Zoning of Oxygen Isotope Ratio Within Single Zircons

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

This renewal proposal is to support in situ ion microprobe studies of single zircons from extensive collections at the University of Wisconsin-Madison, which include the oldest zircons on Earth, plus many zircons from peraluminous magmas and nested caldera complexes. The major scientific goals for this next increment of funding will include the analysis of Earth's oldest zircons, 4.4-3.8 Ga, and their mineral inclusions; investigations of maturation and evolution of continental crust through time; analysis of zircon, garnet and titanites from Archean and younger granitoids; correlation of growth zoning of magmatic d18O with other geochemistry in zircon to determine mechanisms and timing of contamination in felsic magma chambers; and undergoing empirical tests of oxygen diffusion rate in zoned zircon. Analysis of oxygen isotopes in zircon provides a valuable petrologic tool. The Wisconsin Secondary-Ion Mass-Spectrometer facility will add a new, potentially revolutionary, dimension to these studies of zircon. In situ analysis of oxygen isotope ratios will provide quantitative measurement of intra-crystalline diffusion profiles, growth zoning, inheritance, and radiation damage. Such information has not previously been available. These studies will provide new insight into processes of magma genesis, anatexis, metamorphism, and the maturation and growth of continental crust. Studies of early Archean (4.4-3.8 Ga) zircons provide the only direct evidence of the earliest conditions on Earth. This project will test the recent hypotheses of a "Cool Early Earth", and the presence of continental crust, liquid water, and oceans 200 million years after the formation of the Solar System and the accretion of the Earth. This information is necessary to answer the question: When was Earth first habitable for life? Broader impacts of this project include research and publication of scholarly work, graduate and undergraduate education, outreach through the University of Wisconsin Geology Museum, and development of new techniques for studies of problems of societal importance including CO2 sequestration and global warming.