Description du sujet de thèse

The Earth's continental crust is distinguished from other planetary crusts by its predominantly felsic composition (Si- and Al-rich). The presence of a rigid, stable felsic crust could be considered one of the consequences of the existence of abundant liquid water on Earth, but also on other rocky planets. Current models predict that an ocean of liquid water could be produced on planet's surface over a wide range of planetary surface temperatures, atmospheric compositions, and distance from the Sun. The isotopic variability in the relative abundances of long and short-lived nuclide decay products, such as 176Hf (from 176Lu), 142Nd (produced from 146Sm), and 182W (from 182Hf) provides evidence that primordial reservoirs were involved in the formation of the first continental crust. Yet, most models consider the recycling of basaltic protocrust and garnet-bearing residues instead of a Hadean serpentinized peridotitic protocrust and olivine-chromite-bearing residue evidenced by recent work. Preliminary data suggest that the shallow 4.5 – 4.0 Ga terrestrial protocrust was predominantly ultramafic and hydrated due to alteration by liquid seawater-derived fluids. As a consequence, the geochemical behavior of commonly used isotopic systems (e.g., Sm-Nd, Lu-Hf and Hf-W) in such environment has yet to be understood in order to shed new light on early terrestrial crust processes.

Contexte de travail

The planned PhD work will have three main objectives which are: (1) sampling of recently discovered ultramafic Eoarchean samples from Southwest Greenland, (2) acquiring new high-precision and high-quality isotopic data on these samples using 146,147Sm-142,143Nd, 176Lu-176Hf and 182Hf-182W isotope systematics, and (3) making numerical models of early Earth evolution that can reproduce and account for newly acquired data together with available data from the literature. The successful candidate should have a strong background in elemental and isotope geochemistry, petrology and numerical modeling. The collaboration involves LMV, Clermont-Ferrand, France. The present PhD project is an essential part of the PLANETAFELSIC project financed by ERC ADG.

Contraintes et risques

Specialist and skills: Isotope Geochemistry, Mass spectrometry, geochemical modelling