This project will investigate the magmatic history of the Earth's crust over the past 4.4 billion years. The mineral zircon will be analyzed for oxygen isotope ratio, U-Pb age, and rare earth element concentration. The high melting temperature, chemical inertness, and hardness of zircon creates a time capsule of geochemical information that lasts virtually forever in samples with low amounts of radiation damage. New ion microprobe and laser techniques for high precision and accuracy, and/or high spatial resolution will be employed. Single domains within zoned zircons will be analyzed. Igneous minerals will be studied from selected terranes of early Archean, Proterozoic and Mesozoic through Cenozoic age. Archean samples will include detrital zircons from the Jack Hills, Western Australia, where we discovered the oldest known sample of the Earth in the past grant period. The younger samples will document processes of crustal reworking in the Idaho and Sierra Nevada batholiths of the western U.S. Analyses of zircon will be compared to new data for magmatic garnet, baddeleyite, titanite, epidote, rutile and quartz. In combination, these results will provide a record of evolving compositions including pre-magmatic xenocrysts, magmatic phenocrysts and a range of post magmatic alteration minerals.
These studies of zircons and associated igneous minerals will have broad significance for understanding the origin and evolution of the Earth's crust. Studies of the Archean (4.0 to 4.4 Ga) zircons provide the only evidence of the earliest conditions on Earth. The recent hypothesis of a "Cool early Earth" will be tested, which has implications for the timing of the first oceans and the origin of life.
