The enigmatic formation of the first continental crust

The continents currently represent nearly 30% of the earth’s surface. If the tectonic plates has reworked and moved them many times, it appears that the amount of continental crust has remained relatively stable over time. If the first continental crust appeared very early in the history of EarthHowever, the question of the mechanisms and conditions that led to its formation is still very much debated. The present-day relics of this first felsic crust indicate that it formed duringHadean which marks the original period of the Earth, between 4.5 and 4 billion years. These relics are found in the form of zircons, from minerals silicates.

Hadean Earth: an ocean of magma

Many models suggest that in Hadean, the Earth was characterized by the presence of a huge ocean of magma. No continents or plate tectonics at this time. Only rock in fusion. The way in which a felsic crust, that is to say composed mainly of silicate rocks such as granites, could have formed from this magmatic ocean of very different composition, is not clear.

Several processes are proposed to explain the origin of the first continental rocks. One hypothesis in particular suggests that a proto-crust mainly composed of peridotites would have floated above the ocean of magma primitive. Currently, peridotites are the characteristic minerals of earthly mantle. This proto-crust would have quickly hydrated on contact with thehydrosphere already present, this process leading to the formation of a surface layer of serpentinized peridotite. The residual part of the magmatic ocean would then have evolved below this primitive crust, producing magmas basaltic, enriched with incompatible elements, which have difficulty reacting with minerals and therefore preferentially enter the phase liquid, magma. The composition of these magmas would also be quite similar to that of some basalts found on the Moon.

Laboratory studies then showed that the interaction between serpentinized peridotites and basaltic magmas can produce tonalites and granodiorites, which are felsic rocks. These rocks could have been formed at a shallow depth, less than 10 km below the surface. The process of partial fusion at the origin of these rocks characteristic of the continental crust would have been generated by the dehydration associated with the intrusion of basaltic magmas in the serpentinite or by meteorite impacts.

Hadean zircons, witnesses to the formation of the first continental crust

To validate this hypothesis, researchers were interested in the crystallization conditions of Zircons of Hadean age. These minerals, more than 4 billion years old, in fact represent the oldest witnesses to the formation of a continental crust. Most hadean zircons come from Jack Hills in Australia. Usually, zircons are formed during the genesis of magmatic rocks with felsic component. It is one of the first minerals to crystallize from a primary magma. If these minerals give reliable information concerning their crystallization environment, there is however no consensus concerning the conditions of genesis of the magma from which they come.

The international team of researchers led by Anastassia Borisova from the University of Toulouse and the University of Moscow, therefore carried out a series of laboratory experiments and modelization digital to reproduce the characteristics of the magma that gave rise to the hadean zircons. The results, published in the journal Geology, show that the hypothesis of a serpentinized proto-crust is compatible with the characteristics of hadean zircons found in Australia and that the processes mentioned above could have led to the formation of the first felsic rocks.

One of the important points is that the formation mechanisms of this first continental crust would not have required an environment associated with plate tectonics, even primitive. These results are all the more interesting as they could also explain the formation of the primitive felsic crust of Mars.