HIGHLY PRECISE RE-OS DATING FOR MOLYBDENITE USING ALKALINE FUSION ANDNTIMS

The technique described in this paper represents the modification and combination of two previously existing methods, alkaline fusion and ne gative thermal ion mass spectrometry (NTIMS). We have used this techni que to analyze repeatedly a homogeneous molybdenite powder used as a r eference standard in our laboratory. Analyses were made over a period of 18 months, using four different calibrations of two different spike solutions. The age of this standard reproduces at a level of +/- 0.13 %. Each individual age analysis carries an uncertainty of about 0.4% t hat includes the uncertainty in the decay constant for Re-187. This ne w level of resolution has allowed us to recognize real differences in ages for two grain-size populations of molybdenite from some Archean s amples. (C) 1998 Elsevier Science B.V.