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This picture shows the divisions between the layers of the Earth. Ancient rocky regions the size of a continent encircle the outer liquid core. Credit: Lawrence Livermore National Laboratory
According to new research, ancient, distinct and continent-sized regions of rock, isolated from the collision that created the Moon 4.5 billion years ago, are hundreds of kilometers crust, opening a window to the constituent elements of our planet.
The new study at AGU Geochemistry, Geophysics, Geosystems Journal used models to trace the location and origin of volcanic rock samples found throughout the world and dating back to two solid continents in the deep mantle. The new research suggests that specific giant rocky regions have existed for 4.5 billion years since the beginning of the Earth.
Previously, scientists had hypothesized that the continents separated in the deep mantle originated from subducted oceanic plates. But the new study indicates that these distinct regions may have been formed from an ancient ocean of magma that solidified early in the formation of the Earth and may have survived the massive impact of the Moon.
Determining the origin of the masses reveals more details about their evolution and composition, as well as clues about the history of the primordial Earth in the early Solar System, according to the authors of the study.
It's amazing that these areas have survived most of the Earth's volcanic history, "said Curtis Williams, a geologist at the University of California, Davis, Davis, California and lead author of the study.
Look inside
The mantle is a layer of rock extending over 2,900 kilometers (1,802 miles) to the interior of the Earth. The melted, liquid metallic core of the Earth is under the mantle. The core-mantle boundary is where the solid mantle meets the metallic liquid core.
Scientists knew from previous seismic imaging studies that two individual rock bodies existed near the core-mantle boundary. A solid rocky body is found under Africa and the other under the Pacific Ocean.
Seismic waves, the vibrations produced by earthquakes, traverse these masses differently from the rest of the mantle, suggesting that they have physical properties distinct from those of the surrounding mantle. However, geologists were unable to determine whether seismic waves moved differently across mantle mantle continents due to differences in temperature, mineral composition or density, or a combination of these properties. This meant that they could only make assumptions about the origin and history of the separate rock masses.
"We had all these geochemical measurements on the surface of the Earth, but we did not know how to connect these geochemical measurements to the regions of the Earth's interior. We had all these geophysical images of the Earth's interior, but we did not know how to associate them with geochemistry on the surface of the Earth, "said Williams.
Primitive material and feathers
Williams and his colleagues wanted to determine the origin and evolution of different masses to learn more about the composition and past of the Earth. To do this, they had to be able to identify samples on the Earth's surface with higher concentrations of primitive materials and then trace these samples back to their origin.
Scientists often collect rock samples from volcanic areas such as Hawaii and Iceland, where deep mantle plumes or extremely hot rock columns emerge from near-core areas, melt into the shallow mantle, and emerge far away. tectonic fault lines. These samples consist of igneous rock created from cooling lava. The authors of the study used an existing database of samples and also collected new samples in areas with volcanic activity such as the Balleny Islands in Antarctica.
Geologists can measure specific isotopes in igneous rocks to learn more about the origin and evolution of the Earth. Some isotopes, such as helium 3, are primordial, which means that they were created during the big Bang. Rocks closer to the Earth's crust contain fewer isotopes than deeper rocks that have never been exposed to air. Specimens containing more helium-3 are thought to come from more primitive rocks in the mantle.
The researchers found that some of the samples studied contained more Helium-3, indicating that they could come from primitive rocks deep within the Earth's mantle.
The researchers then used a new model to determine how these primitive samples could have reached the Earth's surface from the mantle. Geological models assume that plumes rise vertically from the depths of the mantle to the surface of the Earth. But the plumes can move off course, deviated, for various reasons. The new model takes into account this plume deviation, allowing the study authors to trace the samples to the two giant masses near the core-mantle boundary.
The combination of isotopic information and the new model allowed researchers to determine the composition of the two giant masses and to theorize their formation.
According to the authors of the study, understanding the composition of specific rock masses near the boundary between the core and the mantle helps geologists to conceptualize the ancient processes of shaping the Earth that led to the modern mantle.
"It's a stronger framework to try to answer these questions by not assuming these vertically increasing materials, but taking into account the degree of deviation that these plumes have seen," said Williams.
Reference: "Primitive helium comes from seismically slow regions of the lowest mantle" by C. D. Williams, S. Mukhopadhyay, L. Rudolph and B. Romanowicz, July 31, 2019, Geochemistry, Geophysics, Geosystems Journal.
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