Graveyard of Earth’s Plates: Ancient Protocrust of Earth Discovered in Depths of Earth’s Mantle

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Earth’s earliest crust, more than four billion years old, survived deep in the mantle / Traces of ancient Earth’s crust still exist today in volcanic rocks at Earth’s surface / Publication in PNAS

Dr. Jonas Tusch and Carsten Münker, both scientists at the Institute of Geology and Mineralogy at the University of Cologne, along with their colleague Dr. Elis Hoffmann, Freie Universität Berlin, used chemical analyses of magmatic rocks to prove that the remains of the first crust of our planet lie buried at great depths in Earth’s mantle. The findings have just been published in the journal PNAS under the title "Long-term preservation of Hadean protocrust in Earth’s mantle."

Earth’s earliest crust has been lying dormant in a "graveyard" of cooled ancient tectonic plates for more than four billion years, probably ever since the Moon and Earth emerged from a gigantic cosmic event, the collision of two protoplanets. This is very surprising, since in the course of Earth’s history, the old crust was recycled into Earth’s mantle and mixed up there by plate tectonic processes within short geological periods of time. Up to now scientists assumed that witnesses of the earliest geological processes have only survived on other celestial bodies, such as the terrestrial planets (Mercury, Venus, Mars), asteroids, or the moon, where there are no significant plate tectonics. However, they now concluded that parts of the old crust resurfaced from the deepest part of Earth through volcanism. The chemical properties of certain igneous rocks on Earth today provide scientists with detailed information about what the earliest crust was like and how it found its way into the "graveyard"of Earth’s plates.

In cooperation with international colleagues, the German scientists examined rocks from South Africa and Eswatini that were up to 3.55 billion years old. An examination of these rocks revealed that the samples show small anomalies in the isotopic abundance of the chemical element tungsten. The origin of these anomalies, specifically the relative abundance of the isotope tungsten-182 (182W), points to geological processes that must have occurred more than 4.5 billion years ago. Using modeled mantle melts, the German research team shows that the present 182W anomalies can best be explained by recycling extremely old crust into Earth’s mantle. "We assume that the lower layers of the crust, i.e., the roots of the supercontinents, became heavier than their surroundings due to a geological maturing process and sank into the underlying mantle, similar to a lava lamp," explains the Cologne geochemist Dr. Jonas Tusch.

The ancient crust rested in the deep regions of Earth’s mantle, probably at the border of Earth’s outer core. Parts of the material were tapped via so-called mantle plumes, similar to conveyor belts, and transported back to higher regions of the mantle and sometimes even to Earth’s surface. Remarkably, the same isotope patterns are still found today in certain volcanic rocks (so-called ocean island basalts), e.g., on Hawaii, La Reunion, or Galapagos, demonstrating that remnants of Earth’s oldest crust are still buried in the lower mantle. "This fascinating finding opens up the possibility of obtaining a geochemical fingerprint of the early Earth. It enables us to better understand how large continents formed over the course of Earth’s history and how an oxygen-rich atmosphere was able to develop, which is an important prerequisite for the emergence of complex life forms," says Dr. Elis Hoffmann from Freie Universität Berlin.

The geochemical fingerprint of early Earth can also be compared to what scientists have learned about other planets from space missions. For example, data from the Mars missions and studies of Martian meteorites show that Mars still has a very old surface due to the lack of plate tectonics. This may correspond to the composition of young Earth.

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