Acessory Minerals and Crustal Processes

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Intellectual merit. This accomplishment-based renewal (ABR) proposal requests funding to continue a multi-faceted program of research bearing on the properties of accessory minerals in the Earth's continental crust. The overall goal is to enhance capabilities for determining the timing and intensity of crustal geologic events and processes ranging from igneous activity and metamorphism to uplift and exhumation rates. Research during the last funding cycle (2005-2009) focused mainly on development of tools and techniques for characterizing high-temperature processes: i.e., geothermometers for zircon, rutile, titanite and quartz and the diffusion data needed to evaluate their resistance to diffusive resetting. Exploratory efforts were made to investigate the role of defects on oxygen diffusion, and to develop techniques to characterize He diffusion in accessory minerals. Because diffusion of He produced by radioactive decay of U and Th has numerous applications in quantifying time-temperature histories of shallow crustal rocks, future research will shift substantially toward applications in this area. (U-Th)/He thermochronometry is a powerful technique for characterization of thermal histories of geologic systems. Underpinning the technique is knowledge of the mobility (diffusion) of He in accessory minerals (e.g., apatite, zircon, titanite, xenotime and monazite). Measurements of He diffusion in zircon and apatite are generally consistent with information deduced from bulk-release studies of natural crystals containing radiogenic He. In the case of zircon, direct profiling reveals substantial diffusive anisotropy, with faster diffusion rates parallel preferred crystallographic orientations as generally pedicted by molecular dynamics simulations. Future efforts will focus on: 1) identifying the underlying cause for differences between bulk-release and direct-profiling results for apatite; 2) extending the study to other accessory minerals; 3) assessing possible He saturation effects on diffusion; 4) evaluating the role of radiation damage in He diffusion; and 5) development of computer codes for modeling anisotropic diffusion in non-spherical grains. Work will also continue on development of accessory-mineral thermometers, including evaluation of the effect of pressure on the Ti-in-zircon thermometer and calibration of a crystallization thermometer for apatite.The proposed research will provide fundamental information on the behavior of key rare-element minerals in the Earth's crust and enable extraction of information from them regarding crustal processes and evolution. Emphasis is on acquisition of basic experimental data, but systems are strategically targeted in the hope of furthering knowledge in areas of broad significance to the solid-earth sciences. This work provides new tools for addressing petrological and tectonic problems, and will hopefully lead to refinement of molecular dynamics and first-principles modeling of diffusion processes in minerals.Broader impacts. This project will contribute to development of methods to quantify T-t histories of metamorphic and igneous rocks. Because the work focuses upon minerals that contain radioactive elements, results will also be directly relevant the radioactive waste isolation arena (e.g., He diffusion measurements). Moreover, the proposed studies involve development of new experimental methods and crystal-growth techniques that may find applications in other fields (e.g., ceramic engineering). Lastly, this project will support one PhD student and involvement of several undergraduates in aspects of the proposed research.