MSU scientists together with colleagues from Switzerland and Australia revealed tectonic processes that took place on a young land several billion years ago. It turned out that then these processes were very different from modern due to higher temperatures of the upper mantle. The results of the study are published in the Journal Scientific Reports.
It is known that at an early stage of existence, the land was hotter than now. More than 2.5 billion years ago, the planet possessed a thinner lithosphere (solid shell), and the high temperature of the mantle contributed to a higher degree of melting dominant-contained in it – peridotite. From melting peridotitis, such processes as the formation of a crust and volcanic activity are dependent, but little is known about how it could affect the tectonic processes of the past.
In the new work, the researchers simulated oceanic plate tectonics from Archeye (4-2.5 billion years ago) to this day in the temperature range from 1300 to 1550 degrees Celsius (the difference between the ancient and modern temperatures varies from 0 to 250 degrees). It is believed that in the early Earth there was no subduction in the modern form, when one tectonic stove is immersed under another slab. For that time, a bilateral subduction was typical, when both contacting plates were immersed in the mantle.
It turned out that the transition from bilateral to one-sided regime of subducts occurs when the temperature of the upper mantle falls below 1550 degrees Celsius. With one-sided subduction of rocks, immersed in the mantle, are melted, thereby volcanic arcs arise, however, in the case of two-sided subducts, another picture is observed. Instead, a wedge of a depth of 180 kilometers was formed, which consisted of metabazite rocks containing a large amount of water. Such wedges could become the buds of modern continents, but this is to find out in future studies.
This entire process led to the emergence of an overly thickened geochemically exhausted (deprived of many chemical components), a mantle, which peelled from the plates and accumulated under the oceanic lithosphere. Later, part of this mantle was part of the cratones – the ancient parts of modern lithospheric plates. The depleted mantle is present under the lithosphere and at present, but now its thickness is significantly less.