The current continental crust differs from the primitive earth’s crust

The chemical composition of zircons hadeans, which are the oldest minerals terrestrial (4.4 to 4 billion years ago), seemed to indicate that the primitive crust had a granitic composition close to that of the continental crust modern. However, this composition could rather reflect their formation at the very end of the crystallization process of puddles more typical of the Archean crust (4 to 2.5 billion years old), the origin of which does not necessarily imply the existence of plate tectonics as we know it today.

The development of a primitive crust, rich in silica, allowed the advent of conditions favorable to theappearance of life on Earth, but its origin is debated as it was largely destroyed by subsequent geological processes. The only relics of this primitive crust are thus zircons detrital of Hadean age (4.4 to 4 billion years) found in sediment most recent in Jack Hills, Australia.

A process called into question

The chemical composition of these zircons indicates that they crystallized in a liquid magmatic of granitic composition. The granites being the most typical rocks of the recent continental crust, this was heretofore considered to indicate the existence of processes comparable to the tectonic plates current.

A study carried out by researchers from the Géosciences Environnement Toulouse (GET / OMP) and Magmas and Volcanoes (LMV, Clermont-Ferrand / Saint-Étienne) questioned this conclusion. Zircons from magmatic rocks the most common of the aeon Archean (aged 4 to 2.5 billion years), which are not granites and belong to the so-called TTG (Tonalite-Trondhjemite-Granodiorite) suite, exhibit homogeneous chemical compositions on a global scale and above all identical to those Jack Hills hadean zircons.

A model thermodynamics explained this observation, demonstrating that the crystallization process of TTGs resulted in a residual liquid of granite composition in which these zircons are formed. Even if the hadean zircons did crystallize from a granitic liquid, it turns out that this composition was not necessarily representative of the primitive crust as a whole. On the contrary, it may have resembled the Archean TTGs, the origin of which does not necessarily imply the tectonic plates as we know it today.

The enigmatic formation of the first continental crust

Article by Morgane Gillard, published on December 10, 2021

The continental crust that forms the continents we know today was formed during the early stages of Earth’s evolution, in the Hadean style. During this period, the early Earth was characterized by an ocean of magma. However, how the first continental crust emerged from this environment is much debated. A recent study brings new elements to this riddle.

The continents currently represent nearly 30% of the earth’s surface. If plate tectonics have altered and displaced 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 the Earth, the question of the mechanisms and conditions that led to its formation is still very much debated. The current relics of this first felsic crust indicate that it was formed in the Hadean which marks the original period of the Earth, between 4.5 and 4 billion years ago. These relics are found in the form of zircons, silicate minerals.

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. It is not clear how a felsic crust, that is to say composed mainly of silicate rocks like granites, was able to form from this magmatic ocean of very different composition.

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 primeval magma. Currently, peridotites are the characteristic minerals of the earth’s 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 basaltic magmas, enriched in incompatible elements, which have difficulty reacting with minerals and therefore preferentially enter the liquid phase, the 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 a 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.