In-situ LA-ICP-MS U-Pb dating of lower greenschist facies xenotime

Xenotime, a common accessory mineral in metasedimentary rocks, is reactive during metamorphism and can incorporate significant amounts of uranium, making it a promising target for in-situ U-Pb geochonology. However, small grain sizes and cryptic mineral zoning limit the applicability of this tool in low-grade metamorphic rocks thus far. Herein we present results from the dating of two low-grade metamorphic samples collected in the Tirolic-Noric Nappe System at the southern base of Dachstein and Hochschwab massifs (Eastern Alps, Austria). These weakly metamorphosed metasediments, in which sedimentary structures are preserved, underwent metamorphism during the Cretaceous Eo-Alpine event, resulting in a mineral assemblage consisting of chloritoid + pyrophyllite + muscovite + hematite + rutile + quartz that infers P-T conditions of ~350°C and 0.2-0.6 GPa.

Both samples contain accessory xenotime grains that typically range from 10 to 30 µm in diameter. SEM imaging and chemical mapping reveal systematic chemical zoning of most xenotime grains with a heterogeneous core and a distinct, 5-10 µm wide rim that is enriched in MREEs (Sm-Gd). We targeted each chemical domain of xenotime by in-situ LA-ICPMS U-Pb dating using a 5 µm beam diameter. In addition to U and Pb isotopes, Gd and Y concentrations were monitored. The majority of concordant U-Pb dates in both samples range between 632 and 250 Ma. These analyses exhibit low Gd/Y ratios consistent with the xenotime core composition and are therefore interpreted as an inherited population. Both samples exhibit a younger age cluster characterized by high Gd/Y ratios, corresponding to the MREE-rich xenotime rim. In the Hochschwab sample, the youngest cluster yields a concordia age of 134.2 ± 4.0 Ma (MSWD: 2.7, n: 7). Host-inclusion relationships of chloritoid and xenotime suggest coeval growth of the MREE-rich xenotime rim and chloritoid porphyroblasts, linking the U-Pb date to the growth of the main metamorphic assemblage. In the Dachstein sample, the youngest cluster yields a concordia age of 92.4 ± 1.5 Ma (MSWD: 2.9, n: 9). Xenotime and chloritoid are not observed in direct contact, and this sample is characterized by a pervasive crenulation cleavage, which postdates chloritoid growth. From the distribution and morphology of xenotime we conclude that dissolution-precipitation related to crenulation cleavage formation facilitated growth of the MREE-rich rim.

Our results suggest that several stages of xenotime precipitation occurred during protracted or multistage tectono-metamorphic activity in the Tirolic-Noric Nappe System during the Eo-Alpine metamorphic evolution, which is also corroborated by published geochronological data in the same unit. We also demonstrate that the constantly improving spatial resolution of LA-ICP-MS systems allows successful in-situ dating of minute xenotime growth zones, thus providing great potential to improve our understanding of processes during low-grade metamorphism.