Chemical reactions involving iron could have increased oxygen-rich compounds in the mantle
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We may have ancient magma oceans to thank for Earth’s breathable air.
Shortly after the planet’s formation about 4.5 billion years ago, the mantle somehow became much richer in oxygen than it was originally. That rock began leaking molecules like carbon dioxide and water into the oxygen-poor atmosphere — helping to jump-start conditions suitable for life some 2 billion years before the Great Oxidation Event, when the amount of molecular oxygen in the atmosphere skyrocketed (SN: 2/6/17).
The cause of that chemical transition in the mantle has been a mystery. Now, new lab experiments suggest that chemical reactions involving iron in early Earth’s magma oceans tipped the chemical balance of the mantle in favor of more oxygen-rich compounds, researchers report in the Aug. 30 Science.
“This is more than a chemical curiosity.… It’s profoundly important because it really sets the stage for all of Earth’s subsequent evolution,” says Jonathan Tucker, a geochemist at the Carnegie Institution for Science in Washington, D.C., who was not involved in the work. “The oxidation state of the Earth, and planets in general, is a very, very important factor controlling habitability.”
Early in Earth’s history, the planet was pummeled by planetesimals, which could have created oceans of molten rock that dipped hundreds of kilometers deep into the mantle. Scientists have suspected that intense pressure in such magma oceans forced oxygen-containing ferrous iron to split into two different kinds of iron: one richer in oxygen, called ferric iron, and oxygen-free metal iron. This heavy metallic iron would have sunk into the Earth’s core, leaving the mantle dominated by more oxygen-rich ferric iron.
To test that idea, geochemists at the University of Bayreuth in Germany performed lab experiments that simulated conditions about 600 kilometers deep inside a magma ocean. While heating synthetic mantle material to thousands of degrees Celsius, the researchers used anvils to crush the molten samples with pressures up to more than 20 gigapascals.
“That’s the equivalent of putting the entire mass of the Eiffel Tower on an object the size of a golf ball,” says Katherine Armstrong, now at the University of California, Davis.
Armstrong and colleagues measured the amounts of ferrous and ferric iron in samples before and after exposure to these extreme conditions. No matter how much ferrous iron was originally in the rock, at the highest pressures 96 percent of the iron in the final product was the oxygen-rich ferric iron.
That finding indicates that deep in a magma ocean, ferric iron is more stable, Armstrong explains. Any ferrous iron at those depths would be liable to decompose into ferric iron, shedding metallic iron that would sink to the core.
These results are “pretty convincing” evidence that the chemical breakdown of ferrous iron in magma oceans could have helped boost the relative abundance of oxygen in the early Earth’s mantle, Tucker says. But it’s not yet clear whether this chemical process was the only one that contributed to the uptick of oxygen in early Earth’s atmosphere, he adds.
Afu Lin, a mineral physicist at the University of Texas at Austin who wasn’t involved in the work, similarly finds the decomposition of ferrous iron a plausible explanation for Earth’s oxygen-rich atmosphere. Researchers could help validate this account, he says, by searching for chemical signatures of the process in early Earth rocks and super deep diamonds from the mantle (SN: 8/15/19).