ELECTRON-MICROPROBE DATING OF MONAZITE

Because monazite is extremely rich in U and Th, radiogenic Pb (Pb) ac cumulates very quickly, and reaches, in about 100 Ma a level where it is possible to analyse it with the electron probe. Assuming that commo n Pb is negligible, and that partial loss of Pb has not occurred, the simultaneous measurement of U, Th, and Pb allows to obtain a geologica lly meaningful age from a single electron probe analysis. Here we pres ent the results of two years of systematical investigations aiming to define both the limits and potential of this method. A specific statis tical method to deal with the large number of data which can be obtain ed on a single sample is described, and several guidelines, illustrate d by examples, are suggested to optimize the method. Electron probe me asurements 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 iso topically determined age, demonstrating that this method is reliable. The younger age limit is approximately 100 Ma, although it can be youn ger in some favourable cases. In old monazites, extremely high Pb con tents have been found (up to 5 wt%) indicating that monazite can toler ate high radiation doses without experiencing lead loss. The final pre cision on the age, for a 'normal' monazite, is +/-30-50 Ma, for a tota l 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 preserv ed in monazite, either in small relict cores in crystals, or by the co existence 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 mu m can be dated), and because it is possible to work on normal polished thin- sections, the petrographical position of the dated crystal is known. T his method offers a large number of geologists access to an in-situ da ting technique at moderate cost.