The Ar/Ar laser probe in the BGR consists of a 12 W YAG-Nd laser system (Baasel Lasertech), a stainless steel ultra high vacuum line for gas extraction and purification and a Fisons VG 3600 mass spectrometer.
The low-volume mass spectrometer (18 cm radius, 60° extended geometry instrument) is equipped with a Nier-type electron bombardment source and a twin collector system consisting of an off-axis Faraday cup and an axial Johnston secondary electron multiplier.
The line is fitted with a sample chamber, a cold trap (dry ice/alcohol), two Zr-Al getter elements (SAES) and an inlet system for atmospheric argon. The ultra high vacuum is maintained either with a turbo molecular pump (Balzers) or an ion pump (Varian).
The IR laser (l = 1064 nm) is operated in continious mode, usually with a beam diameter of ca. 200 µm. The temperature of the grain heated by the laser can be measured with an IR pyrometer (Heimann) and held constant by computer-controlled adjustment of the laser power.
Installation of a replacement multiplier in August 1994 made it possible to analyse irradiated samples for the first time. We chose sanidine, mica and amphibole from the Miocene Kaiserstuhl volcanic complex (Kraml et al., 1995). To be certain that the total fusion results are meaningful and not influenced, for example, by excess argon, we incrementally heated single sanidine crystals (tephritic t3 tephra from the Limberg volcano). In the case of the hydrous phlogopites from the ultramafic diatreme breccia, we released the gas with several laser spots (200 µm in diameter) distributed over the crystal (4 mm in diameter).
The incremental heating experiments with the laser on the sanidines yielded reproducible results that are in excellent agreement with conventional K/Ar dates published by Lippolt et al. (1963) for that volcanic unit (recalculated with new constants).
The mean value of the spot fusion results and the related isochron date are identical within 2s analytical uncertainty with conventional K/Ar analyses of whole-rock separates from the diatreme breccia, the isochron of phlogopite and amphibole totally fused with the laser, and a Rb/Sr mineral isochron from Schleicher & Keller (1991). Nevertheless, slightly different apparent ages were obtained for the individual laser spots. The differences cannot be interpreted as being different 40Ar* contents in the different parts of the mica grain. The scatter can be attributed to the fact that the contribution of the blank to a spot fusion measurement in this case was not precisely known.
In contrast to Ar-rich Mesozoic or even older samples, it is necessary for future laser spot analysis on Cenozoic samples to have thicker mica crystals or a defocussed laser beam and more blank measurements to obtain the original concentration gradient of Ar (if any) in the single grains.
Kraml, M. et al., Beih. z. Eur. J. Mineral. 7, No.1, 142 (1995).
Lippolt, H.J. et al., Jh. geol. Landesamt Baden-Württemberg 6, 507-538 (1963).
Schleicher, H. & Keller, J., Jh. geol. Landesamt Baden-Württemberg, 33, 33-57 (1991).