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.

科学家和普通大众都为我们所居住的大陆的古老而着迷。第一个大型大陆块体是什么时候出现的？大陆是如何随着时间的推移而生长的？早期地球的环境是怎样的？这些都是许多人思考的问题。具体来说，这些问题没有简单的答案，并引发了激烈的争论。在许多大陆的小区域里保存着最古老的结晶基岩，它们是地球信息的重要来源。美国最早的历史。该项目旨在了解大陆地壳在地球历史的第一个十亿年中是如何生长的，并专注于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）。许多人认为这块地体长有地球上已知最古老的花岗岩，因此它是了解地球早期历史的重要信息来源。然而，AGC不是一个简单的岩石包，从这些岩石中提取可靠的同位素信息需要一种面向细节的方法。这些岩石值得注意的是，它们在任何方面都没有出现异常，这在一定程度上支持了产生这些岩石的过程与今天正在进行的过程相似的观点。然而，它们中的许多都超过39亿年，有些甚至有40亿年的历史。考虑到地球的年龄被认为是45亿年多一点，这些岩石可以让我们追溯到很久以前的地球历史。研究小组希望在这项研究中发现更古老的岩石。这一提议将两位对地壳形成的早期历史有着根本不同看法的研究者聚集在一起，但他们对早期地球有着强烈的好奇心。地球历史的前10亿年不仅对了解大陆岩石圈的生长、成熟和保存至关重要，而且对评估现代地球上保存的地球化学储层的性质和年龄也至关重要。地球?地球最早的历史是一个有争议的问题，争论的焦点是在约4.0 Ga之前是否有大量的大陆地壳和相应的大型全球衰竭地幔储层。在一组成分不同的岩石上进行Hf和Nd同位素系统研究对于理解地球最初10亿年的形成至关重要。年代进化。本研究旨在确定锆石的U-Pb年代和Hf同位素组成，以及年龄在3.4 - 4.0 Ga之间的一组岩石的Hf和Nd全岩石同位素组成。我们将采用多管齐下的方法：通过ID-TIMS确定锆石和寄主岩石的U-Pb日期；ID-TIMS U-Pb工作遗留溶液中锆石的Hf同位素组成；LA-MC-ICPMS分析得到的锆石Hf同位素组成与分流法得到的U-Pb数据吻合；从锆石U-Pb和Hf同位素组成最不复杂的锆石开始，研究整个岩石的Hf和Nd同位素组成。这些方法将有助于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，通过确定约束良好的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。目标是提取出这些岩石来源于的地壳和地幔储层性质的可靠约束。