Résumé
Recent models on continental growth suggest that ca. 65% of the present volume of the continental crust was present by 3 Ga, and that the rates of continental growth were significantly higher before 3 Ga than subsequently. This change has been tentatively linked to the onset of subduction-driven plate tectonics and discrete subduction zones. If correct this represents a fundamental change in the evolution of the Earth, with implications for the nature of the magmas generated, the efficiency with which crustal material is returned back into the mantle and the cooling history of the Earth. Because of the poor preservation of rocks and minerals after billions of years of crustal evolution, a major uncertainty remains about the composition of new, juvenile continental crust in the Hadean and the Archaean and hence the conditions and the tectonic setting(s) in which it was generated. One way forward is to evaluate the composition of new continental crust from the time-integrated parent/daughter ratios of isotope systems in magmatic rocks subsequently derived from that new crust. Crustal differentiation processes produce a large range of highly fractionated Rb/Sr ratios because of the highly incompatible character of the Rb-Sr system. As a consequence mafic crust typically has Rb/Sr at least five times lower than intermediate/felsic bulk crust. We calculated time-integrated Rb/Sr in crustal material with pre- and post-3 Ga Nd model ages. Preliminary data indicate that time-integrated Rb/Sr were, on average, much lower in the Hadean/Mesoarchaean than subsequently. This suggests that new continental crust was principally mafic over the first 1.5 Ga of Earth evolution, that a large volume of pre-3 Ga crust may have been associated with intraplate magmatism, and that approximately 3 Ga may indeed mark the onset of plate tectonics on Earth.