Reproducibility of Re-Os molybdenite ages depends on sample size and homogeneity, suggesting that Re and Os are decoupled within individual molybdenite crystals and do not remain spatially linked over time. In order to investigate the Re-Os systematics of molybdenite at the subgrain (micron) scale, we report LA-ICP-MS Re-Os ages for an Archean molybdenite crystal from Aittojarvi, Finland, analyzed in situ in a white aplite matrix. A related Aittojdrvi molybdenite (A996D), in the form of a very fine-grained mineral separate, is used as one of our in-house NTIMS standards, and thus its age of 2760 +/- 9 Ma is well established. Measurements of (Re-187 + Os-187)/Re-185 on micron scale spots along 200 mum traverses across the crystal yield a wide range of ages demonstrating that, in this case, microsampling of molybdenite does not produce geologically meaningful ages. Experimentation with mineral separations and sample size over a 7-yr period predicted that this would be the outcome. We suggest that Os-187 is more likely to be the mobile species, based on its charge and ionic radius, and that Os-187 becomes decoupled from parent Re-187 with time on the micron and larger scale. Incompatible charge and ionic radius for Os ions formed during reduction of molybdenite-forming fluids may explain the widely observed absence of common (initial) Os in molybdenite. Geologically accurate ages for molybdenite can only be obtained for fully homogenized crystals (or crystal aggregates) so that any post-crystallization Re-187-Os-187 decoupling is overcome. A growing number of geologically accurate ID-NTIMS Re-187-Os-187 ages for homogenized molybdenite suggest that postcrystallization mobility of radiogenic Os-187 must be limited to within the molybdenite mineral phase. We suggest that radiogenic Os-187 may be stored in micron scale dislocations, kink bands, and delamination cracks produced by deformation, and that the unusual structure and deformation response of molybdenite results in an increased chemical stability in this mineral. Migration Of Os-187 into adjacent silicate phases is highly unlikely, but other contacting sulfides may take in Os. In an example from a Proterozoic skarn deposit at Pitkaranta (western Russia), we demonstrate minor loss of radiogenic Os-187 from molybdenite and a corresponding gain in adjacent chalcopyrite such that the molybdenite age is not perceptibly disturbed, whereas the resulting chalcopyrite ages are impossibly old. Therefore, it is unadvisable to perform Re-Os analytical work on any sulfide in contact or intimate association with molybdenite. In addition to large errors in the age, if the isochron method is employed, initial Os-187/Os-188 ratios could be erroneously high, leading to seriously errant genetic interpretations. Copyright (C) 2003 Elsevier Ltd.
