The role of inclusions in U-Pb and Sm-Nd garnet geochronology: Stepwise dissolution experiments and trace uranium mapping by fission track analysis

Abstract

The U-Pb and Sm-Nd dating of garnet are important tools for understanding 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 rocks may be dominated by contributions from inclusions of monazite or zircon. A combined fission track and stepwise dissolution technique is proposed for evaluating the role of inclusions of monazite and zircon in the budgets of uranium, lead, samarium, and neodymium in garnets used for U-Pb and Sm-Nd chronology. Variations of Th/U, U/Nd, and Sm/Nd ratios between successive dissolution steps reveal the contributions of monazite and zircon inclusions. The stepwise dissolution procedure does not induce any apparent artifacts on 207Pb- 206Pb and Sm-Nd ages. The technique has been applied to garnets from three metamorphic terranes. Almandine garnet samples from the high grade Pikwitonei Granulite Domain (Manitoba, Canada) and the Wind River Range (Wyoming, USA) have the majority of their uranium, samarium, neodymium, and radiogenic lead (Pb*) hosted by micrometer-scale inclusions of monazite. Fission track densities reveal that uranium is 10 8 times more abundant in inclusions than in garnets. Stepwise dissolution of the samples shows that neodymium and uranium are associated with the dissolution of monazite. In contrast, grossularandradite garnets from Cascade Slide (Adirondack Highlands, NY, USA) show little variation in fission track density and similar isotopic ratios between dissolution steps, indicating that the budgets of uranium, samarium, neodymium, and Pb * are not significantly influenced by inclusions. The demonstrated success of these techniques leads us to recommend similar procedures as a routine matter in U-Pb and Sm-Nd garnet geochronology of metasedimentary rocks.