These continents were able to rise above sea level, necessary for terrestrial life, because the continental crust is poorer in iron and more oxidized compared to that under the ocean. This means that these continents are less dense and more buoyant, which means they are higher.
But the new study eliminates this hypothesis, while also providing a better understanding of the continents. The team behind this study sought a way to test this hypothesis regarding garnet crystallization by replicating a version of that heat and pressure in the laboratory.
They did this using “piston and cylinder presses,” which can apply a huge amount of force to small samples, as well as using a heating assembly that heats them up at the same time. In 13 different experiments, the researchers grew garnet samples from molten rock under conditions similar to those inside the Earth’s crust.
They then collected garnet samples from around the world. These had been analyzed, so that their composition was known, and the researchers knew how much oxidized iron they had and how little oxidized they had.
The researchers then compared the two, with the help of X-rays that were used to understand their composition. They found that the garnets did not contain enough unoxidized iron to represent the composition present on Earth.
„Athese results make the garnet crystallization model an extremely unlikely explanation for why magmas from continental arc volcanoes are oxidized and iron-depleted” said Elizabeth Cottrell, one of the researchers who participated in the new work, in a statement. “Eit is much more likely that the conditions in the Earth’s mantle, below the continental crust, establish these oxidized conditions”.
That means one of the leading theories can now be dismissed. But it’s not yet known exactly what happens, and researchers hope to work to conduct further research to understand what happens in this mysterious process.
An article describing the findings, “Garnet crystallization does not drive oxidation at arcs,” is published in the journal Science.
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