Why did the Earth’s mantle cool down 700 million years ago?

Since its formation, the Earth has evacuated its internal heat through mantle convection. But the researchers observed that 720 million years ago, the upper mantle temperature dropped significantly. This episode would be associated with the entry into the mantle of cold material in the subduction zones.

Since its formation, the Earth has constantly lost heat, which is evacuated through the different layers of the earth to the surface. This flow of heat from the inside to the outside of the planet is one of the drivers of mantle convection, which is the basis of plate tectonics.

More precisely, the mantle temperature exerts a first order control on the rheology of the lithosphere (in other words on the physical behavior of rocks) and on its composition.

720 million years ago, the mantle experienced a sudden drop in temperature

It was therefore assumed that the upper mantle had cooled more or less linearly for 3 billion years. However, a new study shows that this is not the case. If the cooling is there, however, it is not regular. The history of the Earth seems to be marked by a period during which the cooling of the mantle increased significantly.

This information was obtained through the analysis of basalt rocks emitted into the continental environment by hot spots. The composition of these rocks makes it possible to trace the temperature of the mantle and therefore to study its variations over time.

It is the study conducted by a team of scientists from the Institute of Oceanology of the Chinese Academy of Sciences. Through statistical analyzes on the geochemistry of intracontinental basalt rocks, the researchers were able to reconstruct the evolution of potential temperature over the last billion years. They were thus able to observe a particular event during the Cryogenian and Ediacaran (-720 to -541 million years). During this time, it appears that the mantle actually cooled much more significantly, with a potential temperature of between 50 and 1,400 ° C.

So what happened at that moment to explain this thermal regime change?

The appearance of low-temperature metamorphic rocks

Researchers note that this period corresponds to the diffuse appearance of a certain type of rock: low-temperature metamorphic rocks.

Recall that metamorphic rocks are rocks resulting from the transformation of pre-existing rocks under the effect of a change in pressure and temperature. There is therefore a very large number of metamorphic rocks, which can be classified according to different degrees of metamorphism during which some typical minerals will crystallize, in well-defined pressure-temperature ranges.

Metamorphism therefore occurs when there is a burial of rocks. This can happen when there is the formation of a mountain range and some rock units are compressed and then covered by other units, but metamorphism is mainly a process associated with zones of subduction and burial of one plate under another plate.

The appearance of low-temperature metamorphic rocks around 720 million years ago is therefore a clear signal of a change in the tectonic regime and the establishment of numerous “cold” subduction zones, characterized by the sinking of an oceanic lithosphere, stuffed with water and sediment, under another plate. A classic situation today, as can be observed in particular at the level of the circumference of the Pacific Ocean.

The beginnings of the “modern” tectonic regime.

The results, published in Earth and planetary sciences letters, therefore suggest that the “Cryogenic-Ediacaran” period would correspond to the establishment of a modern plate tectonic regime, which, through the subduction of cold and hydrated material in the mantle, caused a considerable cooling of the upper mantle. This effect on the mantle temperature would have been further increased by the presence of large quantities of oceanic sediments, resulting from the high sedimentation rates that characterize the Sturtian glaciation (episode of the snowball Earth), in progress at the same time.

This new study allows us to date with greater precision the beginning of modern plate tectonics and to define its influence on the thermal state of the Earth’s mantle.

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