Rhenium-osmium dating

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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 [1][2] 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).

[edit] 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,
(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]

[edit] References

  1. ^ 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
  2. ^ Bosch (1996). "Observation of bound-state β– decay of fully ionized 187Re:187Re-187Os Cosmochronometry". Physical Review Letters 77 (26): 5190–5193. doi:10.1103/PhysRevLett.77.5190. 
  3. ^ 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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