Zircon is an indispensable tool for precise dating of events in Earth history, and recent advances in zircon oxygen isotope analysis have laid the foundation for its further use as a precise archive of solid and surface Earth and planetary evolution including the first appearance of hydrospheres. Full application of this technique is hampered, however, by uncertainty as to whether primary zircon oxygen isotope compositions survive the 'wet' (water vapor present in trace amounts) high temperature regional metamorphism common in ancient and active continental crust. Moreover, very little is known of oxygen behavior in purely metamorphic zircon widely recognized to grow in common rock types such as basalt. Our project will employ field and analytical research in a cross-section through the North American crust to determine oxygen diffusion behavior in zircon, a key mineral for exploring the nature, origin and evolution of the Earth's crust and mantle. The project will deliver answers to two outstanding questions in the field: 1) Does zircon exhibit 'wet' (i.e. rapid) oxygen diffusion in nature? Calculated and experimentally determined 'wet' diffusion rates vary widely; 2) What are the oxygen systematics of metamorphic zircon? Oxygen isotope studies of metamorphic zircon are presently few in number and combined in situ U-Pb and oxygen analysis has the potential to date different stages in the evolving metamorphic fluid, or, alternately, provide unique new information on the duration of metamorphic cycles. Our definitive test for rapid, 'wet' oxygen diffusion in zircon is to make in situ U-Pb, oxygen, and trace element measurements of initially low d18O zircon crystals in a high d18O metasediment unit at three places in the Kapuskasing Uplift (Superior Province) - one of the rare such exposures where this is possible worldwide. The zircon in our metasediment samples will be the benchmark monitor of oxygen diffusion across the exposed crustal cross-section. To achieve objective 2, we will apply the same analytical tools plus a new Ti geothermometer to metamorphic zircon from metabasalt adjacent to the metasediment samples. To the best of our knowledge, no such comprehensive tests of calculated, experimental, and empirical predictions for oxygen diffusion in zircon, igneous or metamorphic, have been conducted in regionally metamorphosed rocks. The broader impacts include comprehensive skills transfer to a doctoral student in the areas of field analysis, petrology and/or geochemistry, in addition to technical writing and communication skills. This will lead to development of new researchers trained in the careful integration of field analysis and control with state of the art micro-imaging and micro-geochemistry.
