Metamorphic processes that take place deep in the earth's crust or in its upper mantle cannot be observed directly, and they span time scales much longer than human lifetimes, so knowledge of them must come from study of the rocks they produce. To reconstruct geologic histories -- to learn, for example, how long it takes for a mountain belt to form, how ancient it is, how rapidly it arose or was eroded -- we must be able to read the record of those processes that is encoded in minerals that formed at great depth and have been brought to the surface by tectonic processes. The central goal of this research is to learn better how to read such records in garnet, a mineral with a remarkable ability to capture details of its history in its chemical composition. Garnet, during growth, commonly develops differences in the concentrations of constituent elements from the cores of crystals out to their rims, and these variations are modified by diffusion, the movement of atoms through the solid crystal structure. Diffusional modifications occur to varying degrees, reflecting the various lengths of time that the crystals spent at different temperatures and pressures during their post-growth histories. The key to transforming these diffusional modifications into detailed information on geologic processes is quantitative knowledge of the rates and mechanisms of diffusion of different elements through the garnet structure.
Prior work has produced robust measurements of diffusion rates for elements that are abundant in garnet, but a much more nuanced interpretation of geologic history would be possible if comparable data were available for trace elements, those with very low abundance. Some progress toward this goal has also been made, by measuring and modeling the effects of diffusion in garnet crystals that have been partially resorbed at very high temperatures. But data are needed at lower temperatures, and for a broader suite of trace elements than have so far been investigated. This project will obtain those data by developing and deploying high-spatial-resolution methods of chemical analysis that will extend prior work to crystals in which the effects of diffusion span a narrower range of distance because they have been resorbed at lower temperatures. The research will investigate various means by which trace elements are incorporated into the garnet structure, and how different means of incorporation may affect diffusion rates, both for individual elements and for pairs of elements that may diffuse together in order to maintain local balance of electrical charges in the crystal. New data on trace-element diffusion in garnet should add rigor to applications as diverse as high-temperature heating/cooling histories and timescales of high-temperature thermal and metasomatic events, Sm-Nd and Lu-Hf garnet geochronology, understanding of metamorphic equilibration between garnet and accessory minerals, and interpretation of rare-earth patterns of mantle minerals and melts.
