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

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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.