RELIABLE RE-OS AGE FOR MOLYBDENITE

Re-Os ages for four molybdenites were determined by our newly develope d method with isotope dilution-mass spectrometry. Special caution was given to achieve osmium isotopic equilibration between sample and ''sp ike'' during chemical treatment. Re-Os data for molybdenites from Mt. Mulgine (Australia), Matasvaara (Finland), Preissac (Canada), and Clim ax (USA) define ages of 2790 Ma, 2600 Ma, 2680 Ma, and 26.6 Ma, respec tively. The reproducibilities of our Re-Os age determination are shown to be within 2% by the replicate analyses for Mt. Mulgine, Matasvaara , and Preissac samples. The ages obtained are in good agreement with r eference ages estimated for wall rocks of molybdenite deposits concern ed. Thus, with careful experimental procedure, it appears that the Re- Os geochronometer can be used to reliably determine the age of molybde nite.