Origin and composition of the Earth :
The precision obtained with the new mass spectrometers has shown that the Earth and the different groups of chondrites were distinguished by very small isotopic differences. In order to constrain the origin and composition of our planet, we wish to measure precisely the isotopic compositions of the primitive and differentiated meteorites since these two groups participated in the planetary accretion. Moreover, the phases carrying nucleosynthetic anomalies must be characterized. If the mineral phases cannot be separated in sufficient quantity, we use the progressive leaching technique.
The last phase of accretion is marked by the giant impact forming the Moon. Lunar anorthosites are the oldest samples identified on the surface of the Moon. They would be formed by plagioclase flotation during the crystallization of a magmatic ocean. While their age places an upper limit on the age of the giant impact, their composition offers a unique opportunity to characterize the geochemical composition of the lunar mantle and thus to trace it back to the juvenile Earth.
Early evolution of the silicate Earth (differentiation process and chronology):
The first 500 Ma of Earth’s history remains largely unknown since witnesses of this activity are very rare. The coupled analysis of several isotopic systems (146Sm-142Nd, 147Sm-143Nd and 176Lu-176Hf) in rocks from the oldest terrains discovered on the surface of the Earth (e.g. South Africa, Canada, Greenland) allows us to understand the evolution of mantle sources. Thus we can discuss the chemical fractionations that occurred early in the Earth’s history and bring additional information on the crystallization models of the terrestrial magma ocean. The Udachnaya pipe (Siberian Craton) represents a unique opportunity to understand the structure and evolution of a craton since we have a complete vertical section with representative samples of the crust and base of the lithosphere. The results of the petrogeochemical and isotopic study of these samples and the U-Th-Pb ages obtained on zircons and accessory minerals give us access to the chronology of the crustal growth. Thus the processes responsible for the formation of cratons (superplumes vs. subduction) can be discussed.
The transition from Archean to modern Earth (Archean geodynamics):
From about 4 Ga, the Earth differentiated a stable continental crust. This one has a tonalitic, trondhjemitic and granodioritic (TTG) composition, very different from the modern continental crust. This Archean crust results from the melting at depth of hydrated basalts transformed into garnet amphibolites or eclogite. This implies that the geodynamic mechanisms and/or the sources of these magmas were different from what they are today. The transition from these archaic mechanisms to “modern” geodynamics took place around 2.5 Ga, at the Archean-Proterozoic transition. At this period, large volumes of magmas called sanukitoids are formed. These have petrological and geochemical characteristics intermediate between those of the Archean GTS and the modern arc series. Their transitional character is therefore not only temporal but also compositional. The Limpopo Belt in South Africa is unique in that it contains numerous intrusions emplaced fairly regularly between 2.9 and 2.2 Ga, the composition of which ranges from TTG for the oldest to sanukitoid for the most recent. Massifs emplaced at different periods are studied by a geochemical approach (major, trace, isotopes) in order to reconstruct a reliable and realistic petrogenetic model and to discuss the geodynamic changes that occurred at this major period of our planet’s history.