DATING MONAZITE WITH THE ELECTRON-MICROPR OBE

Lead, uranium and thorium can be analyzed with precision in monazite u sing the electron microprobe. The concentrations can be interpreted di rectly as an age with a typical precision of +/- 30 Ma for a 300 Ma ag e and +/- 100 Ma for a 3.0 Ga age. For samples of known age, the discr epancy between the microprobe age and the isotopic age is less than th e errors. This method is an in situ dating technique, which can be use d to date domains at the microscopic scale, with a spatial resolution of a few microns. The first results indicate that, as for zircon, mona zite can display age heterogeneity, preserving old domains in a more r ecent crystal.