『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 5μm
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.』
1. Introduction
2. The method
2.1. Theoretical basis
2.2. Sample preparation
2.3. Analytical procedure
2.4. Data presentation
2.5. Modelling
2.6. Comparison with previous work
3. Examples
3.1. Simple cases
3.2. A more complex case
3.3. An even more complex case
4. Working at the limits of the method
4.1. Young ages
4.2. Old ages
4.3. Small crystals
4.4. Zoned crystals
5. Conclusions after two years of work
5.1. Reliability of the method
5.2. Limits
5.3. Advantages
5.4. General guidelines
Field of application
Recommended procedure
5.5. The paradox of monazite concordant behaviour
5.6. Future work
Acknowledgements
Appendix A. Least-squares modelling of multiple ages
References