If a rock has been affected by thermal metamorphism, aqueous alteration or shock metamorphism some time after crystallization, it is likely that uranium and radiogenic lead accumulated since crystallization moved between the minerals, or at a larger scale, in that process.
The date calculated from the radiogenic 207Pb/206Pb ratio assuming closed system evolution could yield an erroneous date. Knowing the nature of element mobility in the secondary process is important for accurate interpretation of isotopic dates. For example, if we try to apply diffusion formalism to a rock in which the isotopic systems were reset by mineral recrystallization, then we are likely to get a meaningless date.
The student will study the distribution of U and Pb isotopes in a variety of rocks, minerals and meteorites, and search for characteristic geochemical and isotopic patterns predicted by diffusion formalism. He or she will also inspect the existing set of experimental data for diffusion of parent and daughter elements in the minerals used in isotopic dating, and plan and perform experiments necessary to obtain the missing data.