Many questions remain about Earth’s formation around 4.6 billion years ago. Why does Earth store so much carbon in the mantle, but not other elements, like hydrogen and nitrogen? How did the first oxidized compounds necessary for life, like carbon dioxide and water, leak from Earth’s interior to accumulate at the surface? Now a new study may explain both these mysteries.
DCO scientists Daniel Frost, Catherine McCammon (both at University of Bayreuth, Germany), and colleagues replicated the “magma ocean” stage of Earth’s evolution that occurred when a giant impact blasted off enough rock to create the Moon and melted the rocky planet. They discovered that under high-pressure and temperature conditions deep in the molten magma ocean, iron likely displayed some surprising chemical behavior. Its unusual activity may have helped to oxidize the surface and create a “carbon pump” that moved carbon dioxide into the mantle where it formed diamonds. This behavior may be one reason why Earth supports life, while its neighbor Venus, which never experienced a magma ocean phase, suffocates inside a thick, carbon dioxide rich atmosphere. The novel findings are published in a new paper  in Science.
To recreate the early magma ocean in the lab, the researchers melted the mineral bridgmanite at temperatures of more than 2000 degrees Celsius within a multi-anvil press, which squeezed the sample to intense pressures. Bridgmanite is believed to be the most abundant material in Earth and makes up most of the mantle.
They discovered that under these conditions, the iron atoms inside bridgmanite essentially react with each other. Half of the iron atoms lose electrons and become fully oxidized, just like the form of iron in rust, while the other half gain those electrons to become fully reduced, which turns them into molten iron metal. If such reactions occurred in the magma ocean, the oxidized iron would travel to the surface, carried by the vertical currents that drive mantle convection, and the molten iron would sink deeper to build up the core.
Iron’s strange behavior could create a gradient through Earth, with oxidized compounds necessary for life, like water and carbon dioxide, accumulating at the surface and more reduced conditions persisting deep in the interior. Once carbon dioxide formed at the surface, it could dissolve into the magma ocean, like a giant carbon sponge. As the carbon dioxide sank deeper through convection, the more reduced conditions would convert it to diamonds.
“It’s like a carbon pump that enables the carbon dioxide from the atmosphere to be captured into Earth’s interior, like a diamond factory,” said McCammon. “A lot of diamond would have been formed very early in Earth’s history.”
“This may be an explanation for why Venus has this hostile atmosphere whereas Earth’s became habitable,” said McCammon. Venus never experienced an impact large enough to create a deep magma ocean, and so its carbon dioxide stayed at the surface.
While vast stores of diamonds likely exist in the deep mantle, McCammon doesn’t think these deep gems have erupted at Earth’s surface. And if they have, it would be difficult to recognize them, since determining the age and depth of a diamond’s origins typically relies on identifying the minerals inside blobs of mantle material that get trapped as the diamonds form.
If scientists do identify deep, ancient diamonds in the future, it would not only provide evidence for the importance of iron in the magma ocean for Earth’s evolution, but would also support a very early start for the movement of carbon into the rocky subsurface. “This provides a mechanism for the deep carbon cycle to have started right at the start of Earth’s history, 4.6 billion years ago,” said McCammon.
Main image: The giant impact to Earth that produced the Moon likely created a magma ocean on both bodies and altered Earth’s evolution as a planet. Credit: NASA/SVS/GSFC