A new scientific study provides a deeper understanding of the Earth’s crust by testing and proving a common hypothesis about why the continental crust has less iron and more oxidation compared to oceanic crust.
The extremely poor composition of the landscape is a major reason why vast areas of the Earth’s surface remain dry land above sea level, making terrestrial life possible today.
And in advance study Published May 4 in the journal Science, laboratory experiments show that iron-reducing oxidation chemistry in the Earth’s continental crust does not come from the crystallization of the mineral chalcedony, as a popular explanation suggested in 2018.
The mineral garnet hypothesis
For years, scientists assumed that the crystallization of garnet in magma beneath volcanoes was responsible for stripping iron from the Earth’s crust and floating it above the oceans.
Earth’s crust is divided into two types: the older, thicker continental crust and the younger, thicker oceanic crust. New continental crust forms when its building blocks are sent from continental arc volcanoes to the Earth’s surface.
These arc volcanoes are found in parts of the world where oceanic plates sink beneath continental plates, areas called subduction zones.
Continental crust is characterized by a low amount of iron in it, thus capable of floating and forming the land that forms the continents we live on above sea level.
To test the Agate hypothesis, the team simulated conditions of extreme pressure and temperature at the base of continental arc volcanoes using compressors from the Smithsonian and Cornell University’s Hyperbaric Laboratory.
Made of steel and tungsten carbide, these small presses exert tremendous pressure on small rock samples when heated by a roller furnace.
According to the results, the resulting pressures were 15,000 to 30,000 times greater than Earth’s atmosphere, and the temperatures generated ranged from 950 to 1,230 degrees Celsius, hot enough to melt rocks.
Agate analysis in the laboratory
The authors conducted 13 different laboratory experiments in which garnet samples of molten rock were exposed to pressures and temperatures that simulated conditions inside magma chambers deep in the Earth’s crust. This garnet was analyzed in the laboratory using X-ray absorption spectroscopy, and the results were then compared to garnets with known concentrations of oxidized and unoxidized iron.
The results of those experiments revealed that agate did not bind to unoxidized iron from rock samples to account for the degree of iron decay and oxidation in the magma that forms the building blocks of Earth’s continental crust. The results indicate that the agate crystal form is an unlikely explanation for why magma from continental volcanoes is oxidized and iron-depleted.
Study co-author Elizabeth Cottrell, a research geologist at the Smithsonian National Museum of Natural History, said the results make the agate crystal sample a highly unlikely explanation for the oxidation and depletion of iron in magma from continental arc volcanoes. And the conditions in the Earth’s crust are likely to be below the crust. It is the continent that sets those oxidation states.
Cottrell added Press release Published on the “EurekAlert” website, the study’s results raise additional questions regarding explanations of the causes that lead to oxidation or iron decomposition, and ask: “If the crystallization of opal is not in the crust, how did the magma come from the mantle, and what is happening?” Mantle? How have its vehicles been modified?
The researcher points out that it is difficult to answer these questions at the moment, but the main theory now is that oxidized sulfur oxidizes iron.
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