Abstract
Unravelling the evolution of the continental crust trough time is fundamental to understand the interaction between Earth’s geochemical reservoirs as well as global changes in tectonic regimes. The composition and the tectonic setting(s) of formation of the crust during the earliest stages of Earth’s evolution remains debated, and Archaean rocks and minerals preserved in cratonic environments constitute the primary means for investigating early crust. Three geochemical proxies for crustal evolution are tested in this thesis: i) a new proxy for the depth of crystallisation of differentiated magmas, based on the variation of the 176Lu/177Hf ratio in zircon; ii) a widely-used proxy for changes in the tectonic settings of crust formation, based on the identification of vertical trends in zircon εHf(t) versus crystallisation age plots; and iii) a recent proxy for changes in the composition of the juvenile crust through time, based on combined information from U–Pb and Hf isotopes in zircon and from Sr isotopes in apatite inclusions within zircon.The development of LA-MC-ICP-MS techniques to measure U–Pb, Hf and Sr isotopes with the highest precision in small volumes of samples (i.e. typically 700–70000 µm3) constitutes a key aspect of this work. In zircon, a precision better than 1% (2 s.e.) was obtained for U–Pb ages measured with a laser ablation beam of 12x12 µm, and a precision better than 150 ppm (2 s.e.) was achieved for 176Hf/177Hf ratios measured at 25x25 µm. In apatite, a precision better than 2000 ppm (2 s.e.) was reached for the 87Sr/86Sr ratio at only 5x5 µm. This high level of precision can only be achieved when the 9 Faraday cups measuring Sr isotopes and their interfering species are connected to 1013 ohm amplifiers.A worldwide compilation of 176Lu/177Hf ratios in >100,000 zircons with Hadean to Phanerozoic crystallisation ages reveals the presence of cyclical oscillations that are broadly in tune with the δ18O record in zircon, and with periods of continental collision and supercontinent amalgamation. The mean depth of crystallisation of differentiated magmas has decreased with time over the last 3 billion years, as indicated by lower 176Lu/177Hf ratios. Prior to ~3.0 Ga, the nature of tectonic interactions was different from what is observed today in a plate tectonics context.The vertical shifts of Hf isotopes in the εHf(t) versus crystallisation age space can be caused by the resetting of the Lu–Hf system in response to heating and metamorphic reactions on a mineral scale. Such vertical shifts do not necessarily involve juvenile additions to the continental crust, since they may be produced by the breakdown of high-Lu/Hf phases (i.e. garnet and pyroxene) in high-grade metamorphic conditions. High grade metamorphic events in the early Earth may be related to the generation of the first cratons in a geodynamical context that does not necessary require plate tectonics.Granites and TTGs from the Mount Edgar Dome (East Pilbara Terrane, Western Australia), taken as representative of the Palaeoarchaean evolution of the continental crust, were analysed for their major and trace elements in whole rocks, U–Pb, Hf and O isotopes in zircon, and Sr isotopes in matrix apatites and apatite inclusions in zircon. Both granites and TTGs were emplaced episodically from 3.47 Ga to 3.23 Ga. Oxygen and Hf isotopes indicate that they were formed via the protracted intracrustal reworking of two distinct crustal reservoirs extracted from a depleted mantle source at ~3.5 Ga, with no evidence for subduction-related magmatism. The least radiogenic 87Sr/86Sr ratios measured in matrix apatites and apatite inclusions in zircon suggest that mantle–crust differentiation processes older than 3.5 Ga formed crustal reservoirs with intermediate to felsic compositions. Overall, the geochemical data indicate a large heterogeneity of the mantle beneath the East Pilbara Terrane in the early Archaean.