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

科学家和普通大众都对我们生活的大陆的古老着迷。第一批大型陆块是什么时候出现的？大陆是如何随着时间的推移而成长的？早期地球上的情况是怎样的？这些都是许多人深思的问题。具体而言，这些问题并没有简单的答案，并引发了太多激烈的辩论。保存在许多大陆小区域中的最古老的结晶基底岩是地球最早历史上的重要信息来源--S。该项目旨在了解大陆地壳在地球历史的最初10亿年中是如何生长的，并专注于对地球上一些最古老的岩石的研究，这些岩石是加拿大北部奴隶省的Acasta片麻岩杂岩(AGC)。这个地体拥有被许多人认为是地球上已知的最古老的花岗岩，因此是关于地球最早历史的至关重要的信息来源。然而，AGC并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个以细节为导向的方法。这些岩石值得注意的是，它们看起来并没有任何不同寻常的地方，这在一定程度上支持了产生这些岩石的过程与今天的操作过程相似的观点。然而，它们中的许多都超过了39亿年，有些甚至超过了40亿年。考虑到地球的年龄被认为是45亿年多一点，这些岩石让我们回到了地球的历史非常久远。研究团队希望，在这项研究中，甚至可以发现更古老的岩石。这一提议汇集了两位对早期地壳形成历史有着根本不同看法的研究人员，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，而且对于评价现代地球上保存下来的地球化学库的性质和时代也是至关重要的。地球？S最早的历史是一个有很大争议的问题，争论的焦点是大约4.0Ga是否存在大量的陆壳和与之对应的大型全球亏损地幔储集层。一套成分多样的岩石上的Hf和ND同位素系统学对于理解地球第一个十亿年的演化--S来说是至关重要的。这项建议旨在确定AGC年龄为3.4~4.0Ga的一组岩石的锆石U-Pb年龄和Hf同位素组成，以及Hf和Nd全岩同位素组成。我们的多管齐下的办法将是确定：ID-TIMS的锆石和主岩的U-Pb年龄；ID-TIMS U-Pb工作剩余溶液中锆石的Hf同位素组成；LA-MC-ICPMS获得的锆石的Hf同位素组成与使用分裂流法获得的U-Pb数据一致；以及全岩的Hf和ND同位素组成，从最不复杂的锆石U-Pb和Hf同位素组成开始。这些方法相结合，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品。相反，通过确定严格约束的锆石U-Pb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。其目标是提取对这些岩石来源的地壳和地幔储集层的性质的强有力的限制。

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 Earthhistory 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 rocksare 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. Thisproposal 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 compromiseinterpretations. 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.