研究者は地球の奥深くに「ダイヤモンド工場」を発見しました

研究者たちは、地球のコア層の境界でさびとダイヤモンドを発見しました。

地球の表面では、鋼は水と空気によって錆びます。 しかし、地球内部の深さはどうでしょうか?

地球上で最大の炭素貯蔵量は地球のコアであり、炭素の 90% が埋もれています。 科学者たちは、構造プレートの上に位置し、内陸に落ちる海洋地殻には、水和鉱物が含まれており、コアとマントルの境界に到達する場合があることを示しています。 コアとマントルの境界では、温度は溶岩の温度の少なくとも 2 倍であり、水が水和鉱物から逃れるのに十分な高さです。 その結果、地球のマントルの境界付近で鉄の錆びに似た化学反応が起こる可能性があります。

Byungkwan-koo は、最近博士号を取得しました。 から出る アリゾナ州立大学、および同僚は最近、ジャーナル Geophysical Research Letters でコアとマントルの境界に関する発見を報告しました。 彼らは高度な光子源で実験を行いました アルゴンヌ国立研究所圧力と加熱水と炭素鉄[{” attribute=””>alloy to conditions similar to those at the Earth’s core-mantle boundary, melting the iron-carbon alloy.

The iron-carbon alloy reacted with water at high pressure and high-temperature conditions related to the Earth’s deep mantle in a diamond-anvil cell. Credit: Arizona State University

The scientists discovered that water and metal react to form iron oxides and iron hydroxides, just like rusting on Earth’s surface. However, they observed that at the core-mantle boundary conditions, carbon separates from the liquid iron-metal alloy and forms diamonds.

“Temperature at the boundary between the silicate mantle and the metallic core at 3,000 km depth reaches to roughly 7,000 F, which is sufficiently high for most minerals to lose H2O captured in their atomic-scale structures,” said Dan Shim, professor at ASU’s School of Earth and Space Exploration. “In fact, the temperature is high enough that some minerals should melt at such conditions.”

Because carbon is an iron-loving element, significant carbon is expected to exist in the core, while the mantle is thought to have relatively low carbon. However, scientists have found that much more carbon exists in the mantle than expected.

“At the pressures expected for the Earth’s core-mantle boundary, hydrogen alloying with iron metal liquid appears to reduce the solubility of other light elements in the core,” said Shim. “Therefore, the solubility of carbon, which likely exists in the Earth’s core, decreases locally where hydrogen enters into the core from the mantle (through dehydration). The stable form of carbon at the pressure-temperature conditions of Earth’s core-mantle boundary is diamond. So the carbon escaping from the liquid outer core would become diamond when it enters into the mantle.”

“Carbon is an essential element for life and plays an important role in many geological processes,” said Ko. “The new discovery of a carbon transfer mechanism from the core to the mantle will shed light on the understanding of the carbon cycle in the Earth’s deep interior. This is even more exciting given that the diamond formation at the core-mantle boundary might have been going on for billions of years since the initiation of subduction on the planet.”

Ko’s new study shows that carbon leaking from the core into the mantle by this diamond formation process may supply enough carbon to explain the elevated carbon amounts in the mantle. Ko and his collaborators also predicted that diamond-rich structures can exist at the core-mantle boundary and that seismic studies might detect the structures because seismic waves should travel unusually fast for the structures.

“The reason that seismic waves should propagate exceptionally fast through diamond-rich structures at the core-mantle boundary is that diamonds are extremely incompressible and less dense than other materials at the core-mantle boundary,” said Shim.

Ko and the team will continue investigating how the reaction can also change the concentration of other light elements in the core, such as silicon, sulfur, and oxygen, and how such changes can impact the mineralogy of the deep mantle.

Reference: “Water-Induced Diamond Formation at Earth’s Core-Mantle Boundary” by Byeongkwan Ko, Stella Chariton, Vitali Prakapenka, Bin Chen, Edward J. Garnero, Mingming Li and Sang-Heon Shim, 11 August 2022, Geophysical Research Letters.
DOI: 10.1029/2022GL098271