The onset of plate tectonics 3 billion years ago? - New insights from Ca-Sr-Pb-Nd isotopes and implications for large-scale crustal evolution

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The continental crust is the archive of the processes and events that have controlled our planet's evolution for the last 4.4 billion years. Its formation modified the composition of the mantle and the atmosphere, it supports life, and it remains a sink for carbon dioxide through weathering and erosion. The continental crust has therefore had a key role in the evolution of the Earth, and yet the timing of its generation, and the processes that shaped its evolution, remain a topic of considerable debate.Views on the evolution of the continental crust have changed dramatically as ideas on geological processes have evolved and as methods to interrogate the rock record have advanced through developments in stratigraphic analysis, petrography, paleontology, geochemistry, geochronology, geophysics and modelling. Crucially our understanding of the processes involved in the generation and the evolution of the continental crust has grown enormously through the latter part of the 20th and beginning of the 21st centuries following on from the development and acceptance of plate tectonic theory.A fundamental question is to determine when plate tectonics started, and a few studies have recently argued that this major change in Earth's evolution may have happened 3 billion years ago during the Mesoarchaean. This model of Earth's evolution remains highly controversial and much debated by the community. The validation of this model is therefore a key step in understanding the large-scale evolution of the Earth, through repeated interaction between the mantle, the continental crust and the atmosphere. This proposal is to explore the processes of generation, maturation and growth of the continental crust through time, using a new geochemical approach based on the analysis of various isotopes systems with different geochemical properties and decay constants (K-Ca, Rb-Sr, U-Pb and Sm-Nd), in a large range of rocks, minerals and sediments with pre- and post-3 Ga ages. The data will allow to establish the composition of the continental crust at different stages of its evolution, as well as the variation in the balance between subduction-related and intraplate- related magmatism through time, with a confidence that has not been achieved hitherto. A sharp transition from intraplate-related to subduction-related magmas at 3 billion years ago, along with high rates of crust recycling from this time, would constitute clear evidence for the global onset of plate tectonics at this time.