ELECTRON PROBE DATING OF MONAZITE
M. VESCHAMBRE, S. FORET, J.M. MONTEL, C. NICOLLET,
CNRS URA 10, UnIversité B. Pascal,
5 rue Kessler 63038, Clermont-Ferrand, France
The development of various micro-analytical techniques has shown that geochronology at the crystal scale can bring new and extremely useful informations in Earth Sciences. Using the ion probe, and laser-coupled methods it is now possible to obtain ages on domains as small 20-100 mm. Here we present a method for dating monazite with the electron probe, with all the advantages of this technique, in particular its unsurpassed spatial résolution. Monazite (LREE phosphate) is extremely rich in U (100 ppm-3 %) and in Th (3- 25 %). Radiogenic Pb (Pb) mainly 208Pb, accumulates very quickly, and reaches, in about 100 Ma, a level where it is possible to analyse it with the electron probe (several hundreds or several thousands of ppm). Assuming (1) that common lead is negligible relative to Pb*, and that no partial loss of Pb occurred, the simultaneous measurement of U, Th, and Pb allows to calculate an age from a single electron-probe analysis. This method is now fully operational at Clermont-Ferrand, and after more than one year of measurements the following conclusions can be inferred :
- this method is reliable. Measurement carried out on samples
of known ages, from 200 Ma to 3.1 Ga indicates that the isotopic age always
falls inside the confidence interval of the electronic age.
- monazile can stand extremely high Pb* content (up to 2-3 wt %) without suffering lead loss.
- monazite as young as 100 Ma can be dated confidently.
- the précision that can be reached is usually, for a "normal" monazite, ± 30-50 Ma. For this précision, a the total counting time necessary is 20 x 30 sec. and the total time necessary for obtaining the age is less than 1 hour.
This method has all the advantages of the electron probe :
- it is non-destructive ;
- it has an excellent spatial résolution (monazites as small as 5 mm can be dated ;
- it is possible to work on normal, polished thin sections. This means that it is possible to date the crystals, knowing exactly their petrogaphical position ;
- it is possible to perform age mapping of a crystal by using the profiling ability of the microprobe.
This method is fast, non-expensive, non-destructive, moderately precise, and reliable. lt is applicable to any rock containing monazite: aluminous granites, metasediments, some pegmatites and carbonatites.