The Paleoarchean: A Pivotal Time in Earth Crustal Evolution--the View from the Zimbabwe and Sao Francisco Cratons.

几十年来，地球大陆的形成和生长一直是地球科学家们着迷的话题，也引发了激烈的争论。关于地球最早期稳定大陆的形成和演化的问题在文献中已经激烈辩论了50多年，并继续成为当前研究的焦点。太古代岩石（年龄超过25亿年）代表了地球近一半的历史，但只出现在大陆的少数地区。古太古代（36亿至32亿年）和始新世（40亿至36亿年）的岩石更为罕见，仅占全球所有保存的太古代岩石的一小部分。这些古太古代和更古老的岩石代表了早期地球和古代大陆的形成，性质和演变的窗口。该提案旨在研究津巴布韦和弗朗西斯科（巴西）的地壳形成，修改和稳定的记录，这两个是地球上保存最完好和暴露的古太古代大陆地壳碎片。对早期地球的研究，以及有关现代板块构造开始的时间和过程的问题，已被列为几个基于社区的文件的优先事项，包括“&构造学研究的挑战和机遇”（亨廷顿&Klepeis，2018年）和“NSF地球科学2020-2030年愿景：美国国家科学、工程和医学院（National Academies of Sciences，Engineering，and Medicine，2020）本提案采取的综合办法将扩大太古代古生物研究的地理范围，改进地球最早期历史的记录，并将有助于了解早期地球的全貌。该提案还将通过支持一名拉丁裔博士生从事科学工作，为建立一个更加多样化的地球科学社区做出贡献。与巴西和南非同事的合作是该研究项目的一个组成部分。这项研究计划旨在整合同位素地球化学和地质年代学，研究3.65-2.70 Ga津巴布韦和弗朗西斯科（SF）的火山，这两个重要的太古代火山，尽管有广泛的太古代地质记录，但相对较少受到关注。津巴布韦和旧金山是地球上保存最完好的古太古代大陆地壳的暴露部分。这些岩石还保存了圆顶和龙骨结构，这是太古代地壳的原型。本研究的目的是限制津巴布韦克拉通和Gavião块体和片麻岩杂岩从北方和南方SF克拉通的中央部分的演变。研究的方法是：1）确定这些地区古太古代地壳形成的时间尺度; 2）确定这些片麻岩的Hf和Nd同位素记录及其随时间的演化; 3）评价片麻岩杂岩的Lu-Hf和Sm-Nd同位素系统，以确定它们是否忠实地记录和保存了它们的初始同位素组成。这项工作将采用锆石、钛铁矿和其他富稀土副相的原位U-Pb年龄和Lu-Hf/Sm-Nd同位素分析的综合方法，结合大块岩石样品的溶液Lu-Hf和Sm-Nd同位素稀释分析来完成。在可能的情况下，石榴石Lu-Hf和Sm-Nd地质年代学，结合年龄和同位素数据从辅助阶段，将被用来记录后岩浆构造热事件。这种综合的地球化学和地质年代学方法将提供一个时间框架的地壳形成事件和一个强大的Hf和Nd同位素记录的津巴布韦和SF火山。最终，这将有助于该社区评估古太古代地壳形成，修改和稳定的过程，这是地球地壳演化的关键时期。这项拟议的研究代表了PI，学生和博士后在格陵兰岛，在Pastasta片麻岩复合体中所做的工作的延续，以及在澳大利亚西北部的Pilbara地区所做的工作（并且仍在进行中）。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

The creation and growth of Earth’s continents is a topic that has captivated geoscientists—and engendered spirited debate—for decades. Questions about the formation and evolution of stable continents during Earth’s earliest history have been vigorously debated in the literature for over 50 years and continue to be a focus of current research. Archean rocks (those older than 2.5 billion years) represent nearly half of Earth’s history, but only occur in a few areas on the continents. Rocks from the Paleoarchean (3.6 to 3.2 billion years) and Eoarchean (4.0 to 3.6 billion years) are considerably more rare and represent only a small percentage of all preserved Archean rocks worldwide. These Paleoarchean and older rocks represent windows into the early Earth and the formation, nature, and evolution of the ancient continents. This proposal seeks to study the record of crustal formation, modification, and stabilization of the Zimbabwe and São Francisco (Brazil) cratons, two of the best preserved and exposed fragments of Paleoarchean continental crust on Earth. The study of the early Earth, as well as questions regarding the time and processes involved in the initiation of modern plate tectonics, have been included as a priority in several community-based documents, including the “Challenges & Opportunities for Research in Tectonics” (Huntington & Klepeis, 2018) and, “A vision for NSF Earth Sciences 2020-2030: Earth in time” report of the NSF (National Academies of Sciences, Engineering, and Medicine 2020). The integrated approach taken in this proposal will enhance the geographic coverage of studies on Archean cratons and improve the record from the earliest history of the Earth and will contribute to the understanding of the whole picture of the early Earth. This proposal will also contribute to the building of a more diverse geoscience community by supporting women in science with one PhD Latinx student. Collaboration with Brazilian and South African colleagues is an integral part of this research project. This research proposal seeks to integrate isotope geochemistry and geochronology to study the 3.65-2.70 Ga Zimbabwe and São Francisco (SF) cratons, two important Archean terranes that have received relatively little attention despite their extensive Archean geological record. The Zimbabwe and SF cratons are two of the best preserved and exposed portions of unmodified Paleoarchean continental crust on Earth. These cratons also preserved dome and keel structures, archetypal of Archean crust. The aim of this research is to constrain the evolution of the central portion of the Zimbabwe craton and the Gavião block and gneiss complexes from the northern and southern SF craton. The approach in this research is to: 1) determine the timescale of crust formation through the Paleoarchean in these areas; 2) determine the Hf and Nd isotope record of these cratons and how they have evolved through time; and 3) evaluate the Lu-Hf and Sm-Nd isotope systematics of the gneiss complexes to determine if they have faithfully recorded and preserved their initial isotope compositions. This work will be done using an integrated approach of in-situ U-Pb age and Lu-Hf/Sm-Nd isotope analysis of zircon, titanite, and other REE-rich accessory phases, combined with solution Lu-Hf and Sm-Nd isotope dilution analyses of bulk rock samples. Where possible, garnet Lu-Hf and Sm-Nd geochronology—in conjunction with age and isotope data from accessory phases—will be used to document post-magmatic tectono-thermal events. This integrated geochemical and geochronological approach will provide a timeframe of crust formation events and a robust Hf and Nd isotope record of the Zimbabwe and SF cratons. This, ultimately, will help the community to evaluate processes of crust formation, modification, and stabilization in the Paleoarchean—a pivotal time in Earth’s crustal evolution. This proposed research represents a continuation of the work that has been done by the PI, students, and post-docs in Greenland, in the Acasta Gneiss Complex, and what has been done (and is still ongoing) in the Pilbara Craton of NW Australia.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.