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Plate tectonics, a very old story

Researchers at IPGP have demonstrated for the first time that plate tectonics were already active on Earth 4 billion years ago. Their study shows that Archean samples display a signature typical of the subduction phenomenon.

Plate tectonics, a very old story

Publication date: 01/10/2019

Press, Research

Related themes : Origins

Our planet’s telluric phenomena (volcanic eruptions, earthquakes, etc.) remind us that the Earth is an active planet. This activity is the result of cooling by convection in its mantle, which is now reflected on the surface by the movement of tectonic plates in relation to each other, carried by this convection.

But the origin in time of this plate tectonics is still hotly debated in the scientific community. Did the Earth have a youth without plate tectonics? And if so, when did plate tectonics begin? These major scientific questions have yet to be resolved.

Fragments of the oldest continents preserved to this day, such as the granitoid rocks formed in the Archean period between 3 and 4 billion years ago, are crucial witnesses to the mechanisms at work in the first few billion years of our planet. However, the lack of chemical or isotopic tracers means that for the moment it is impossible to decide between the two hypotheses put forward to explain the origin of these primitive rocks: the melting of a hydrous oceanic crust plunging into the Earth’s mantle (subduction implying tectonic activity) or melting at depth at the base of an oceanic plateau (without tectonic activity).

Acasta gneiss, the source of the Archean samples (© Martin Guitreau)

A study conducted by researchers from the Institut de Physique du Globe de Paris in collaboration with the University of Clermont Auvergne and two American universities (University of Chicago and University of Maryland) and published in Nature Geoscience on  August 26th 2019 has shown that the composition of Archaean granitoids, and in particular their silicon isotope ratios, are typical of the chemistry of primitive oceans. And more specifically of the specific siliceous marine sediments of the Archaean oceans (cherts). The presence of this typical isotopic signature in magmas emplaced in the Earth’s crust can only be explained by the melting of the hydrous oceanic crust plunging into the mantle during a subduction process.

This observation removes any doubts about the origin of these rocks and shows that the Earth was already experiencing plate tectonic activity 4 billion years ago!

Ref: Deng et al. An oceanic subduction origin for Archaean granitoids revealed by silicon isotopes. Nature Geoscience. doi.org/10.1038/s41561-019-0407-6

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