Isotope dating of rocks file profile

Unravelling the full geological history of basement terranes using geochronological methods is not a simple task, because regional metamorphism may obscure, or even obliterate, older crystallization stages.

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 demonstrate that even very small uraninite crystals can survive an amphibolite facies overprint without their U-Th-Pb systematics being significantly disturbed or reset.

Many crystals that we measured during this study did not show Pb loss despite having been subjected to temperatures of ∼500–550 °C during a much younger, penetrative regional metamorphic event.

Zircon dating (Eichhorn et al., 2000; Kozlik et al., 2016) defines the first two rocks as early Carboniferous granites (339.6 ± 1.2 Ma, 338.5 ± 1.3 Ma), whereas the Granatspitz and Reichenspitz gneisses are younger Permian intrusions (271 ± 4 Ma, 292 ± 6 Ma). Some are included in feldspars, epidote, or titanite (Fig. Swarms of uraninite microcrystals are present in some quartz-chlorite veins (Fig. Most uraninite crystals display a uniform grayscale in backscattered electron (BSE) images, implying compositional homogeneity.

On average, 10–20 uraninite microcrystals were observed in a single thin section. However, a few grains exhibit a weak internal zonation with a slightly brighter rim, or irregular and diffuse patchy zonation (Figs. Uraninite microcrystals were analyzed with a Zeiss Ultraplus SEM equipped with a field emission cathode (FE-SEM) and an EDX large-area silicon-drift detector (Oxford X-Max50).

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.

Using the Tauern Window of the Eastern Alps as an example, we show that uraninite microcrystals (.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).A small but systematic blank value of −0.16 ± 0.06 wt% Pb O was recognized and externally corrected based on the assumption that it was constant during every uraninite analysis.The uncertainty of the blank value was incorporated in the total Pb error and the age error.

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