Isotope dating of rocks file profile
Three novel points of generic geochronological importance are raised in this study.
215 Ma) age that predate medium-grade regional metamorphism were identified using high spatial resolution field emission–scanning electron microscopy–energy dispersive X-ray (FE-SEM-EDX) U-Th-Pb dating of uraninite microcrystals in basement rocks of the Tauern Window, Eastern Alps.
The successful application of in situ U–Th–total Pb dating of uraninite by means of electron beam excitation and X-ray spectroscopy dates to before the more popular U–Th–total Pb dating of monazite (Parslow et al., 1985; Bowles, 1990), but the method has never been widely used.
It has been restricted to studies of uraninite in uranium deposits (Alexandre and Kyser, 2005; Cross et al., 2011; Pal and Rhede, 2013) and to accessory magmatic uraninite in granitic rocks (Förster, 1999; Kempe, 2003; Cocherie and Legendre, 2007; Förster et al., 2012).
An analytical precision of ∼0.9 and 0.1 wt% (1σ) was achieved for U (Mα) and Pb (Mα), respectively, using an accelerating voltage of 15 k V (2 n A beam current, ∼200 nm beam width) and a counting time of 3–5 min per spot.
This analytical uncertainty corresponds to an age error of ∼20 m.y. Quantification of concentrations is based on intensity values taken from the Oxford Instruments INCA Energy internal standard data bank and a 180 s calibration measurement on copper before every analytical session.
Precise temporal constraints on, and identification of, thermal events that predate medium-grade regional metamorphism is generally only possible if these early events involved crystallization of robust datable minerals such as zircon, monazite, or garnet.We briefly comment on the potential of the field emission–scanning electron microscopy–energy dispersive X-ray method for U–Th–total Pb uraninite dating.We propose that the special construction, particularly the high beam stability, of modern FE-SEMs is advantageous for high spatial resolution measurements, as is the ability of EDX detectors to analyze all elements simultaneously.Electron beam analysis offers a much finer spatial resolution (∼1 µm at 15 k V), and may thus be the first choice for the total Pb dating of micron-sized uraninite crystals.Using the Tauern Window of the Eastern Alps as an example, we show that uraninite microcrystals (.