ELECTRON MICROPROBE DATING OF MONAZITE

Abstract

Because monazite is extremely rich in U and Th, radiogenic Pb (* Pb) accumulates very quickly, and reaches, in about 100 Ma, a level where it is possible to analyse it with the electron probe. Assuming that common Pb is negligible, and that partial loss of Pb has not occurred, the simultaneous measurement of U, Th, and Pb allows to obtain a geologically meaningful age from a single electron probe analysis. Here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method. A specific statistical method to deal with the large number of data which can be obtained on a single sample is described, and several guidelines, illustrated by examples, are suggested to optimize the method. Electron probe measurements carried out on samples of known age, from 200 Ma to 3.1 Ga, yield ages that always fall inside the confidence interval of the isotopically determined age, demonstrating that this method is reliable. The younger age limit is approximately 100 Ma, although it can be younger in some favourable cases. In old monazites, extremely high * Pb contents have been found (up to 5 wt%) indicating that monazite can tolerate high radiation doses without experiencing lead loss. The final precision on the age, for a 'normal' monazite, is +/- 30-50 Ma, for a total counting time of 600 s. A complete dating procedure can be completed in less than 1 h. First results indicate that old ages can be preserved in monazite, either in small relict cores in crystals, or by the coexistence of several generations of monazites in a sample. This method has all the advantages of the electron probe: it is non-destructive, has an excellent spatial resolution (monazites as small as 5 μm can be dated), and because it is possible to work on normal polished thin-sections, the petrographical position of the dated crystal is known. This method offers a large number of geologists access to an in-situ dating technique at moderate cost.