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Rhenium-osmium dating

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

where:

t is the age of the sample,
λ is the decay constant of 187Re,
(eλt-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

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