AGE AND CHEMISTRY OF MEGACRYSTIC ZIRCONS FROM ZIRCONIA, NORTH CAROLINA

Data from multiple LA-ICP-MS sessions at Vanderbilt University suggest that megacrystic zircons obtained from the same locality as samples studied by Callahan et al. (2007, Southeastern Geology 45: 1-13) can be used as a geochronologic standard. Samples were collected at a mine dump associated with the inactive Freeman Mine, which is located in the Zirconia Mine District of SW North Carolina within the western Inner Piedmont. Loose, mostly whole, zircon grains can be easily obtained in large volumes from mine dump material located on privately owned property. Most of the collected zircons are between 0.5 and 1 cm and were separated in the field from dump material by sieving and panning; some were also identified under shortwave UV light. This study was conducted in conjunction with work on zircons of unknown age from the southern Appalachians, southern Brazil, and Norway. All data were collected on a Perkin Elmer 6100 DRC ICP-MS coupled with a New Wave/Merchantek 213 nm Nd:YAG laser ablation unit. Raw data were imported into Glitter software for reduction. Harvard 91500 zircon (1065 ± 0.4 Ma ID-TIMS; Wiedenbeck et al., 1995) was used as the primary standard. Zirconia analyses of several grains obtained over a 3 day session yield a Concordia age of 328 ± 3 Ma (2σ; MSWD=0.31; ρ=0.58; n=32), in good agreement with a SHRIMP ²⁰⁶Pb/²³⁸U age of 327 ± 2 Ma (2σ; MSWD=1.5; 9/11 analyses) obtained by Bream (2003, Ph.D. Thesis, University of Tennessee). The same data set was also reduced with Zirconia as a standard; pooling all 55 LA-ICP-MS analyses of standard 91500 yields a Concordia age of 1059 ± 7 Ma (2σ). Age and trace element data (including REE) from additional sessions suggest some variability in the Zirconia grains, in particular trace element zoning. However, these initial results suggest that the Zirconia zircons yield reasonable and consistent U-Pb age data. The abundance of material makes Zirconia zircons particularly useful as secondary LA-ICP-MS standards.