Computer Science
Scientific paper
Aug 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.4119y&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 57, Issue 16, pp.4119-4128
Computer Science
Scientific paper
A Re-containing sample is irradiated with thermal neutrons in a nuclear reactor. The following reactions occur with high cross sections: 185 Re( n , ) 186 Re and 187 Re( n , ) 188 Re. Both 186 Re (half-life 90.6 h) and 188 Re (16.7 h) -decay to the stable isotopes 186 Os and 188 Os. Thus 186 Os and 188 Os are enriched in proportion to the neutron fluence and Re/Os ratio of the sample being irradiated. Analysing merely the isotopic composition of the sample, we get two ages that should be consistent. In one irradiated molybdenite sample from Kingsgate molybdenum mine in New South Wales, Australia, the measured 186 Os / 192 Os and 188 Os / 192 Os are 0.3284 and 0.6299, whereas in an unirradiated sample they are assumed to be 0.0390 and 0.3248. These ratios are measurable with negative thermal ionization mass spectrometry (N-TIMS) to a precision of 1 . The derived ages for this sample are t 186 = 220.5 ± 8.7 Ma and t 188 = 214.9 ± 8.9 Ma , respectively (or ±1.4 Ma and ±1.7 Ma, respectively, when excluding the uncertainty of 187 Re half-life). These values are in close agreement with the 221-238 Ma K-Ar ages of the leucogranites emplacement that causes the Mo mineralization at Kingsgate (Evernden and Richards, 1962; Kleeman, 1988; Gilligan and Barnes, 1990). The major limiting factor on the precision of this age determination is the uncertainty in the half-life of 187 Re. The errors associated with the irradiation parameters are greatly reduced by choosing an appropriate neutron flux monitor. The advantage of this method is that the Re / Os ratio is determined on the same sample and that only one measurement of the isotopic composition of osmium is required, thus overcoming some of the experimental difficulties of the conventional Re-Os analysis. The problem of sample inhomogeneity (which is a serious problem for the Re-Os system) and the need to measure the absolute concentrations of rhenium and osmium are thus eliminated. With this method, it is potentially possible to obtain an age spectrum of the disturbed Re-Os system with selective leaching steps or by in situ analysis.
Jagoutz Emil
Verkhovskiy A. B.
Wanke Heinrich
Yin Q.-Z.
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