AN IMPROVED U-TH-PB AGE CALCULATION FOR ELECTRON MICROPROBE DATING OF MONAZITE

Abstract

Determination of U-Th-Pb ages using the electron probe microanalyser (EPMA) is an inexpensive alternative method for dating monazite. The method is rapid and reliable, both for simple monogenenetic monazite and for complex polygenetic monazite having undergone metamorphic events involving fluid interaction and recrystallization. The main limitation of the method is its rather poor precision, i.e., +/- 45 to +/- 120 Ma for ages ranging from 300 to 3000 Ma calculated on each spot. The precision is limited by Pb content and by counting statistics, which cannot give precision better than 2% for individual determinations on U, Th, and Pb even at high levels. A procedure that uses the new Th/Pb vs. U/Pb diagram to improve the calculated precision on U-Th-Pb ages gives results to within +/- 5 to +/- 15 Ma for ages ranging from 300 to 3000 Ma. With complex polygenetic monazite, in which either the points show large scattering indicated by a large MSWD or the regression line exhibits a slope very different from neighbour theoretical isochrons, the procedure must be applied separately on homogeneous domains only. This makes it possible to distinguish between events separated by a gap of ~20 to 60 Ma, according to the range of ages concerned (i.e., 300 to 3000 Ma). Several examples are given to illustrate these systematics.