U-Pb ID-TIMS dating applied to U-rich inclusions in garnet

Garnet has long been recognized as an important mineral in metamorphic petrology thanks to its widespread distribution and successful use in the reconstruction of P-T-t paths. However, the potential of garnet to solve geological problems may have been underestimated. In this paper, we make use of the role of garnet as a safe container of U-rich inclusions that, despite being metamict, are screened from Pb loss-causing processes and can be dated by isotope dilution-thermal ionization mass spectrometry. Garnet itself is not a true U-Pb geochronometer, as it normally contains no uranium, but it represents a vessel that protects U-rich inclusions from later disturbances. Garnet is virtually free of common Pb, it is clean and transparent, and provides a good control for the selection of suitable grains. In this study, inclusions such as uranitite in garnet were successfully used to obtain a precise U-Pb age of 318.36+ or -0.32 Ma of a pegmatite vein (Evora, Portugal), which otherwise would have been undatable due to the absence and/or strong alteration of common geochronometers such as zircon.