Image: Caroline McNiel / National MagLab
Scientists at Florida state University in the US found that in the new earth’s crust there are fragments of ancient crust, which long ago sank into the mantle in the subduction zone and almost all melted. The number of these parts indicates that the planet has started to update its surface since its formation 4.5 billion years ago, contrary to prevailing views. Article researchers, which razvalivaetsya the mystery of the evolution of the earth’s crust, published in the journal Science Advances.
Geologists analyzed the chemical analysis 500 samples of basaltic rocks, taken in 30 sites of mid-oceanic ridges (MORB), where the formation of new crust in the process of spreading. In these zones of mantle flows (plumes) rise to the surface, forming igneous rocks and pushing lithospheric plates. Simultaneously, other, more ancient areas of oceanic crust dives under continental crust and sink into the mantle.
Previously, researchers found that the molten part of the crust may return to mid-ocean ridges and to fit into the new bark. These areas are characterized by abnormally low ratio of germanium and silicon in comparison with the new species.
The researchers compared the ratio between different types of basalts ranges — chemically depleted (D-MORB) and enriched (E-MORB). Depleted basalts are formed by low degrees of melting, when the lattices of minerals are extracted are incompatible with the cationic centers of the elements and the enriched basalts, on the contrary, these incompatible elements are more common. Previously it was thought that E-MORB are formed from melts in the area of mantle plumes, but it was later revealed that some of the enriched basalts occur away from the places where the flows of the mantle rise to the surface. The researchers tested could these basalts belong to the ancient bark.
It turned out that for E-MORB characteristically low ratio of germanium and silicon, which proves that these anomalous areas are recycled in the mantle fragments of ancient crust. Their number indicates that 5-6% of the Earth’s mantle presented to the melted bark. In turn, this means that the formation of crust in the past, the pace could not significantly differ from the current process.
Video, photo All from Russia.