LASER-ABLATION ICP MS MEASUREMENTS OF Re/Os IN MOLYBDENITE AND IMPLICATIONS FOR Re Os GEOCHRONOLOGY

Laser-ablation quadrupole ICP-MS analyses of molybdenite samples from Preissac, Quebec, and Mokrsko, Belcice and Kasperske Hory gold deposits in the Bohemian Massif, Czech Republic, for Re and radiogenic 187 Os suggest the presence of strong zoning in Re and in 187 Os/ 187 Re values within grains. The zoning patterns indicate decoupling of daughter 187 Os from parent 187 Re after molybdenite crystallization. Whereas Re is not homogeneously distributed in the molybdenite grains studied, the lack of correlation between the calculated 187 Os/ 187 Re value and the Re content also reflects the mobility of 187 Os within individual grains of molybdenite. Such decoupling of Re and Os could affect the accuracy of Re-Os ages if disturbed portions of molybdenite are sampled for dating by conventional isotope-dilution N-TIMS or ICP-MS techniques. The results re-emphasize the critical need for homogenization of quantities of molybdenite sufficiently large to capture the bulk Os/Re of the sample to be dated by these methods. Laser-ablation sampling, when used with a multicollector magnetic-sector ICP-MS instrument, is able to produce Re-Os ages with a precision better than 2% (1 � m) from single spots in certain old and Re-rich molybdenite grains, but would be difficult to use as a dating tool given the extreme 187 Os/ 187 Re heterogeneity seen in individual grains.