This project aims at: a) A better understanding of the behaviour of lanthanides, actinides and their isotopes within selected accessory minerals when these undergo fluid-rock interactions and; b) The development of the analytical tools used (i.e. mainly the Laser Ablation-Inductively Coupled Plasma-Mass Spectrometer (LA-ICP- MS), and the Thermal Ionisation Mass Spectrometer (TIMS)). Both parts will build upon what has already been achieved during a HCM fellowship. An improved knowledge of the hydrothermal mobility of these elements will help 1) to trace and date the fluids within the earth's crust, and 2) to assess the suitability of monazite or sphene-like ceramics as nuclear waste forms. The reduction of elemental fractionation and the improvements of the precision in isotope ratio measurements by LA-ICP-MS, together with the enhancement of the spatial resolution of TIMS will be of benefit for the scientific and industrial users of these techniques.