The present generation of electron microprobes can precisely determine U, Th and Pb concentrations in accessory minerals such as monazite, xenotime, uraninite and zircon. This caused a renewed interest in chemical age dating in the 1990's. The principal advantages of the electron microprobe are the high spatial resolution (2-3µm) and the ability to perform non-destructive in-situ measurements in a relatively short time at low expense.
Currently, three methods are used to extract age information from U, Th, Pb analyses. The methods of Suzuki et al. (1991) and Rhede et al. (1996) employ least-squares regression to define isochrons. Suzuki's method combines the contributions of Th and U to radiogenic Pb assuming that
the U-Pb and Th-Pb ages are identical. Rhede et al. (1996) determine the best-fit plane to data in three dimensions and calculate independent U-Pb and Th-Pb ages. Both regression methods require a compositional spread. In contrast, Montel et al. (1994), calculate an apparent age and age error from each individual analysis and apply stastistical methods to the resulting data set to distinguish different age populations. Each method has its advantages and disadvantages and should be combined in data interpretation.
In order to select samples appropriate for U-Pb isotope dating, which is currently in progress, several Variscan granites of the Erzgebirge-Vogtland-Fichtelgebirge domain have been studied for accessory mineral chemical ages.
The three-dimensional method is particularly useful in case of monazite showing high and variable Th and U concentrations. Chemical Th/Pb- and U/Pb monazite ages obtained for the G4 granite (Fichtelgebirge, Germany) are concordant at 323 ± 20 Ma and 304 ± 15 Ma, respectively, and are close to published K/Ar biotite ages (298.2 ± 0.6 Ma) as well as Rb/Sr whole-rock ages (289 ± 4 Ma).
A suitable spread in Th and U concentration of a sample can also be achieved by analyzing more than one type of mineral. For example, dating of monazite, xenotime and uraninite resulted in a Th/Pb-age of 316 ± 14 Ma and an U/Pb-age of 317 ± 16 Ma for the Pobershau granite suite in the central Erzgebirge (306 ± 7 Ma according to Rb/Sr isotope data). The same mineral assemblage yielded Th/Pb- and U/Pb-ages of 333 ± 13 Ma and 311 ± 11 Ma, respectively, for the Bergen composite granite pluton (Vogtland). For this pluton, an Rb/Sr whole-rock age of 313 ± 7 Ma and an K/Ar biotite age of 323.6 ± 2.6 Ma have been reported by Gerstenberger et al. (1995)
Application of the three-dimensional method to monazites from a Moldanubian paragneiss, whose conventional isotopic U-Pb age is 322 ± 3 Ma, resulted in Th/Pb- and U/Pb-ages of 329 ± 6 Ma and 289 ±37 Ma, respectively.
Th-U-Pb dating by microprobe appears to give reasonable results and deserves further attention. It can never replace isotopic measurements but is a valuable method for reconnaisance age studies and for pre-selection of samples suitable for isotopic age determinations. Different age populations can be recognized both within finely zoned single grains and at the thin-section scale, and even smallest sized samples such as enclaves can be tentatively dated.
Gerstenberger, H., Haase, G. & Wemmer, K., Terra Nostra, 36-41 (1995).
Montel, J-M., Veschambre, M. & Nicollet, C., C.R. Acad. Sci. Paris 318, serie II, 1489-1495 (1994).
Rhede, D., Wendt, I. & Förster, H-J., Chem. Geol., submitted (1996).
Suzuki, K., Adachi, M. & Tanaka, T., Sediment. Geol. 75, 141-147 (1991).