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IntroducTIon: Since the introducTIon of highprecision negaTIve thermal ionizaTIon massspectrometry for Re and Os isotopic measurementsin 1991[1,2,3], two problems persisted in obtainingan absolute chronology with the Re-Os system. Thefirst problem centers on the precision and accuracyof Os spike calibration. The main difficulties arisefrom the (1) absence of stoichiometric Os metal orsalts of high purity to serve as primary standards and(2) the lack of isotopic equilibrium between Os ofdifferent oxidation states and of different chemicalforms in the spike and standard. Re-Os isochronshave been determined to per-mil range precision inboth slope and initial [4,5,6]. But because theaccuracy of Os spike is currently limited to 0.5 to1.6%, the isochron may be horizontally off by~1.6%. This will have a significant effect of thecalculated initial Os isotopic composition. Thesecond problem is that the half-life of 187Re is onlyknown to ~3% based on the direct measurement [7].Any age calculated from the slope of an isochronhas an uncertainty of ~3%. The uncertainty of the187Re half-life ultimately depends on the accuracy ofthe Os spike calibration. It is thus clear that this isan area that needs further improvement, so that theRe-Os chronometer as an absolute dating tool can beapplied to its fullest potential.
