Resolving the Youngest Episode of Zircon Rim Growth with High-Spatial Resolution SIMS: U-Pb Ages and Trace Element Analyses from <1 um Thick Metamorphic Zircon Rims from the Zanskar Shear Zone and Tso Morari UHP Complex, NW Himalaya

Abstract:

Dating the youngest phase of metamorphic or magmatic zircon growth by in-situ techniques on sectioned zircons can be challenging when rims are only a few microns thick; in the worst case, geologically meaningless measurements result from mixing between different age and/or compositional growth domains. This is especially true for zircons from the Zanskar Shear Zone and Tso Morari UHP Complex, NW Himalaya, which contain Eocene to Miocene rims that grew over Paleozoic and Proterozoic protolith cores during peak and retrograde metamorphism. These metamorphic rims are typically only 0.5 to 3.0 microns thick and require high-spatial resolution to resolve. We used the SHRIMP-RG ion-microprobe to perform new U-Pb depth-profiling analyses on zircon surfaces (non-polished) pressed into indium metal. Zircons from a leucogranite dike from Malung Tokpo along the Zanskar Shear Zone, which have 2000-5000 ppm U rims, were selected for detailed depth-profiling to evaluate the depth-resolution by SIMS for U-Pb and trace element analyses. Due to the high U concentration, we were able to decrease the primary beam intensity (sputter rate) and the count times for U and Pb isotopes, and increase the number of cycles through the run-table (45 peak-hopping scans). As a result, each cycle yielded a 0.04 micron depth-resolved zircon age and trace element composition. The youngest 13 scans yielded a U-Pb age of 21.3 ± 0.5 Ma, representing a 0.6 micron rim with uniform U, Th, and Hf concentrations. At 1.15 microns, the primary beam sputtered into a ~600 Ma core; this older age is complicated by the fact that it reflects mixing between the bottom of the sputter volume and the Miocene ages of the pit margins and rim, because the diameter of the spot tends to increase with time. Trace element analyses on zircon surfaces from the Tso Morari UHP Complex are highly reproducible, showing enriched HREE profiles with negative Eu anomalies – a result that is difficult to reproduce by analyses of sectioned crystals.