THE ROLE OF INCLUSIONS IN U-PB AND SM-ND GARNET GEOCHRONOLOGY - STEPWISE DISSOLUTION EXPERIMENTS AND TRACE URANIUM MAPPING BY FISSION-TRACKANALYSIS

The U-Pb and Sm-Nd dating of garnet are important tools for understand ing rates of tectonometamorphic processes and have been widely applied in studies of metamorphic terranes. However, the budgets of uranium, lead, samarium, and neodymium in garnet separates from metamorphic roc ks may be dominated by contributions from inclusions of monazite or zi rcon. A combined fission track and stepwise dissolution technique is p roposed for evaluating the role of inclusions of monazite and zircon i n the budgets of uranium, lead, samarium, and neodymium in garnets use d for U-Pb and Sm-Nd chronology. Variations of Th/U, U/Nd, and Sm/Nd r atios between successive dissolution steps reveal the contributions of monazite and zircon inclusions. The stepwise dissolution procedure do es not induce any apparent artifacts on Pb-207-Pb-206 and Sm-Nd ages. The technique has been applied to garnets from three metamorphic terra nes. Almandine garnet samples from the high grade Pikwitonei Granulite Domain (Manitoba, Canada) and the Wind River Range (Wyoming, USA) hav e the majority of their uranium, samarium, neodymium, and radiogenic l ead (Pb) hosted by micrometer-scale inclusions of monazite. Fission t rack densities reveal that uranium is 10(8) times more abundant in inc lusions than in garnets. Stepwise dissolution of the samples shows tha t neodymium and uranium are associated with the dissolution of monazit e. In contrast, grossular-andradite garnets from Cascade Slide (Adiron dack Highlands, NY, USA) show little variation in fission track densit y and similar isotopic ratios between dissolution steps, indicating th at the budgets of uranium, samarium, neodymium, and Pb are not signif icantly influenced by inclusions. The demonstrated success of these te chniques leads us to recommend similar procedures as a routine matter in U-Pb, and Sm-Nd garnet geochronology of metasedimentary rocks.