OS-187-OS-186 AND OS-187-OS-188 METHOD OF DATING - AN INTRODUCTION

A Re-containing sample is irradiated with thermal neutrons in a nuclea r reactor. The following reactions occur with high cross sections: Re- 185(n, gamma)Re-186 and Re-187(n, gamma)Re-188. Both Re-186 (half-life 90.6 h) and Re-188 (16.7 h) beta-decay to the stable isotopes Os-186 and Os-188. Thus Os-186 and Os-188 are enriched in proportion to the n eutron fluence and Re/Os ratio of the sample being irradiated. Analysi ng merely the isotopic composition of the sample, we get two ages that should be consistent. In one irradiated molybdenite sample from Kings gate molybdenum mine in New South Wales, Australia, the measured Os-18 6/Os-192 and Os-188/Os-192 are 0.3284 and 0.6299, whereas in an unirra diated sample they are assumed to be 0.0390 and 0.3248. These ratios a re measurable with negative thermal ionization mass spectrometry (N-TI MS) to a precision of 1 parts per thousand. The derived ages for this sample are t186 = 220.5 +/- 8.7 Ma and t188 = 214.9 +/- 8.9 Ma, respec tively (or +/-1.4 Ma and +/-1.7 Ma, respectively, when excluding the u ncertainty of Re-187 half-life). These values are in close agreement w ith the 221-238 Ma K-Ar ages of the leucogranites emplacement that cau ses the Mo mineralization at Kingsgate (EVERNDEN and RICHARDS, 1962; K LEEMAN, 1988; GILLIGAN and BARNES, 1990). The major limiting factor on the precision of this age determination is the uncertainty in the hal f-life of Re-187. The errors associated with the irradiation parameter s 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 composi tion of osmium is required, thus overcoming some of the experimental d ifficulties of the conventional Re-Os analysis. The problem of sample inhomogeneity (which is a serious problem for the Re-Os system) and th e need to measure the absolute concentrations of rhenium and osmium ar e thus eliminated. With this method, it is potentially possible to obt ain an age spectrum of the disturbed Re-Os system with selective leach ing steps or by in situ analysis.