The objective of the proposed research is to analyze quanti- tatively the magnitude of volume diffusion in compositionally zoned garnets and examine the impact that the process of diffusion has on the determination of pressure-temperature-time paths for metamorphic rocks from medium-grade terranes. Specific research goals include 1) development of numerical models to describe accurately the lattice diffusion of cations in chemically zoned minerals during, as well as after, growth; 2) measuring the change in P-T trajectories caused by diffusion during the growth of compositionally zoned garnet; 3) evaluation of the importance of diffusion to P-T path calculations previously obtained from a nappe and dome terrane in west-central New Hampshire. The fundamental goal of this research is to clarify in a meaningful way the conditions under which pressure-temperature histories can be retrieved from zoned metamorphic porphyroblasts. In addition, this study will week to improve our understanding of the effect of volume diffusion on metamorphic parageneses. It is anticipated that these results will lead to an improved capability for accurately describing the thermo-tectonic behavior of the crust in orogenic belts.
