Southeastern Section–55th Annual Meeting (23–24 March 2006)

Paper No. 10
Presentation Time: 11:20 AM


TRACY, Robert J. and LOEHN, Clayton W., Geosciences, Virginia Tech, Blacksburg, VA 24061,

Two in situ analytical techniques have become the methods of choice for determining ages of monazite (mnz): SHRIMP U-Pb isotopic and Electron Microprobe (EMP) "chemical" (non-isotopic) techniques. Debate remains concerning comparability of ages obtained by the two techniques. SHRIMP has an advantage in analytical precision whereas the EMP has an advantage in spatial resolution. We report examples of individual monazite grains that have been dated by both techniques, covering a range of ages from 300 to 2850 Ma. SHRIMP analyses were done at the Geological Survey of Canada or at the USGS-Stanford SHRIMP-RG lab; EMP analyses were all done at Virginia Tech. Grains display complex zoning in compositional maps, indicating complex growth and recrystallization histories. In most cases, ages obtained by the two techniques agree within the 2-sigma associated error of each technique, except where SHRIMP pits cross age boundaries and resolve a mixed age of two domains, or where EMP spots fall near cracks or pits in the grain surface. Prominent examples discussed include PreC mnz from the Tobacco Root Mountains, MT (TRM), Paleozoic mnz from the Black Mountain Granite, VT, from Candlewood Lake Granite, CT, from a pegmatite at Wendell, MA, and from the Maidens Gneiss, Goochland Terrane VA. To summarize one example, the TRM mnz contains a low-Th older core (ca. 2.85 Ga), a higher-Th mantling domain of about 2.45 Ga, and a low-Th rim of 1.78 Ga. This single grain has 6 SHRIMP pits: 2 fall totally within an age zone delineated by EMP analyses and compositional maps as the middle age zone: 2451 (±4 2-s) and 2432 (±10). The weighted mean EMP age of this domain is 2452 (±6 2-s). SHRIMP spots aimed at the older core yielded 2619 (±11) and 2785 (±9); the actual core age from EMP analyses is 2859 (±14). EMP analyses and elemental maps of magmatic mnz indicate intricate zoning of both chemical and age zones, and presence of both cryptic xenocrystic cores and metamorphic overgrowths, indicating the possibility of mixed ages. The most important message of this comparative study is that SHRIMP and EMP dating of monazites are complementary techniques and in many cases both may be necessary for the best interpretation of complex thermal histories. (Data, samples and collaboration provided by P.S. Dahl, J. N. Aleinikoff, J.Wooden, M.A. Hamilton, F. Mazdab, G. Walsh, C. Shirvell, B.E. Owens)