ABSTRACT

Intellectual merit. Zircons that are older than 4 billion years (> 4 Ga) provide the only known geochemical record of conditions on earliest Earth, and their study has stimulated considerable interest and controversy. They are variously interpreted to have formed in parent rocks and tectonic regimes ranging from wet melting of true granites (sensu stricto) in fully developed continental crust to forming in mafic rocks in gabbroic crust. Resolution of this question would help constrain the emergence on Earth of plate tectonics, full-size sialic continents, and true granites. Zircons are widely studied as time capsules of igneous, metamorphic, and sedimentary processes in younger rocks. Lithium compositions and isotope ratios in zircon are potentially a powerful new tool for coupling fluid-rock interaction to geologic time. Our preliminary lithium studies in zircons show that [Li] is enriched by a factor of over 1000 in zircons from continental crust vs. zircons from ocean crust and that values of _7Li are much more fractionated in continental crust. The compositions of Jack Hills zircons support the hypothesis, based on oxygen isotope ratios, that oceans and extensive weathering existed on Earth as early as 4.3 Ga. If correct, this indicates that Earth was hospitable for life much earlier than previously thought. We propose new studies of lithium composition [Li] and isotope ratio (_7Li) in zircons and of mineral inclusions in zircons. Zircons will be analyzed in situ by ion microprobe at 1 - 10 micron resolution. The primary goals will be to:
1) better understand the systematics of Li substitution and exchange in zircon
2) systematically identify mineral inclusions in zircons as a tool to determine host rock for detrital zircons, and
3) constrain the genesis of 4.4 to 4.0 Ga detrital zircons from the Jack Hills, Western Australia. Zircons from selected younger terranes, including mafic and felsic ocean crust, will also be studied as possible analogs for the now missing >4 Ga parent rocks. These studies and data will be correlated to other geochemical parameters of zircons and host-rocks including: age, oxygen isotope ratio, trace elements, zoning, and other radiogenic isotope systems.

Broader impacts. These include training of graduate students and postdoctoral fellows. This program has been very successful in training academic researchers; 20 graduates have research or teaching positions and 10 have established their own stable isotope laboratories. This grant will be the main operating grant of the UW Geology Stable Isotope Laboratory and will help develop the new capabilities of the Wisc-SIMS ims-1280 ion microprobe that are required by the stable isotope research proposed here.

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