Building Earth's oldest crust: geochemistry and geochronology of Archean cratons and early mantle

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Ancient terrains provide the only constraints on the nature and evolution of Earth's earliest crust. Only few terrains older than 3.5 billion years old (Ga) are preserved, limiting our understanding of the processes that formed our first continents. Moreover, with its active tectonics, the Earth is very efficient at reworking and recycling its own crust. The primitive crust, formed shortly after Earth's formation, has therefore mostly been destroyed and re-melted to form younger rocks. My main objective is to constrain the chemical nature and geological processes that formed the first terrestrial crust as well as the impact on the early chemical evolution of Earth's mantle. Archean cratons are the most ancient cores of our continents. They are mostly composed of rocks formed between 2.5 to 3 Ga, leaving more than the first billion years of Earth's crustal history difficult to study. This Archean crust can, however, contain the isotopic signature of its older precursor. With the increasing precision of analytical techniques, we can use new isotopic tools to better extract information from ancient terranes. This will allow us to build an improved picture of how and when our first continents formed and investigate the nature of Earth's primitive crust. This project combines multiple geochemical and isotopic tools to understand the earliest crustal history, leading to the stabilization of the first continents. Isotopic systems such as 147Sm-143Nd and 176Lu-176Hf have successfully been used to study the evolution of the crust. However, because these tracers are also sensitive to geological processes occurring well after the formation of the early Archean crust, they can often be disturbed and thus make the earliest crustal history difficult to decipher. This approach also often requires that researchers make assumptions about the nature of the older crust that served as a precursor source to build Archean cratons. Coupling conventional geochronology and long-lived isotopic systems to novel short-lived isotopic systems such as 146Sm-142Nd can provide a much better understanding of the early crustal history. The 146Sm-142Nd system is only sensitive to geological processes occurring before 4 Ga and is thus the ideal tracer to study the involvement of primitive crust in the formation of Archean cratons. This system allows us to clearly establish whether Hadean crust or mantle were involved in the formation of the Archean continents. Combining short-lived and long-lived isotopic systems will give us the most complete understanding of how our oldest continents formed. The main areas of research will be 1) the Superior craton (Northern Quebec, Ontario and Manitoba) representing one of the largest extents of Archean crust and possibly including the oldest rocks on Earth; 2) the Nain Province (Labrador) with rocks as old as 3.9 Ga; 3) the Archean crust from Brazil, which is still largely unstudied; and 4) the Pilbara craton (Australia), as old as 3.5 Ga.