Zircon and monazite are the two most important accessory minerals in metamorphic and igneous rocks that can be dated by the U-Th-Pb method. Garnet is a key-mineral to establish metamorphic or magmatic conditions encountered by rocks. These minerals all contain significant amounts of trace elements such as Y and REE. The partitioning of these elements can be used to determine equilibrium between monazite, zircon and garnet, thus creating a strong link between absolute age and P-T conditions experienced by the rock. The partitioning of trace elements between these minerals is beeing established in experiments as well as using natural rocks.