Re-Os isochron method is a study on the dating of a gold deposit in the Witwatersrand mining camp, South Africa.^[3]

References

1. Smoliar, M.I.; Walker, R.J.; Morgan, J.W. (1996). "Re-Os ages of group IIA, IIIA, IVA, and IVB iron meteorites". Science 271 (5252): 1099–1102. Bibcode:1996Sci...271.1099S. doi:10.1126/science.271.5252.1099.
2. Bosch, F.; Faestermann, T.; Friese, J.; Heine, F.; Kienle, P.; Wefers, E.; Zeitelhack, K.; Beckert, K.; et al. (1996). "Observation of bound-state β– decay of fully ionized ¹⁸⁷Re:¹⁸⁷Re-¹⁸⁷Os Cosmochronometry". Physical Review Letters 77 (26): 5190–5193. Bibcode:1996PhRvL..77.5190B. doi:10.1103/PhysRevLett.77.5190. PMID 10062738.
3. Kirk, J.; Ruiz, J.; Chesley, J.; Walshe, J.; England, G. (2002). "A major Archean, gold- and crust-forming event in the Kaapvaal Craton, South Africa". Science 297 (5588): 1856–1858. Bibcode:2002Sci...297.1856K. doi:10.1126/science.1075270. PMID 12228713.