Evolution of Earth's Earliest Crust

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Earth's continental and oceanic crust, the combination of which is likely unique in the solar system, plays a central role in the operation of our planet's lithosphere, atmosphere, and biosphere. The mechanisms by which this crust formed and stabilized early in Earth history remains one of the outstanding questions in the geological sciences. In this application, I propose to address this question by expanding my on-going research program on the Acasta Gneiss Complex (AGC) of the Northwest Territories. The AGC arguably contains the oldest rocks in the world (4.02 billion years old) and is certainly one of the few locations worldwide where evidence can be brought to bear on the first one billion years of Earth history. Despite its status as an old crust locality, much of the AGC remains unstudied. In particular, all previous studies of the AGC have focused exclusively on a ~30 km2 area, which makes up a small fraction of the complex as a whole (~1300 km2). The geology of the broader AGC is largely unknown but reconnaissance work by my group indicates that at least 700 km2 of the complex comprises rocks older than 3.3 Ga. I propose to increase our understanding of this important terrane through two complementary studies. The first study involves careful bedrock mapping of two heretofore unmapped 5 x 5 km areas in the broader AGC so as to document the field relationships between various rock units and identify the optimal samples for subsequent laboratory work. A sound field context is essential for the correct interpretation of state-of-the-art geochemical data that will be obtained on samples collected of the major rocks units in the complex. These include major- and trace-element and neodymium, lead and strontium isotope data on whole-rock samples, and uranium-lead, oxygen and hafnium isotope data on grains of the mineral zircon extracted from representative samples. The aim of these geochemical studies is to constrain the age and formation history of AGC rocks and in turn gain insights on crust-formation processes in the early Earth. The second study will take a novel approach of using the zircon age distributions and strontium-neodymium-lead isotope compositions of glacial sediment collected from across the AGC to derive an estimate of the relative proportions of different rocks suites present in the complex as a whole. These data will provide insight into when the majority of the AGC formed as well as the dominant rock compositions represented in the complex.**