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, but studies using fully ionised ¹⁸⁷Re atoms have found that this can decrease to only 33 y. 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.