Collaborative Research: Early Earth Evolution: Hf and Nd Isotopic Constraints from the ca 3.4->4.0 Ga Acasta Gneisses

合作研究：早期地球演化：来自 ca 3.4- 的 Hf 和 Nd 同位素约束

• 批准号: 1321998
• 负责人: Jeffrey Vervoort
• 金额: $ 18.76万
• 依托单位: Washington State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-07-15 至 2017-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1321998&HistoricalAwards=false
• 关键词: Collaborative; Research; Early; Earth; Evolution

Scientists and the general public alike are fascinated by the antiquity of the continents on which we live. When did the first large continental masses appear? How did the continents grow through time? What were conditions like on the early Earth? These are all questions that many ponder. In detail, these questions do not have simple answers and stimulate much heated debate. The oldest crystalline basement rocks preserved in small regions on many of the continents are an important source of information in Earth?s earliest history. This project is designed to understand how continental crust grew during the first billion years of Earth history and is focused on a study of some of the oldest rocks in the planet, which is the Acasta Gneiss Complex (AGC) of the Slave Province in northern Canada. This terrane hosts what are thought by many to be the oldest known granites on Earth and therefore is critically important source of information on the earliest Earth history. The AGC is not a simple package of rocks, however, and extracting reliable isotopic information from these rocks requires a detailed-oriented approach. What is remarkable about these rocks is that they do not appear unusual in any way and lend some support to the idea that the processes that produced these rocks are similar to those operating today. However many of them are older than 3.9 billion years and some as old as 4.0 billion years. Considering that the age of the Earth is thought to be a little more than 4.5 billion years old, these rocks get us very far back in the history of the planet. The team of investigators are hopeful that even older rocks will be identified during this study. This proposal brings together two investigators with fundamentally different views on the early history of crust formation, but who are intensely curious about the early Earth. The first billion years of Earth history is crucial not only for understanding the growth, maturation, and preservation of continental lithosphere, but also for evaluating the nature and age of geochemical reservoirs preserved in the modern earth. Earth?s earliest history is a subject of much controversy, which centers on whether or not there were large volumes of continental crust by ca. 4.0 Ga and a corresponding large global depleted mantle reservoir. Both Hf and Nd isotope systematics on a compositionally diverse suite of rocks are essential for understanding the first billion years of Earth?s evolution. This proposal seeks to determine U-Pb dates and Hf isotopic compositions of zircon as well as Hf and Nd whole rock isotopic compositions from a suite of rocks ranging in age from ca. 3.4 to 4.0 Ga from the AGC. Our multi-pronged approach will be to determine: the U-Pb dates of zircons and host rocks by ID-TIMS; the Hf isotopic composition of the zircons on the solutions remaining from the ID-TIMS U-Pb work; Hf isotopic composition of the zircons by LA-MC-ICPMS obtained coincidently with the U-Pb data using the split stream approach; and the Hf and Nd isotopic compositions of whole rocks starting with those with least complex zircon U-Pb and Hf isotopic compositions. These methods, in concert, will be able to help identify samples where complexities in the age and isotopic compositions could compromise interpretations. Conversely, by determining well-constrained U-Pb zircon crystallization ages and corresponding isotopic compositions, they will be able to provide an unambiguous isotopic record for these rocks. The goal is to extract robust constraints on the nature of the crustal and mantle reservoirs from which these rocks were derived.

科学家和公众都对我们所生活的大陆的古老着迷。第一个大大陆块何时出现？大陆是如何随着时间的推移而增长的？早期地球的情况是什么样的？这些都是很多人思考的问题。具体来说，这些问题没有简单的答案，并引发了激烈的争论。许多大陆的小区域中保存的最古老的结晶基岩是地球最早历史的重要信息来源。该项目旨在了解地球历史最初十亿年期间大陆地壳如何生长，重点研究地球上一些最古老的岩石，即加拿大北部奴隶省的阿卡斯塔片麻岩杂岩 (AGC) 。该地体被许多人认为是地球上已知最古老的花岗岩，因此是有关地球最早历史的极其重要的信息来源。然而，AGC 并不是简单的岩石包，从这些岩石中提取可靠的同位素信息需要采用面向详细的方法。这些岩石的非凡之处在于它们在任何方面都没有显得异常，并且为这样的观点提供了一些支持：产生这些岩石的过程与今天的操作过程相似。然而，其中许多的年龄超过 39 亿年，有些甚至超过 40 亿年。考虑到地球的年龄被认为略高于 45 亿年，这些岩石让我们可以追溯到地球的历史。研究小组希望在这项研究中能够识别出更古老的岩石。这项提议汇集了两位对地壳形成的早期历史有着根本不同观点的研究人员，但他们对早期地球抱有强烈的好奇心。地球历史的前十亿年不仅对于了解大陆岩石圈的生长、成熟和保存至关重要，而且对于评估现代地球中保存的地球化学库的性质和年龄也至关重要。地球最早的历史是一个备受争议的话题，争议的焦点是大约在公元1世纪左右是否存在大量的大陆地壳。 4.0 Ga和相应的大型全球贫化地幔储层。一组成分多样的岩石中的铪和钕同位素系统学对于了解地球最初十亿年的演化至关重要。该提案旨在确定锆石的 U-Pb 年代和 Hf 同位素组成，以及一组年龄从 100 到 100 岁的岩石的 Hf 和 Nd 全岩同位素组成。来自 AGC 的 3.4 至 4.0 Ga。我们的多管齐下方法将确定： 通过 ID-TIMS 确定锆石和母岩的 U-Pb 年代； ID-TIMS U-Pb 工作剩余溶液中锆石的 Hf 同位素组成； LA-MC-ICPMS 获得的锆石 Hf 同位素组成与使用分流方法获得的 U-Pb 数据一致；以及从具有最复杂的锆石 U-Pb 和 Hf 同位素组成的岩石开始的整个岩石的 Hf 和 Nd 同位素组成。这些方法协同作用，将能够帮助识别年龄和同位素组成的复杂性可能影响解释的样本。相反，通过确定严格约束的 U-Pb 锆石结晶年龄和相应的同位素组成，他们将能够为这些岩石提供明确的同位素记录。目标是提取对这些岩石所来源的地壳和地幔储层性质的严格约束。