Rhenium-osmium dating

Rhenium-osmium dating

Rhenium-Osmium dating is a form of radiometric dating based on the beta decay of the isotope 187Re which usually has a half life of 4.16×1010 y [ Smoliar, M.I., Walker, R.J. and Morgan, J.W. (1996). Re-Os ages of group IIA, IIIA, IVA, and IVB iron meteorites. Science 271, pp. 1099 - 1102] cite journal
doi = 10.1103/PhysRevLett.77.5190
title = Observation of bound-state β– decay of fully ionized 187Re:187Re-187Os Cosmochronometry
author = Bosch
year = 1996
journal = Physical Review Letters
volume = 77
issue = 26
pages = 5190–5193] to 187Os. These two elements are strongly siderophilic (iron loving) and chalcophilic (sulfur loving) making them 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 ight)_{mathrm{present = left(frac}^{187}mathrm{Os}^{188}mathrm{Os ight)_{mathrm{initial + left(frac}^{187}mathrm{Re}^{188}mathrm{Os ight) cdot (e^{lambda t}-1),

where: : "t" is the age of the sample, : λ is the decay constant of 187Re, : ("e"λ"t"-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. [Kirk, J., Ruiz, J., Chesley, J., Walshe, J. and England, G. (2002). A major Archean, gold- and crust-forming event in the Kaapvaal Craton, South Africa. Science 297 pp. 1856-1858]

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