The early infancy of terrestrial planets began about 4.5 Gyr ago in a chaotic and turbulent environment
consisting of dust and gas, the protoplanetary disk. The terrestrial planets grew by accretion of small dust
particles to form kilometer-sized planetesimals and then Moon to Mars-sized planetary embryos. A final
violent collision between a Mars-sized body and the Earth lead to the formation of the Moon. The accretion
process provided enough energy to entirely melt terrestrial planets and induced the so-called early planetary
differentiation. This process resulted in the creation of multiple layers in terrestrial planets with a metallic core,
a rocky primitive mantle, and an outer layer, the primitive crust. First, iron-rich droplets sunk through a global
magma ocean into the center of the planet to form the metallic core. Then, the magma ocean solidified to
generate a rocky unstable primitive mantle. Due to gravitational instability, the mantle will partially melt
resulting in the extraction of the outer layer, the primitive crust. These early differentiation events had a strong
influence on the long-term evolution of the terrestrial planets, such as controlling the onset of plate tectonic
regime on Earth. In addition, the establishment of a stable primordial crust is a requirement for the emergence
of life. It is thus crucial to understand the nature and the timing of such events on terrestrial planets.
There is no preservation of rocky material at the surface of the Earth that formed in these earliest
evolutionary stages due to their recycling during billions of years of plate tectonic regime. Mars could have
preserved such ancient crustal material because of the lack of plate tectonic regime for the majority of its
history. Therefore, Mars represents an important analogue of Earth in its infancy. To characterize the nature
and the timing of the early planetary differentiation events, we sought for variations at the atomic scale in rock
or mineral samples from the oldest terranes on Earth and from martian meteorites. These so-called radiogenic
variations can be caused by the accumulation of a daughter isotope due to the radioactive decay of a parent
isotope. The amount of the daughter isotope depends on the time and the relative initial abundances of the
daughter and parent isotopes. Interesting, these two factors are intimately related to the nature and/or the age
of the mantle or crustal source from which the rock or the mineral are derived from. For instance, we combined
the Lu-Hf (Lutetium/Hafnium) isotope system with the U-Pb (Uranium/Lead) isotope systems of zircon
minerals extracted from a martian meteorite, NWA 7034. This meteorite originates from the Southern
hemisphere of Mars and preserves fragments as old as ~4.43 Gyr-old. We determined that these zircons derived from an ancient crust formed within 20 Myr after the beginning of the Solar System. A corollary is Mars
formed extremely rapidly such as the accretion and the planetary differentiation were completed in less than
20 Myr after Solar System formation.
To further constrain the nature and the age of this primordial crust, we applied an additional
chronometer, the 92Nb92Zr (92-Niobium/92-Zirconium) isotope system. This geochronometer is extremely
sensitive to early episodes of planetary differentiation given that the short-lived 92Nb nuclide survived in our
Solar System only during the first 180 Myr. Any radiogenic 92Zr variations are consequently related to events
that occurred when 92Nb was still extant, namely core formation or mantle-crust differentiation. We determined
that two of the seven NWA 7034 zircons previously analyzed yield radiogenic 92Zr variations. Our results
reflect the mechanism of formation of the crust from which the NWA 7034 zircons derived although additional
work is required to decipher the nature of this mechanism.
To track early differentiation events on Earth, we applied the 92Nb92Zr chronometer to the oldest
terrestrial samples, the Jack Hills zircons from Western Australia and for two primitive meteorites. The
detection of radiogenic 92Zr variations in >3.8 Gyr old terrestrial zircons could be attributed to early crust
formation. We determined that these ancient zircons does not display any radiogenic 92Zr variations, which
confirm a late formation of the crust from which they derived (> 70 Myr after Solar System formation). The
two primitive meteorites called Orgueil and Murchison are designated as carbonaceous chondrites and
represent fragments of planetesimals that avoided differentiation. The detection of 92Zr variations in these
primitive meteorites compared to Earth can be attributed to core formation. We determined that Earth displays
a deficit in 92Zr compared to Orgueil but not compared to Murchison or other primitive carbonaceous
chondrites. This could be either attributed to the terrestrial core formation but also to some heterogeneous
distribution of 92Nb nuclide among the carbonaceous chondrites. As such, the 92Zr deficit in Earth compared
to Orgueil can be alternatively caused by the enrichment in 92Nb nuclide in this latter.