Subduction already at work on Earth 4 billion years ago
Geochemical analysis of 4-billion-year-old samples has identified the processes that led to their formation. A team led by IPGP scientists has demonstrated that plate tectonics were already operational very early in the planet's history, and that the early Earth's atmosphere was then rich in CO2.
Publication date: 05/05/2021
Press, Research
Related teams :
Cosmochemistry, Astrophysics and Experimental Geophysics (CAGE)
Related themes : Origins
The Earth is the only known planet in our solar system with active plate tectonics and a so-called continental crust, which differs from oceanic crust in its high silica content. Plate tectonics, or continental drift, is characterised by the creation of new oceanic crust at ridges and by the burial of this crust beneath the continents in so-called subduction zones.
Many geological phenomena, such as earthquakes and certain volcanic eruptions, are linked to this process of plate tectonics, which has helped to shape the Earth’s surface into the morphology we know today.
But the period in the Earth’s history at which subduction became operational is still debated within the scientific community. Higher temperatures in the early Earth could have delayed the onset of subduction during the planet’s first billion years. Witnesses of the processes at work on the young Earth are rare, precisely because of the recycling of the crust by plate tectonics, but very old granitoids are still present on the surface of the globe and represent precious samples of the formation of the first continents.
In a study published on 5 May in the journal Nature Communication, an international team of scientists from the Institut de Physique du Globe de Paris, the Universities of Paris, Clermont-Auvergne and Waterloo in Canada, and the Massachusetts Institute of Technology in the USA, have studied the abundance of calcium isotopes in samples of ancient granitoids, using a cutting-edge geochemical analysis technique that provides two new pieces of information about the deep processes and environmental conditions that prevailed on the early Earth.
By measuring the abundance of calcium isotopes, the study shows that the granitoid rocks forming the oldest known continents (4 billion years ago) were produced at geothermal gradients of between 500 and 750°C/GPa, the same as those observed in certain modern subduction zones, indicating that subduction processes were under way at least 4 billion years ago.
In addition, the calcium isotope signature in other granitoid samples, 3.8 billion years old, shows that the oceanic crust recycled by subduction to form these granitoids contained oceanic carbonate sediments. Marine carbonates were therefore already present on the Earth’s surface at this time. These results predate the oldest oceanic carbonate units preserved today by 100 million years.
This discovery is particularly important because for these carbonates to precipitate and settle on the ocean floor, the Earth’s atmosphere had to contain a large quantity of CO2. This is in line with the atmospheric models currently proposed to explain the temperatures required to explain the presence of liquid water in the primitive Earth, when the Sun’s luminosity was lower at that time (the young Sun paradox).
This study confirms that the processes of subduction and precipitation of oceanic carbonates began to occur at least 4 billion years ago, suggesting that plate tectonics and the silicate-carbonate cycle began very early in the Earth’s history.
Ref: Antonelli, M.A., Kendrick, J., Yakymchuk, C. et al. Calcium isotope evidence for early Archaean carbonates and subduction of oceanic crust. Nat Commun 12, 2534 (2021). https://doi.org/10.1038/s41467-021-22748-2
