MONAZITE U-PB AND TH-PB GEOCHRONOLOGY BY ION MICROPROBE, WITH AN APPLICATION TO IN SITU DATING OF AN ARCHEAN METASEDIMENTARY ROCK

Abstract

Monazite grains from nine different Precambrian rock samples and having concordant isotope dilution U-Pb and Th-Pb ages were used to develop and evaluate techniques for U-Pb, Th-Pb, and Pb-Pb isotopic dating using an ion microprobe operating at high mass resolution (R=5500, 1%). An unidentified isobar at ~203.960 amu complicated the determination of 204Pb in the monazites with higher Th, but it appeared to have been eliminated when using a slight energy offset. The 207Pb*/206Pb* ratios of individual spots were determined in 10 min analyses to +/-0.3-0.5% (1σ) of the true values using a 20-μm-wide O2- primary beam, whereas the uncertainty in the mean of several spots on a single generation of monazite was typically +/-0.1-0.4% (2σ). Instrumental bias in the measured Pb+/UO+ and Pb+/ThO+ values of individual analyses was monitored using linear relationships between 206Pb+/UO+ vs. UO2+/UO+ and 208Pb+/ThO+ vs. UO2+/UO+. The bias appears to vary somewhat with monazite Th content, requiring the use of compositionally matched standards. Individual spot measurements of the 206Pb/238U and 208Pb/232Th ratios in unknown monazites of low to moderate Th can be determined to +/-2% (1σ).The dating technique was applied to in situ analysis of small monazite inclusions hosted in an amphibolite-grade metapelite of Archean origin. Both the contextual setting and the morphology of the monazite grains were found to be important factors in interpreting the timing of growth of the host metamorphic mineral (e.g., garnet, staurolite, plagioclase). Euhedral monazite grains armoured in low-CaO garnet and matrix plagioclase yielded an age of 2548+/-17 Ma, interpreted as dating an early episode of low-pressure metamorphism. Partially resorbed monazite within a high-CaO garnet rim yielded ages of ca. 2500 Ma, but are interpreted as inherited from the first event rather than syn-genetic with respect to the growth of this garnet. An age of 1755+/-30 Ma was obtained for anhedral monazite associated with late, chlorite-filled fractures, apparently reflecting post-tectonic retrogression.