What are we dating? The role of micro-inclusions for in situ garnet geochronology

In situ garnet U-Pb geochronology by laser ablation-inductively coupled mass spectrometry (LA-ICPMS) is a powerful tool for rapid and high-spatial resolution dating of metamorphic pressure-temperature–time histories. However, ultra-low U and Pb contents in many metamorphic garnets pose a significant analytical challenge, including the risk of contamination by inclusions. Here we use split-stream analysis to simultaneously measure U, Th, and Pb isotopes and trace-element contents of garnet from eclogite-facies metamafic rocks (As Sifah, Oman). The data show strong linear correlations in U vs Zr contents, the slopes of which can only be explained by zircon contamination. Time-resolved U, Th, and Pb signals show some irregularities, but often lack sharp spikes typically diagnostic of inclusions. Abundant micro-zircon (<2 µm diameter) inclusions were observed by SEM in all five samples. Interestingly, garnet–zircon mixing lines in Tera-Wasserburg space project to well-defined concordia intercept dates of 94–89 Ma, whereas a sample with sufficient zircon-free analyses gave an intercept date of 71 ±7 Ma. The latter date overlaps within uncertainty of published Sm-Nd garnet–whole rock isochron ages (81–77 Ma) and U-Pb zircon ages (82–78 Ma), whereas ‘garnet’ dates dominated by the zircon-inclusion are 8 to 17 Myr older. The discrepancy between zircon inclusion and published zircon ages from the same samples likely arises from the fact that such small zircon size fractions are rarely dated and may represent a different age population from that of lager grains. We suggest that the <2 µm zircon fraction formed by diffusion-limited local nucleation from the expulsion of Zr from igneous precursor minerals during (sub-)greenschist facies metamorphism. Recrystallization and Ostwald ripening of matrix grains produced zircons that record peak metamorphic ages, whereas the micro-zircon grains remained isolated and were shielded by garnet. We therefore suggest that (1.) the ablation of inclusions may produce linear trends in Tera-Wasserburg space that may be easily misinterpreted as U-Pb garnet ages, and (2.) bulk inclusion dating of zircons by LA-ICPMS is a promising technique that requires further investigation. Finally, we establish more rigorous criteria for the screening of samples for inclusions prior to and during LA-ICPMS U-Pb garnet geochronology analysis.