Thesis defense of Zhengbin DENG – Wednesday, September 19
Zhengbin DENG collaborates with the UnivEarthS I6 team: From dust to planets in which he completed his doctorate. UnivEarthS is pleased to inform you of the defense of its thesis entitled:
“Silicon and titanium stable isotope perspectives on the Earth’s mantle-crust evolution”.
It will be held on Wednesday, September 19 at 14:00 in the IPGP amphitheatre, in front of the next jury:
Rapporteur – Richard W. Carlson (Staff scientist, Carnegie Institution of Washington, USA)
Rapporteur – Roberta Rudnick (Professor, University of California, Santa Barbara, USA)
Examiner – Janne Blichert-Toft (Research Director, CNRS, ENS Lyon)
Examiner – Catherine Chauvel (Research Director, CNRS, IPGP)
Thesis Director – Marc Chaussidon (Research Director, IPGP)
Thesis Director – Frédéric Moynier (University of Paris Diderot, IPGP)
The chronology of the emergence of plate tectonics in the Earth’s history is still debated, proposed timings for its onset ranging from > 4.2 Ga to ≈ 0.85 Ga. This debate arose from the unsettled origins of Hadean zircons, Archean tonalite-trondhjemite-granodiorite (TTGs) and continental crust in general. In this PhD thesis, three approaches using high-precision Si and Ti stable isotope data have been carried out to study the Earth’s mantle-crust evolution:
(i) A Si isotopic study of Archean TTGs and komatiites, as well as Phanerozoic granites, reveals the extensive contamination in the sources of TTGs by sedimentary cherts, pointing clearly to an oceanic subduction origin for the Archean TTGs.
(ii) A 47Ti-49Ti double spike method has been implemented to measure precisely Ti isotopic variations in nature, including chondritic matter. Its application on komatiites and MORBs shows that the present-day depleted MORB mantle (DMM) likely formed by an efficient mixing into the mantle of melting residues from the generation of Archean continental crust.
(iii) Another Ti isotopic study of differentiated igneous rocks demonstrates that the combination of Ti isotopes with SiO2 content provides a distinctive signature for either tholeiitic or calc-alkaline magmas. The application of this proxy shows that the continental crust was generated under plume setting at 3.8-3.5 Ga, and started to be dominated by island arc rocks after ≈ 3.5-3.0 Ga, thus showing the prevalence of plate tectonics at this early epoch.
These results together imply that plate subduction has been locally operating on the Earth since ≈ 4.0 Ga, but that plate tectonics became globally prevalent only after 3.0 Ga, resulting in the development of a felsic continental crust and in the formation of the DMM, and participating to the decrease of the Earth’s thermal gradient during late Archean.