Soutenance de thèse GM : Anda BUZENCHI

à 14h amphi 36.07 campus Triolet – Université de Montpellier

Anda BUZENCHI présentera ses travaux de thèse

Secular changes in the composition of the continental crust and implications for Earth dynamics: New insights from high-precision analyses of Sr, U–Pb and Hf isotopes by LA-MC-ICP-MS »

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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 constitutes a key aspect of this work. High precision and high spatial resolution were obtained for U–Pb ages and ¹⁷⁶Hf/¹⁷⁷Hf ratios measured in zircon and for ⁸⁷Sr/⁸⁶Sr measured in apatite. 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.

 

 

Composition du jury :

Maud BOYET, Directrice de recherche CNRS, Rapportrice

Craig STOREY, Professeur University of Portsmouth, Rapporteur

Émilie BRUAND, Chargée de recherche CNRS, Examinatrice

Pascal PHILIPPOT, Directeur de recherche CNRS, Examinateur

Bruno DHUIME, Chargé de recherche CNRS, Directeur de thèse

Olivier BRUGUIER, Ingénieur de recherche CNRS, Co-directeur de thèse