Monazite inclusions in garnet grains were analyzed for Th-Pb isotopes by laser-ablation ICPMS. The monazite inclusions, generally 10-20 microns in diameter, were analyzed from within separated garnet crystals of 0.5 to 5.0 mm in diameter. The position of each inclusion relative to the interior or rim of the host garnet grain is indicated in Table ___. Inclusions were identified and mapped with a LINK EDS detector attached to a Cameca Camscan Series II SEM. The analytical methodology for garnets resembles that for zircons except that (1) the laser beam diameter is 8 to 10 microns, (2) ages are calculated on the basis of the measured 208Pb/232Th, 206Pb/238U, 206Pb/207Pb, and 208Pb/204Pb, (3) common Pb composition is adapted from Stacey and Kramers (1975) with an uncertainty of 2.0 for 208Pb/204Pb, 1.0 for 206Pb/204Pb, and 0.3 for 207Pb/204Pb, and (4) fractionation is monitored by frequent (every fourth) analysis of standard monazite from the Wissahickon Formation in Delaware (sample # 44069), which was provided by J. Aleinikoff (US Geological Survey). This monazite has been dated by ID-TIMS at 424 ± 1 Ma (2-sigma: Aleinikoff et al., 2006). Uncertainties of the reported ages are based on the quadratic sum of measurement errors (from 208Pb/232Th, 206Pb/238U,206Pb/207Pb, and 208Pb/ 204Pb) and systematic errors (from calibration correction, age of standard, U and Th decay constants, and composition of common Pb).
Aleinikoff, J.N., Schenck, W.S., Plank, M.O., Srogi, L., Fanning, C.M., Kamo, S.L., Bosbyshell, H., 2006, Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington Complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite, Geological Society of Americ Bulletin, v. 118, p. 39-64.
Ludwig, K.R., 2003, Isoplot 3.00, Berkeley Geochronology Center, Special Publication 4, 70 p.
Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth and Planetary Science Letters, v. 26, p. 207-221.