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

锆石中的锂同位素和矿物包裹体 - 4.4 Ga 至最近

• 批准号: 0838058
• 负责人: John Valley
• 金额: $ 40.44万
• 依托单位: University of Wisconsin-Madison
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-01-01 至 2011-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0838058&HistoricalAwards=false
• 关键词: Lithium; Isotopes; Mineral; Inclusions; Zircons

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.

智力上的优点。超过40亿年(4Ga)的锆石提供了地球上唯一已知的关于最早的地球条件的地球化学记录，他们的研究引发了相当大的兴趣和争议。它们形成于母岩和构造体制中，从完全发育的陆壳中真正的花岗岩(严格意义上说)的湿熔到辉长岩壳中的镁铁质岩石。这个问题的解决将有助于限制板块构造、全尺寸唾液大陆和真正的花岗岩在地球上的出现。锆石被广泛研究为年轻岩石中火成岩、变质作用和沉积过程的时间胶囊。锆石中锂的组成和同位素比值是一种潜在的将流体-岩石相互作用与地质时代联系起来的新工具。我们对锆石中锂的初步研究表明，陆壳中的锆石比洋壳中的锆石富集了1000多倍的[Li]，并且~7Li的值在陆壳中的分馏程度更高。杰克山锆石的成分支持了基于氧同位素比率的假设，即地球上早在4.3Ga就存在海洋和广泛的风化作用。如果是正确的，这表明地球比之前认为的更早适合生命生存。我们建议对锆石中锂的组成[Li]和同位素比(~7Li)以及锆石中的矿物包裹体进行新的研究。锆石将由1-10微米分辨率的离子微探针进行现场分析。主要目标将是：1)更好地了解锆石中Li替代和交换的系统学2)系统地识别锆石中的矿物包裹体，以此作为确定碎屑状锆石赋存岩石的工具，以及3)限制来自西澳大利亚杰克山的4.4-4.0Ga碎屑状锆石的成因。来自选定的年轻地体的锆石，包括镁铁质和长英质洋壳，也将被研究为目前失踪的4Ga母岩的可能类似物。这些研究和数据将与锆石和寄主岩石的其他地球化学参数相关联，包括：年龄、氧同位素比率、微量元素、分带和其他放射性成因同位素系统。其中包括研究生和博士后研究员的培训。该项目在培养学术研究人员方面非常成功；20名毕业生拥有研究或教学岗位，10名毕业生建立了自己的稳定同位素实验室。这笔赠款将是华盛顿大学地质稳定同位素实验室的主要业务赠款，并将帮助开发WISC-SIMS IMS-1280离子微探测器的新能力，这些能力是这里提出的稳定同位素研究所需的。