Lithium Isotopes and Mineral Inclusions in Zircons- 4.4 Ga to Recent

知识价值。年龄超过40亿年（4ga）的锆石提供了唯一已知的早期地球条件的地球化学记录，它们的研究引起了相当大的兴趣和争议。它们被解释为形成于母岩和构造体系，从完全发育的大陆地壳中真花岗岩的湿熔融（严格意义上的）到辉长岩地壳中的基性岩石。这个问题的解决将有助于限制地球上板块构造、全尺寸的唾液大陆和真正的花岗岩的出现。锆石作为火成岩、变质岩和沉积过程的时间胶囊被广泛研究。锆石中的锂成分和同位素比值可能是研究流体-岩石相互作用与地质年代耦合的有力新工具。我们对锆石中锂元素的初步研究表明，大陆地壳锆石中[Li]的富集程度是洋壳锆石的1000倍以上，大陆地壳中[Li]的分馏程度要高得多。杰克山锆石的组成支持了基于氧同位素比率的假设，即早在4.3 Ga时地球上就存在海洋和广泛的风化作用。如果正确的话，这表明地球适合生命存在的时间比之前认为的要早得多。我们提出了新的锆石锂组成[Li]和同位素比值（_7Li）以及锆石矿物包裹体的研究。锆石将用1 - 10微米分辨率的离子探针原位分析。主要目标是：1)更好地了解锆石中Li取代和交换的系统学；2)系统地识别锆石中的矿物包裹体，作为确定碎屑锆石寄主岩石的工具；3)限制西澳大利亚杰克山4.4 ~ 4.0 Ga碎屑锆石的成因。还将研究从较年轻的地层（包括基性和长英质海洋地壳）中提取的锆石，作为目前缺失的4亿年母岩的可能类似物。这些研究和数据将与锆石和寄主岩的其他地球化学参数相关联，包括：年龄、氧同位素比、微量元素、分带和其他放射性成因同位素系统。更广泛的影响。其中包括培养研究生和博士后。这个项目在培养学术研究人员方面非常成功；20名毕业生从事科研或教学工作，10名毕业生建立了自己的稳定同位素实验室。这笔拨款将成为威斯康星大学地质稳定同位素实验室的主要运营拨款，并将帮助开发威斯康星-西姆斯ims-1280离子微探针的新功能，这是这里提出的稳定同位素研究所需的。

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 zircon2) systematically identify mineral inclusions in zircons as a tool to determine host rock for detrital zircons, and3) 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.