Trace and Rare Earth Element Mapping of Megacrystic Zircon Grains Using GIS

Data from numerous LA-ICPMS traverses of megacrystic zircons ranging in size from 0.5 to 1 cm were collected and imported to GIS. Zircon grains were obtained as part of an earlier study (Callahan et al., 2007) from near Zirconia, North Carolina at the Freeman Mine dump site located within the western Inner Piedmont. This study was designed as a more detailed complement to published electron microprobe, SHRIMP, and bulk chemical analyses determined for zircon grains from the same ~325 Ma syenitic pegmatite body. In addition to U, Th, Hf, and Ti variation at the whole-grain level, Callahan et al. (2007) noted Y concentrations >750 ppm and Dy >100 ppm; however, the distribution within individual grains is unknown and the suite of trace and rare earth elemental data is limited. A Perkin Elmer 6100 DRC ICP-MS coupled with a New Wave/Merchantek 213 nm Nd:YAG laser was used to collect individual 80 µm spots along traverses with a minimum of 25 points in the shortest dimension and to ~75 points in the longest dimension for most of the mounted grains at each grain mount polish level. At each polish level, analyses were both averaged and separated into depth profile intervals for U-Pb ages and a suite of trace- and rare earth-element concentrations. This analytical information was imported into GIS for each polish and/or depth profile interval and was spatially referenced to the proper location on the grain mount. Numerous pre- and post-ablation scanned, reflected light, and SEM (cathodoluminescence and backscattered electron) images were utilized for the spatial referencing and provided an important secondary check for locating individual laser ablation pits. This study was undertaken to provide a systematic, efficient, interactive, and robust method for collecting, interpreting, and displaying a moderately complex analytical data set on a relatively simple spatial reference frame.