Discovering Hadean Earth

知识价值。地球历史的前5亿年——冥古宙——是一段“失落”的时间，因为实际上没有任何岩石记录保存下来。然而，PI和同事们有来自西澳大利亚（杰克山）的碎屑和异晶锆石样本，年龄在4.0到4.4 Ga之间。一项大规模的分析运动已经确定了成千上万个这样的古老锆石，对这些颗粒的研究已经被解释为地球表面存在液态水，大陆地壳的显著增长，成熟的沉积物循环系统，以及在冥古宙期间可能发生的板块边界型过程。这些证据表明，这颗非常早期的行星可能比以前认为的更像今天的地球，这刺激并证明了新的测量是正确的。提出了分析发展、分析程序和实验，以解决以下问题：1)冥古宙杰克山锆石可能是由撞击形成的吗？2)冥古宙锆石是否记录了与~3.9 Ga月球大灾变相对应的地球事件？3) 3.9 ~ 4.3 Ga锆石高富集（低Lu/Hf）储层发生了什么变化？4) 4ga锆石的矿物包裹体是否保留了原始冥古宙记录？5)现有的气压计是否适用于冥古宙锆石的包裹体？6)冥古宙锆石的Pu-U变化是否反映了水介导的分馏作用？7)这些同位素、矿物学和测温数据是否是冥古宙大陆再循环和板块边界相互作用的证据？建议的调查包括：1)激光烧蚀ICPMS测量3.9-3.6 Ga锆石中初始176Hf/177Hf， 2) 4 Ga锆石的U-Th-Pb离子探针深度剖面，3)陆地撞击熔体中形成的锆石的表征，4)裂变Xe同位素的冥古宙Pu-U地球化学，5)Jack Hills锆石的Nd同位素系统，6)微米级白云母中Si/Al的NanoSIMS分析，7)冥古宙白云母包裹体的K-Ca定年，8)过铝花岗岩的Si-in-白云母气压计的校准。更广泛的影响。这项研究将直接惠及加州大学洛杉矶分校的研究生。一旦接受了离子微探针的培训，这些学生反过来为nsf支持的凯克同位素地球化学中心（见下文）的访客（包括许多研究生）提供支持。已经确定了几个小型项目，将用于产生本科生的研究机会。如果冥古宙范式被推翻，研究成果将引起国际关注，可能类似于行星探索任务的结果。该项目展示了全球关注领域的科学和技术领导地位，有助于激发考虑从事科学和技术职业的年轻人的想象力，并为年轻科学家提供培训机会。研究成果将通过高引用影响力的科学期刊传播给科学界，并通过持续的外联活动传播给公众（见http://sims.ess.ucla.edu/outreach/outreach.php）。

Intellectual merit. The first 500 million years of Earth history - the Hadean Eon - is a period of 'lost' time as there is virtually no rock record preserved from this interval. However, the PI and colleagues have samples of detrital and xenocrystic zircons from western Australia (Jack Hills) with ages between 4.0 and 4.4 Ga. A massive analysis campaign has identified many thousands of these ancient zircons, and investigations of these grains have been interpreted to suggest the presence of liquid water at the Earth's surface, significant growth of continental crust, a mature sediment recycling system, and perhaps operation of plate boundary-type processes during the Hadean. Such evidence suggests the possibility that the very early planet was much more similar to the present day Earth than previously thought and this stimulates and justifies new measurements. Analytical developments, analysis programs, and experiments are proposed to address the following questions: 1) Could Hadean Jack Hills zircons have been formed by impacts? 2) Do Hadean zircons record the terrestrial counterpart to the ~3.9 Ga lunar cataclysm? 3) What happened to the highly enriched (low Lu/Hf) reservoir seen in 3.9- 4.3 Ga zircons? 4) Do mineral inclusions in 4 Ga zircons retain a pristine Hadean record? 5) Are available barometers applicable to inclusions in Hadean zircons? 6) Do Pu-U variations in Hadean zircons reflect water-mediated fractionation? and 7) Are the collective isotopic, mineralogical, and thermometry data evidence of Hadean continental recycling and plate boundary interactions? Proposed investigations include: 1) Laser ablation ICPMS measurement of initial 176Hf/177Hf in 3.9-3.6 Ga zircons, 2) U-Th-Pb ion microprobe depth profiling of 4 Ga zircons, and 3) characterization of zircons formed in terrestrial impact melts, 4) Hadean Pu-U geochemistry from fission Xe isotopes, 5) Nd isotopic systematics of Jack Hills zircon, 6) NanoSIMS analysis of Si/Al in micron-sized muscovite, 7) K-Ca dating of Hadean muscovite inclusions, and 8) calibration of the Si-in-muscovite barometer forperaluminous granitoids.Broader Impacts. This research will directly benefit the UCLA graduate students supported by this project. Once trained on the ion microprobe, these students in turn provide support to visitors (including numerous graduate students) to the NSF-supported Keck Center for Isotope Geochemistry (see below). Several small projects have been identified that will be used to engender undergraduate research opportunities.Research outcomes are of international interest and could be akin to the results of a planetary exploration mission if the Hadean paradigm is overturned. This project showcases scientific and technical leadership in a field of global interest, aids in capturing the imagination of young people considering a career in science and technology, and provides training opportunities for young scientists. Research results will be communicated to the scientific community via high citation impact scientific journals and to the public through sustained outreach activities (see http://sims.ess.ucla.edu/outreach/outreach.php).