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

科学家和公众都对我们所居住的大陆的古老着迷。第一个大的大陆块是什么时候出现的？大陆是如何在时间中成长的？早期地球的情况是怎样的？这些都是许多人思考的问题。具体而言，这些问题没有简单的答案，并引发了激烈的辩论。在许多大陆的小区域中保存的最古老的结晶基底岩石是地球信息的重要来源。最早的历史。该项目旨在了解大陆地壳在地球历史的头十亿年是如何生长的，重点是研究地球上最古老的岩石，即加拿大北方奴隶省的阿里斯塔片麻岩复合体。这座花岗岩被许多人认为是地球上已知最古老的花岗岩，因此是最早的地球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。这些岩石的非凡之处在于，它们看起来并不罕见，并为产生这些岩石的过程与今天的操作类似的想法提供了一些支持。然而，它们中的许多年龄超过39亿年，有些年龄高达40亿年。考虑到地球的年龄被认为是45亿年多一点，这些岩石让我们在地球的历史上非常遥远。研究人员希望在这项研究中能够识别出更古老的岩石。这一提议汇集了两个对地壳形成的早期历史有着根本不同观点的研究者，但他们对早期地球非常好奇。地球历史的前10亿年不仅对于理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储层的性质和年龄也至关重要。地球？中国最早的历史是一个有很大争议的话题，争论的焦点是大约在公元前是否有大量的大陆地壳。4.0 Ga和相应的大型全球亏损地幔储集层。Hf和Nd同位素系统的组成不同的岩石套件是必不可少的了解地球的第一个十亿年？的进化。本文对一套年龄约为1000 ~ 10000年的岩石进行了锆石U-Pb年龄、Hf同位素组成和全岩Hf、Nd同位素组成的测定。3.4从AGC到4.0 Ga。我们将从多个方面着手：用ID-TIMS测定锆石和寄主岩石的U-Pb年龄，用ID-TIMS U-Pb工作的剩余溶液测定锆石的Hf同位素组成，用LA-MC-ICPMS测定锆石的Hf同位素组成，并与用分流法测定的U-Pb数据相吻合;全岩的Hf、Nd同位素组成从锆石U-Pb、Hf同位素组成最简单的岩石开始。这些方法，在协调一致，将能够帮助确定样品的年龄和同位素组成的复杂性可能会损害解释。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，他们将能够提供这些岩石的明确的同位素记录。我们的目标是提取强大的约束性质的地壳和地幔水库，这些岩石的衍生。

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.