Probing Iron Chemistry in the Deep Mantle

Laboratory experiments, along with the discovery of tiny bits of carbonate impurities in lower mantle diamonds, indicate that carbonates could withstand the extreme pressures and temperatures of not only the upper mantle, but the lower mantle as well.

Carbonates are a group of minerals that contain the carbonate ion (CO32-) and a metal, such as iron or magnesium. Carbonates are important constituents of marine sediments and are heavily involved in the planet’s deep carbon cycle, primarily due to oceanic subduction. During subduction, carbonates interact with other minerals, which alter their chemical compositions. The nature of these interactions is critical for understanding the deep carbon cycle. 

It was thought that carbonates, known constituents of the upper mantle, could not withstand the more extreme conditions of the lower mantle. However, laboratory experiments, along with the discovery of tiny bits of carbonate impurities in lower mantle diamonds, indicated that carbonates could withstand the extreme pressures and temperatures of not only the upper mantle, but the lower mantle as well.

A team of DCO researchers including Sergey Lobanov, Alexander Goncharov (Carnegie Institution of Washington, USA), and Konstantin Litasov (Russian Academy of Science and Novosibirsk State University, Russia) focused on the high-pressure chemistry of a carbonate mineral called siderite, which is an iron carbonate, FeCO3, commonly found in hydrothermal vents. Their findings, published this week in American Mineralogist, help resolve questions about the presence of iron-containing lower mantle carbonates [1].

Until recently the electron-arrangement change responsible for iron redistribution in the lower mantle had not been measured in the lab. It was previously shown that this change, a phenomenon called a spin transition, took place between about 424,000 and 484,000 times normal atmospheric pressure (43 to 49 gigapascals). The team was able to pinpoint that the spin transition was occurring in iron carbonates under about 434,000 times normal atmospheric pressure (44 gigapascals), typical of the lower mantle. Pressure-induced spin transitions rearrange electrons and change the energy of the chemical bonds. If the change in chemical bond energy is high enough, the spin transition may trigger iron redistribution between coexisting minerals.

To quantify the energy change, Lobanov et al examined siderite’s spin transition using highly sensitive spectroscopic techniques at pressures ranging from zero to about 711,000 times normal atmospheric pressure (72 gigapascals). Their spectroscopic data provide the key ingredient for estimating carbonate composition at pressures exceeding the spin transition-pressure, and suggest that lower mantle carbonates should be iron-rich, unlike upper mantle carbonates which are magnesium-rich and iron-poor (pictured). Similar effects may exist in other lower mantle minerals also undergoing spin transitions.

“As we learn more about how the spin transition affects chemical composition in carbonates, we improve our understanding of all iron-bearing minerals, enhancing our knowledge about lower mantle chemistry,” said Lobanov.

This work was supported by the Deep Carbon Observatory, the Ministry of Education and Science of the Russian Federation, NSF EAR, and the Carnegie Institution for Science. 

Article adapted from Carnegie Institution of Washington, USA. Source.

Image: Illustration of the possible location of carbonates' spin transition in the lower mantle. Credit: Sergery Lobanov.

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